Memory Management in Linux

Memory is very important in linux. We need to be very cautios while managing our memories. We need to figure out bottleneck and make sure we have enough memory so that application running in Linux runs smooth. Few are the parameters we need to keep in mind and tune memory.

vmstat - swap in and swap out should be less.
procps - rpm needed for vmstat
dstat --top-oom -- First process kernal is going to kill so that memory is freed
getconf PAGE_SIZE - To view page size(Memory)
Resident Memory - Is actual memory
Virtual Memory - Each process cant access direct physical memory, so it uses virtual memory. Each process have its own virtual space. Physical memory address space is mapped to virtual memory address space. It is sort of security restriction where process cant directly indulge with Physical RAM. MMU - Memory Management Unit does the mapping between physical address space to virtual address space. pmap 1 shows mapping between VAS to PAS.
pmap 1 - 1st column in virtual memory / 2nd column is Physical memory
Resint column when added comes out more then actual physical memory.
less /proc/1/smaps - It shows dirty read and dirty buffer

Architecture:
1. Free
2. Inactive Clean - File is opened but dint modify. It will read from disk always
3. Inactive dirty - File is opened and made some changes. It will be in RAM, if any power cut, there will be loss, so kernal keep pushing them to disk. BDI is responisble for moving dirty buffers to disk.
4. Inactive

CREATING SWAP in LINUX

FIRST WAY:
Create extra partition and make that partition as SWAP.

Check the present partition disk - fdisk -l
To see the current SWAP - swapon -s
/etc/fstab - We need to make an entry here of partitions so that after reboot, we dont loose the partition created.
To remove Swap - swapoff -v /dev/xvd3
To read from fstab - swapon -a

Step 1:
1. Create partition
   fdisk /dev/sdc   -- main disk name where we will create partition
   After that follow what is on screen, it will help to create a partition for SWAP
   p - will show the partitions
   m - will show menu where all commands and switches are mentioned
   n - add a new partitions."p" will create primary partitions
   partition number - Which number of partitions
   rest few paramter as default enter, enter
   Mention how much partition we need - +2G
   p - will show the new partitions
   t - change partition system ID
   Mention partition number for which we want to change system ID - 3
   Enter HEX code - L and then 82 (Linux SWAP/solaris). L will list the codes
   Then enter W - Write table to disk and exit
   fdisk -l /dev/sdc - Will show the existing partitons on the disk
   
2. Create Swap Partition
   mkswap /dev/sdc3  - It will create the partitions
   swapon -v /dev/sdc3 - It will add the partitions
   swapon -s - will show partition has been added
3. Make entry in Fstab
   device_name mount_point(swap) partition_type(swap) defaults 0 0
4. Check swap
   swapon -a -- It will read from /etc/fstab

SECOND WAY:
Create swap with the existing partition.

#In this tutorial we will learn how to create new swap file on CentOS 7 / RHEL 7. Generally on Cloud Servers ( AWS/ DigitalOcean) or VPS, swap space is not created by default.
#Prerequisites
#Must have free space on mounted disk. You can check by using df -Th command.
#Steps to create/add new swap file on Linux

1. Create swapfile-additional file with dd command in / (root). You can select any other partition but it should be mounted (For eg. /opt, /usr ,/NewMountedPartition)

dd if=/dev/zero of=/swapfile-additional bs=1M count=4048
dd = It is a unix command used for convert and copy a file
if = read from FILE instead of stdin
/dev/zero = /dev/zero is a special file in Unix-like operating systems that provides as many null characters (ASCII NUL, 0x00) as are read from it
of = write to FILE instead of stdout
/swapfile-additional = file named swapfile-additional will be created in /
bs = Read and write bytes at a time but if you do not mention MB or GB like only number it will read as bytes. for eg. bs=1024 means 1024 bytes
count = Copy input blocks in our case it is 1024 (1M * 4048 = 4GB)

2. Run mkswap command to make swap area
mkswap /swapfile-additional

3. Change the permission of file swapfile-additional
chmod 600 /swapfile-additional

4. Permanent mounting the swap space by editing the /etc/fstab file .
Use your file editor, I generally use vi editor.
vi /etc/fstab
Paste below given content in /etc/fstab file
/swapfile-additional swap swap    0   0

5. Now mount the swap area, run below given command.
mount -a

6. Enable the swap area
swapon -a

7. Check the number swap space mounted on your system
swapon -s

8. To check how much is swap space available on system. Run below given command
free -m

TOP Command in Linux

Top is an utility provided by Linux. It gives completes analysis of all the process running in the system like all the active process. Also it tells about system performance like CPU, Memory utilisation. Some of the commands are:

shift + M - List process according to memory usage
shift + P - List the process according to CPU usage
Shift + W - For saving top output to a file.

EXAMPLE:

top - 04:38:00 up 10 days, 13:23,  1 user,  load average: 0.00, 0.01, 0.05
Tasks:  90 total,   2 running,  88 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem :  1014976 total,   116512 free,    93008 used,   805456 buff/cache
KiB Swap:        0 total,        0 free,        0 used.   628588 avail Mem

  PID USER      PR  NI    VIRT    RES    SHR S %CPU %MEM     TIME+ COMMAND
14446 postgres  20   0  279896   2236   1472 S  0.3  0.2   5:04.70 postgres
    1 root      20   0  128092   6700   3944 S  0.0  0.7   0:27.88 systemd
    2 root      20   0       0      0      0 S  0.0  0.0   0:00.10 kthreadd
    3 root      20   0       0      0      0 S  0.0  0.0   0:01.29 ksoftirqd/0
    7 root      rt   0       0      0      0 S  0.0  0.0   0:00.00 migration/0
    8 root      20   0       0      0      0 S  0.0  0.0   0:00.00 rcu_bh
    9 root      20   0       0      0      0 R  0.0  0.0   1:26.88 rcu_sched
   10 root      rt   0       0      0      0 S  0.0  0.0   0:03.87 watchdog/0
   12 root       0 -20       0      0      0 S  0.0  0.0   0:00.00 khelper
   13 root      20   0       0      0      0 S  0.0  0.0   0:00.00 kdevtmpfs
   14 root       0 -20       0      0      0 S  0.0  0.0   0:00.00 netns
   15 root      20   0       0      0      0 S  0.0  0.0   0:00.01 xenwatch
   16 root      20   0       0      0      0 S  0.0  0.0   0:00.00 xenbus
   18 root      20   0       0      0      0 S  0.0  0.0   0:00.14 khungtaskd
   19 root       0 -20       0      0      0 S  0.0  0.0   0:00.00 writeback
   20 root       0 -20       0      0      0 S  0.0  0.0   0:00.00 kintegrityd
   21 root       0 -20       0      0      0 S  0.0  0.0   0:00.00 bioset
   22 root       0 -20       0      0      0 S  0.0  0.0   0:00.00 kblockd
   23 root       0 -20       0      0      0 S  0.0  0.0   0:00.00 md
   28 root      20   0       0      0      0 S  0.0  0.0   0:00.24 kswapd0
   29 root      25   5       0      0      0 S  0.0  0.0   0:00.00 ksmd


Below are the few description of the contents of TOP Command:

Job - Present time, how much time system is up, how many users connected, load average.

Task - How many total tasks, how many tasks running, how many tasks sleeping, how many tasks stopped, how many zombie process.

CPU - % of CPU used by users, % of CPU used by system, % of CPU used by nice value, idle CPU %, % of CPU wating for IO(to write), % of CPU used by Hardware Interupt, % of CPU used by Software Interupt, Steal time - used when there is a virtual box( delay in getting the actual CPU by virtual CPU in virtual systems)

Memory - Total RAM, Memory Used (Actually used - buffer), Memory Free( Actual + buffer + cache from swap) , Buffer( when system have free memory, it extracts few pages and keep it in buffer for caching, if system needs, it will give back the pages from buffer)

Swap- Total Swap, Swap Used, free, caching

PID - Process ID
USER - user through which process are running
PR - Priority - Process priority. 0 - highest priority 40 - Lowest
NI - Nice value - CPU Scheduling Priority. Utility through which we can set the priority manually. -19 -highest priority 20 - lowest priority
Virtual Memory - Each process cant access direct physical memory, so it uses virtual memory. Each process have its own virtual space. Physical memory address space is mapped to virtual memory address space. It is sort of security restriction where process cant directly indulge with Physical RAM. MMU - Memory Management Unit does the mapping between physical address space to virtual address space. pmap 1 shows mapping between VAS to PAS.
Resident Memory - Total Physical Memory that is non swapped.
Shared Memory - Memory that is being shared by other processes
S - state - Sleeping or running
% CPU - % of CPU used by the process
% MEM - % of Memory Used by the process
Time - How long is the process running
Command - What is the command that is being executed.

C - Process with obsolete path
k <pid> - to kill a process
z - Change color of top
r - for re nice a value
1 - shows the number of CPU
d - change refresh rate - Default 3 sec
A - Splits screen into multiple part/screen. Top Right Side says which split screen I am. So accordingly we can sort per split screen. Goto different split screen using a. Shift + a will toggle screen from small to big.
h - help
n - Reduce the number of listed process to n
I - To hide load average line
t - to hide CPU states. 2nd and 3rd line.
m - to hide/show the memory line. 4th and 5th.
top -u postgres or top and press u
top -n 2 stops refresh after 2 sec
top -p pid, pid
top -b -n 1
top -d3 -b -n 2  > top_output.txt.
top -H -p pid
Shift F - add extra column as required. 

Is Truncate DDL or DML

It is very tricky question. Truncate releases the memory. In Oracle, PostgreSQL, MySQL it is DDL. So I believe it will be DDL for other RDBMS also. Most of the RDBMS it drops the table and recreate the schema, thus making truncate faster then Delete. Truncate is fast because there is no where clause like delete and also because the data rows still exists in data pages but extent is marked empty for reuse. Truncate don't scan table like in delete, so the process is faster in truncate. Also truncate don't get logged in log file. We can not truncate table where constraint exists, we need to remove constraints, truncate table and again add back the constraint. Truncate will reset the identity column, which means row id will be 1.

NETSTAT

When we issue netstat command we can see what are the ports listening on my system. Print network connections, routing tables, interface statistics, masquerade connections, and multicast memberships. Netstat prints information about the Linux networking  subsystem.

[root@rahul-linux ~]# netstat -an |grep 5432
tcp        0      0 192.168.140.102:5432        0.0.0.0:*                   LISTEN
tcp        0      0 192.168.160.102:5432        0.0.0.0:*                   LISTEN
tcp        0      0 192.168.198.102:5432        0.0.0.0:*                   LISTEN

-l or --listening shows only the sockets currently listening for incoming connection.
-a or --all shows all sockets currently in use.
-t or --tcp shows the tcp sockets.
-u or --udp shows the udp sockets.
-n or --numeric shows the hosts and ports as numbers, instead of resolving in dns and looking in /etc/services.

netstat -atn           # For tcp
netstat -aun           # For udp
netstat -atun          # For both

NSLOOKUP

Nslookup helps us find possible IP addresses of any site like www.google.com

SAR in Linux

The SAR command extracts and writes to standard output  records  previously saved in a file. The SAR command only reports on local activities. Each time we need to run /usr/lib64/sa/sa1 to get status updated into /var/log/sa/sa27. To install SAR we need to install yum install sysstat

[root@ip-172-31-28-103 sa]# /usr/lib64/sa/sa1
[root@ip-172-31-28-103 sa]# sar
Linux 3.10.0-514.el7.x86_64 (ip-172-31-28-103.ec2.internal)     11/27/2016      _x86_64_        (1 CPU)

[root@ip-172-31-28-103 sa]# /usr/lib64/sa/sa1

[root@ip-172-31-28-103 sa]# sar

Linux 3.10.0-514.el7.x86_64 (ip-172-31-28-103.ec2.internal)     11/27/2016      _x86_64_        (1 CPU)

12:58:31 AM     CPU     %user     %nice   %system   %iowait    %steal     %idle

12:58:53 AM     all      0.04      0.00      0.04      0.00      0.04     99.87

Average:        all      0.04      0.00      0.04      0.00      0.04     99.87

[root@ip-172-31-28-103 sa]# /usr/lib64/sa/sa1

[root@ip-172-31-28-103 sa]# sar

Linux 3.10.0-514.el7.x86_64 (ip-172-31-28-103.ec2.internal)     11/27/2016      _x86_64_        (1 CPU)

12:58:31 AM     CPU     %user     %nice   %system   %iowait    %steal     %idle

12:58:53 AM     all      0.04      0.00      0.04      0.00      0.04     99.87

12:59:08 AM     all      0.07      0.00      0.00      0.00      0.00     99.93

Average:        all      0.05      0.00      0.03      0.00      0.03     99.89

[root@rahul-linux1 ~]# sar  --CPU Info
Linux 3.8.13-118.13.2.el6uek.x86_64 (rcdn9-emaas-rdb-001)       11/18/2016      _x86_64_        (120 CPU)
12:00:01 AM     CPU     %user     %nice   %system   %iowait    %steal     %idle
12:10:01 AM     all      1.86      0.00      0.75      0.02      0.00     97.37
12:20:02 AM     all      2.20      0.00      0.93      0.02      0.00     96.85
12:30:01 AM     all      1.96      0.00      0.79      0.02      0.00     97.23
12:40:01 AM     all      1.81      0.00      0.73      0.01      0.00     97.45
12:50:01 AM     all      2.52      0.00      1.10      0.02      0.00     96.36
01:00:01 AM     all      1.67      0.00      0.75      0.02      0.00     97.56
01:10:01 AM     all      2.23      0.00      0.85      0.02      0.00     96.90
01:20:01 AM     all      1.85      0.00      0.76      0.01      0.00     97.37
01:30:01 AM     all      1.81      0.00      0.73      0.02      0.00     97.44

[root@ip-172-31-28-103 sa]# sar -S  --SWAP
Linux 3.10.0-514.el7.x86_64 (ip-172-31-28-103.ec2.internal)     11/27/2016      _x86_64_        (1 CPU)

12:58:31 AM kbswpfree kbswpused  %swpused  kbswpcad   %swpcad
12:58:53 AM         0         0      0.00         0      0.00
12:59:08 AM         0         0      0.00         0      0.00
01:00:02 AM         0         0      0.00         0      0.00
01:01:42 AM         0         0      0.00         0      0.00
Average:            0         0      0.00         0      0.00

[root@ip-172-31-28-103 sa]# sar -b  --I/O Details read, write, transaction, bytes read
Linux 3.10.0-514.el7.x86_64 (ip-172-31-28-103.ec2.internal)     11/27/2016      _x86_64_        (1 CPU)

12:58:31 AM       tps      rtps      wtps   bread/s   bwrtn/s
12:58:53 AM      0.40      0.00      0.40      0.00      5.42
12:59:08 AM      0.07      0.00      0.07      0.00      1.13
01:00:02 AM      0.30      0.00      0.30      0.00      4.39
01:01:42 AM      0.29      0.00      0.29      0.00      3.81
Average:         0.29      0.00      0.29      0.00      3.97

[root@ip-172-31-28-103 sa]# sar -d  --device
Linux 3.10.0-514.el7.x86_64 (ip-172-31-28-103.ec2.internal)     11/27/2016      _x86_64_        (1 CPU)

12:58:31 AM       DEV       tps  rd_sec/s  wr_sec/s  avgrq-sz  avgqu-sz     await     svctm     %util
12:58:53 AM  dev202-0      0.40      0.00      5.42     13.67      0.00      0.44      0.11      0.00
12:59:08 AM  dev202-0      0.07      0.00      1.13     16.00      0.00      0.00      0.00      0.00
01:00:02 AM  dev202-0      0.30      0.00      4.39     14.88      0.00      0.12      0.12      0.00
01:01:42 AM  dev202-0      0.29      0.00      3.81     13.21      0.00      0.34      0.10      0.00
01:10:01 AM  dev202-0      0.17      0.02      2.77     15.94      0.00      0.33      0.14      0.00
Average:     dev202-0      0.21      0.01      3.10     15.12      0.00      0.32      0.13      0.00

[root@ip-172-31-28-103 sa]# sar -w --Context switch
Linux 3.10.0-514.el7.x86_64 (ip-172-31-28-103.ec2.internal)     11/27/2016      _x86_64_        (1 CPU)

12:58:31 AM    proc/s   cswch/s
12:58:53 AM      0.31     61.01
12:59:08 AM      0.42     68.39
01:00:02 AM      0.18     61.34
01:01:42 AM      0.27     61.04
01:10:01 AM      0.06     56.94
Average:         0.12     58.25

[root@ip-172-31-28-103 sa]# sar -r --Run Queue and Load Average
Linux 3.10.0-514.el7.x86_64 (ip-172-31-28-103.ec2.internal)     11/27/2016      _x86_64_        (1 CPU)

12:58:31 AM kbmemfree kbmemused  %memused kbbuffers  kbcached  kbcommit   %commit  kbactive   kbinact   kbdirty
12:58:53 AM    130192    884784     87.17         0    671728    550932     54.28    385096    340260        20
12:59:08 AM    130120    884856     87.18         0    671732    550932     54.28    385108    340252        20
01:00:02 AM    129492    885484     87.24         0    671784    552864     54.47    385580    340248        16
01:01:42 AM    130088    884888     87.18         0    671748    550932     54.28    385120    340256        24
01:10:01 AM    127892    887084     87.40         0    671816    556896     54.87    386680    340268        28
Average:       129557    885419     87.24         0    671762    552511     54.44    385517    340257        22

[root@ip-172-31-28-103 sa]# sar -p  --CPU Usage
Linux 3.10.0-514.el7.x86_64 (ip-172-31-28-103.ec2.internal)     11/27/2016      _x86_64_        (1 CPU)

12:58:31 AM     CPU     %user     %nice   %system   %iowait    %steal     %idle
12:58:53 AM     all      0.04      0.00      0.04      0.00      0.04     99.87
12:59:08 AM     all      0.07      0.00      0.00      0.00      0.00     99.93
01:00:02 AM     all      0.02      0.00      0.09      0.00      0.42     99.46
01:01:42 AM     all      0.04      0.00      0.05      0.00      0.09     99.82
01:10:01 AM     all      0.01      0.00      0.01      0.00      0.01     99.97
Average:        all      0.02      0.00      0.03      0.00      0.05     99.91

[root@ip-172-31-28-103 sa]# sar -u CPU Usage
Linux 3.10.0-514.el7.x86_64 (ip-172-31-28-103.ec2.internal)     11/27/2016      _x86_64_        (1 CPU)

12:58:31 AM     CPU     %user     %nice   %system   %iowait    %steal     %idle
12:58:53 AM     all      0.04      0.00      0.04      0.00      0.04     99.87
12:59:08 AM     all      0.07      0.00      0.00      0.00      0.00     99.93
01:00:02 AM     all      0.02      0.00      0.09      0.00      0.42     99.46
01:01:42 AM     all      0.04      0.00      0.05      0.00      0.09     99.82
01:10:01 AM     all      0.01      0.00      0.01      0.00      0.01     99.97
Average:        all      0.02      0.00      0.03      0.00      0.05     99.91

Default Gateway in Linux

What is Gateway?
A gateway is a network point that acts as an entrance to another network. On the Internet, a node or stopping point can be either a gateway node or a host (end-point) node. Both the computers of Internet users and the computers that serve pages to users are host nodes. The computers that control traffic within your company’s network or at your local Internet service provider (ISP) are gateway nodes. In the network for an enterprise, a computer server acting as a gateway node is often also acting as a proxy server and a firewall server. A gateway is often associated with both a router, which knows where to direct a given packet of data that arrives at the gateway, and a switch, which furnishes the actual path in and out of the gateway for a given packet.

How to find default gateway?
rahul-linux:~$ ip route show
default via 192.168.1.1 dev eth1  proto static
192.168.1.0/24 dev eth1  proto kernel  scope link  src 192.168.1.100  metric 1

rahul-linux:~$ netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG        0 0          0 eth1
192.168.1.0     0.0.0.0         255.255.255.0   U         0 0          0 eth1

The flag U indicates that route is up and G indicates that it is gateway.

rahul-linux:~$ route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 eth1
192.168.1.0     0.0.0.0         255.255.255.0   U     1      0        0 eth1

How to find gateway?
[root@rhel7 ~]# routel
         target            gateway          source    proto    scope    dev tbl
      10.0.0.0/ 8                        10.1.1.56   kernel     link enp0s3
       10.0.0.0          broadcast       10.1.1.56   kernel     link enp0s3 local
      10.1.1.56              local       10.1.1.56   kernel     host enp0s3 local
 10.255.255.255          broadcast       10.1.1.56   kernel     link enp0s3 local
      127.0.0.0          broadcast       127.0.0.1   kernel     link     lo local
     127.0.0.0/ 8            local       127.0.0.1   kernel     host     lo local
      127.0.0.1              local       127.0.0.1   kernel     host     lo local
127.255.255.255          broadcast       127.0.0.1   kernel     link     lo local

[root@rhel7 ~]# cat /etc/sysconfig/network
GATEWAY="10.1.1.1"

[root@rhel7 ~]# routel | grep default
        default          10.1.1.1                   static          enp0s3
        default        unreachable                   kernel              lo unspec
        default        unreachable                   kernel              lo unspec

IOSTAT in Linux

IOSTAT: Report Central Processing Unit (CPU) statistics and input/output statistics for devices and partitions. The  first  report  generated by the iostat command provides statistics concerning the time since the system was booted. Each subsequent report covers  the time since the previous report. All statistics are reported each time the iostat command is run. A  device  header  row  is displayed followed by a line of statistics for each device that is configured. The iostat command generates two types of reports, the CPU  Utilization report and the Device Utilization report.

[root@rahul-linux ~]# iostat 5
Linux 3.8.13-118.13.2.el6uek.x86_64 (rahul-linux)       11/18/2016      _x86_64_        (120 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.99    0.00    0.24    0.01    0.00   98.76

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
fioa              8.74         0.11     10774.22     312495 30035875404
fiob            165.60       385.39      4082.03 1074372153 11379708364
fioc            220.45       352.78     16401.66  983474378 45723802863
sda               1.90         0.62        91.51    1720980  255100394
dm-0              1.90         0.61        91.49    1708384  255048440
dm-1              0.00         0.00         0.00       3576       1736
dm-2              0.00         0.00         0.02       2456      50200
dm-3            541.44       738.28     31257.91 2058152399 87139386631
sdb               4.78         0.57      7000.35    1576711 19515259748
dm-4              4.73         0.18      7000.35     512943 19515259748
dm-5              6.09         0.18      7000.35     512535 19515259748

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           1.36    0.00    0.72    0.01    0.00   97.92

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
fioa              0.00         0.00         0.00          0          0
fiob             11.00         0.00       418.40          0       2092
fioc            273.20         0.00     11328.00          0      56640
sda               0.00         0.00         0.00          0          0
dm-0              0.00         0.00         0.00          0          0
dm-1              0.00         0.00         0.00          0          0
dm-2              0.00         0.00         0.00          0          0
dm-3            438.60         0.00     11746.40          0      58732
sdb               0.20         0.00        17.60          0         88
dm-4              0.20         0.00        17.60          0         88
dm-5              0.20         0.00        17.60          0         88


CPU Utilization Report
              The first report generated by the iostat command is the CPU Uti-
              lization Report. For multiprocessor systems, the CPU values  are
              global  averages  among all processors.  The report has the fol-
              lowing format:

              %user
                     Show the percentage  of  CPU  utilization  that  occurred
                     while executing at the user level (application).
              %nice
                     Show  the  percentage  of  CPU  utilization that occurred
                     while executing at the user level with nice priority.
              %sys
                     Show the percentage  of  CPU  utilization  that  occurred
                     while executing at the system level (kernel).
              %iowait
                     Show  the  percentage  of  time that the CPU or CPUs were
                     idle during which the system had an outstanding disk  I/O
                     request.
              %idle
                     Show  the  percentage  of  time that the CPU or CPUs were
                     idle and the system did not have an outstanding disk  I/O
                     request.

Device Utilization Report
              The  second report generated by the iostat command is the Device
              Utilization Report. The device report provides statistics  on  a
              per  physical device or partition basis. Block devices for which
              statistics are to be displayed may be  entered  on  the  command
              line. Partitions may also be entered on the command line provid-
              ing that option -x is not used.  If no device nor  partition  is
              entered,  then statistics are displayed for every device used by
              the system, and providing that the kernel  maintains  statistics
              for  it.   If the ALL keyword is given on the command line, then
              statistics are displayed for every device defined by the system,
              including  those that have never been used.  The report may show
              the following fields, depending on the flags used:

              Device:
                     This column gives the device (or partition)  name,  which
                     is  displayed  as  hdiskn  with  2.2 kernels, for the nth
                     device. It is displayed as devm-n with 2.4 kernels, where
                     m  is the major number of the device, and n a distinctive
                     number.  With newer kernels, the device name as listed in
                     the /dev directory is displayed.

              tps
                     Indicate  the  number  of  transfers per second that were
                     issued to the device. A transfer is an I/O request to the
                     device.  Multiple logical requests can be combined into a
                     single I/O request to the device. A transfer is of  inde-
                     terminate size.

              Blk_read/s
                     Indicate the amount of data read from the drive expressed
                     in a number of blocks per second. Blocks  are  equivalent
                     to  sectors with 2.4 kernels and newer and therefore have
                     a size of 512 bytes. With older kernels, a  block  is  of
                     indeterminate size.

              Blk_wrtn/s
                     Indicate   the  amount  of  data  written  to  the  drive
                     expressed in a number of blocks per second.

              Blk_read
                     The total number of blocks read.

              Blk_wrtn
                     The total number of blocks written.

              kB_read/s
                     Indicate the amount of data read from the drive expressed
                     in  kilobytes  per  second. Data displayed are valid only
                     with kernels 2.4 and newer.

              kB_wrtn/s
                     Indicate  the  amount  of  data  written  to  the   drive
                     expressed  in  kilobytes  per  second. Data displayed are
                     valid only with kernels 2.4 and newer.

              kB_read
                     The total number of kilobytes read.  Data  displayed  are
                     valid only with kernels 2.4 and newer.

             

[root@linux-rahul ~]# iostat -c Only CPU
Linux 3.8.13-118.13.2.el6uek.x86_64 (rahul-linux)       11/18/2016      _x86_64_        (120 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.99    0.00    0.24    0.01    0.00   98.75


[root@linux-rahul ~]# iostat -d   Only Device
Linux 3.8.13-118.13.2.el6uek.x86_64 (rahul-linux)       11/18/2016      _x86_64_        (120 CPU)

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
fioa              8.73         0.12     10755.19     333951 30035903001
fiob            166.50       583.31      4079.87 1629000641 11393799263
fioc            220.66       352.32     16417.93  983924234 45850168738
sda               1.90         0.65        91.53    1813636  255626242
dm-0              1.90         0.62        91.48    1726024  255479808
dm-1              0.01         0.03         0.03      78592      96216
dm-2              0.00         0.00         0.02       2456      50200
dm-3            542.70       935.75     31252.98 2613252199 87279871002
sdb               5.13         0.57      7295.49    1579140 20374046075
dm-4              5.09         0.18      7295.49     513484 20374046075
dm-5              6.74         0.18      7295.49     513076 20374046075


[root@rahul-linux ~]# iostat -k in KB
Linux 3.8.13-118.13.2.el6uek.x86_64 (rahul-linux)       11/18/2016      _x86_64_        (120 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.99    0.00    0.24    0.01    0.00   98.75

Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
fioa              8.73         0.06      5377.52     166975 15017951525
fiob            166.50       291.65      2039.94  814508468 5696982119
fioc            220.66       176.16      8208.99  491962261 22925454142
sda               1.90         0.32        45.77     906818  127814321
dm-0              1.90         0.31        45.74     863012  127741104
dm-1              0.01         0.01         0.02      39296      48108
dm-2              0.00         0.00         0.01       1228      25100
dm-3            542.70       467.87     15626.45 1306634391 43640387787
sdb               5.14         0.28      3649.91     789574 10193186452
dm-4              5.09         0.09      3649.91     256742 10193186452
dm-5              6.75         0.09      3649.91     256538 10193186452

[root@rahul-linux ~]# iostat -m in MB
Linux 3.8.13-118.13.2.el6uek.x86_64 (rahul-linux)       11/18/2016      _x86_64_        (120 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.99    0.00    0.24    0.01    0.00   98.75

Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
fioa              8.73         0.00         5.25        163   14665968
fiob            166.50         0.28         1.99     795421    5563516
fioc            220.66         0.17         8.02     480431   22388319
sda               1.90         0.00         0.04        885     124819
dm-0              1.90         0.00         0.04        842     124748
dm-1              0.01         0.00         0.00         38         46
dm-2              0.00         0.00         0.00          1         24
dm-3            542.70         0.46        15.26    1276012   42617804
sdb               5.14         0.00         3.57        771    9957800
dm-4              5.10         0.00         3.57        250    9957800
dm-5              6.76         0.00         3.57        250    9957864


[root@rahul-linux ~]# iostat -p fioa
Linux 3.8.13-118.13.2.el6uek.x86_64 (rahul-linux)       11/18/2016      _x86_64_        (120 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.99    0.00    0.24    0.01    0.00   98.75

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
fioa              8.73         0.12     10753.41     334395 30035905061
fioa1             0.00         0.00         0.00          0          0

[root@rahul-linux ~]# iostat -t 5
Linux 3.8.13-118.13.2.el6uek.x86_64 (rahul-linux)       11/18/2016      _x86_64_        (120 CPU)

11/18/2016 07:20:22 PM
avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.99    0.00    0.24    0.01    0.00   98.75

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
fioa              8.73         0.12     10753.01     334821 30035905341
fiob            166.59       604.30      4079.58 1687969584 11395299639
fioc            220.68       352.40     16418.57  984334427 45861237484
sda               1.90         0.65        91.53    1814468  255672866
dm-0              1.90         0.62        91.48    1726856  255526024
dm-1              0.01         0.03         0.03      78592      96624
dm-2              0.00         0.00         0.02       2456      50200
dm-3            542.82       956.82     31251.16 2672632205 87292442464
sdb               5.20         0.57      7368.63    1589948 20582464808
dm-4              5.15         0.19      7368.63     524076 20582464808
dm-5              6.83         0.19      7368.63     523668 20582464808

11/18/2016 07:20:27 PM
avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           4.43    0.00    1.96    0.14    0.00   93.47


[root@rahul-linux ~]# iostat -V
sysstat version 9.0.4
(C) Sebastien Godard (sysstat <at> orange.fr)


[root@rahul-linux ~]# iostat -x
Linux 3.8.13-118.13.2.el6uek.x86_64 (rahul-linux)       11/18/2016      _x86_64_        (120 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.99    0.00    0.24    0.01    0.00   98.75

Device:         rrqm/s   wrqm/s     r/s     w/s   rsec/s   wsec/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
fioa              0.00     0.00    0.00    8.73     0.12 10752.80  1232.05     0.46  173.23    0.21  173.25   0.34   0.30
fiob              0.00     0.00    3.33  163.29   611.07  4079.53    28.15     0.33    7.59    0.17    7.74   0.01   0.11
fioc              0.00     0.00    1.80  218.88   352.39 16418.47    76.00     1.03    0.49    0.19    0.49   0.06   1.24
sda               0.00     0.00    0.03    1.87     0.65    91.53    48.61     0.06   30.66    0.98   31.13   1.13   0.21
dm-0              0.00     0.00    0.03    1.87     0.62    91.48    48.60     0.06   30.65    1.00   31.10   1.10   0.21
dm-1              0.00     0.00    0.00    0.00     0.03     0.03     8.00     0.01  877.21    1.18 1589.75   8.69   0.01
dm-2              0.00     0.00    0.00    0.00     0.00     0.02    74.06     0.00    5.67    1.04    7.24   2.38   0.00
dm-3              0.00     0.00    5.13  537.72   963.58 31250.80    59.34     0.01    0.01    0.19    0.01   0.04   2.37
sdb               0.00     0.00    0.06    5.15     0.57  7375.37  1416.59     0.26   50.25    1.48   50.78   1.06   0.55
dm-4              0.00     1.69    0.01    5.15     0.19  7375.37  1429.62     0.26   51.33    1.75   51.41   1.08   0.56
dm-5              0.00     0.00    0.01    6.85     0.19  7375.37  1076.25     0.36   52.80    1.75   52.86   0.81   0.56


 kB_wrtn
                     The total number of kilobytes written. Data displayed are
                     valid only with kernels 2.4 and newer.

              rrqm/s
                     The number of read requests merged per second  that  were
                     issued to the device.

              wrqm/s
                     The  number of write requests merged per second that were
                     issued to the device.

              r/s
                     The number of read  requests  that  were  issued  to  the
                     device per second.

              w/s
                     The  number  of  write  requests  that were issued to the
                     device per second.

              rsec/s
                     The number of sectors read from the device per second.

              wsec/s
                     The number of sectors written to the device per second.

              rkB/s
                     The number of kilobytes read from the device per  second.

              wkB/s
                     The number of kilobytes written to the device per second.

              avgrq-sz
                     The average size (in sectors) of the requests  that  were
                     issued to the device.

              avgqu-sz
                     The average queue length of the requests that were issued
                     to the device.

              await
                     The average  time  (in  milliseconds)  for  I/O  requests
                     issued to the device to be served. This includes the time
                     spent by the requests in queue and the time spent servic-
                     ing them.

              svctm
                     The  average  service  time  (in  milliseconds)  for  I/O
                     requests that were issued to the device.

              %util
                     Percentage of CPU time during  which  I/O  requests  were
                     issued  to  the  device  (bandwidth  utilization  for the
                     device). Device saturation  occurs  when  this  value  is
                     close to 100%.

iostat -d 2 6
              Display six reports at two second intervals for all devices.

[root@rahul-linux ~]# iostat -d 6 2 -t
Linux 3.8.13-118.13.2.el6uek.x86_64 (rahul-linux)       11/18/2016      _x86_64_        (120 CPU)

11/18/2016 07:25:11 PM
Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
fioa              8.73         0.12     10751.90     335575 30035906676
fiob            166.65       619.32      4079.29 1730095952 11395684530
fioc            220.76       367.26     16418.48 1025968411 45865736276
sda               1.90         0.65        91.53    1814532  255696042
dm-0              1.90         0.62        91.48    1726856  255549200
dm-1              0.01         0.03         0.03      78656      96624
dm-2              0.00         0.00         0.02       2456      50200
dm-3            542.97       986.70     31249.67 2756393311 87297327482
sdb               5.25         0.57      7403.48    1592940 20681943282
dm-4              5.20         0.19      7403.48     526964 20681943282
dm-5              6.93         0.19      7403.52     526556 20682064482

11/18/2016 07:25:17 PM
Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
fioa              0.33         0.00         2.83          0         17
fiob             60.83       176.00       907.33       1056       5444
fioc           1781.83    273525.33     26716.00    1641152     160296
sda               5.00         0.00       552.00          0       3312
dm-0              5.00         0.00       552.00          0       3312
dm-1              0.00         0.00         0.00          0          0
dm-2              0.00         0.00         0.00          0          0
dm-3           1992.17    273701.33     27626.17    1642208     165757
sdb             311.17         5.33    252283.50         32    1513701
dm-4            311.00         4.00    252283.50         24    1513701
dm-5            527.00         4.00    238552.83         24    1431317

VMSTAT in Linux

Reports Virtual Memory Statistics. It reports information about processes, memory, paging, block IO, taps, and CPU activity. The first report produced gives averages since the last reboot. Additional reports give information on a sampling period of length delay. The process and memory reports are instantaneous in either case.

VMSTAT does not require special permissions. These reports are intended to help identify system bottlenecks.   Linux VMSTAT does not count itself as a running process. All  linux  blocks  are currently  1024  bytes. Old kernels may report blocks as 512 bytes, 2048 bytes, or 4096 bytes. Since procps 3.1.9, vmstat lets you choose units (k, K, m,  M)  default is K (1024 bytes) in the default mode
Other way to get information are /proc/meminfo , /proc/stat,  /proc/*/stat.

Some of the examples are below:

[root@rahul@linux ~] vmstat
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
18  0    708 96000856    464 2555262208    0    0     3   160    0    0  1  0 99  0  0
[root@rcdn9-emaas-rdb-001 ~]#

Procs
       r: The number of processes waiting for run time.
       b: The number of processes in uninterruptible sleep.

Memory
       swpd: the amount of virtual memory used.
       free: the amount of idle memory.
       buff: the amount of memory used as buffers.
       cache: the amount of memory used as cache.
       inact: the amount of inactive memory. (-a option)
       active: the amount of active memory. (-a option)

Swap
       si: Amount of memory swapped in from disk (/s).
       so: Amount of memory swapped to disk (/s).

IO
       bi: Blocks received from a block device (blocks/s).
       bo: Blocks sent to a block device (blocks/s).

System
       in: The number of interrupts per second, including the clock.
       cs: The number of context switches per second.

CPU
       These are percentages of total CPU time.
       us: Time spent running non-kernel code. (user time, including nice time)
       sy: Time spent running kernel code. (system time)
       id: Time spent idle. Prior to Linux 2.5.41, this includes IO-wait time.
       wa: Time spent waiting for IO. Prior to Linux 2.5.41, shown as zero.


[root@rahul@linux ~]# vmstat -a          Active/Inactive Memory
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
 r  b   swpd   free  inact active   si   so    bi    bo   in   cs us sy id wa st
 3  0    708 95259808 1758343168 822516864    0    0     3   160    0    0  1  0 99  0  0

[root@rahul@linux ~]# vmstat -f
     31309949 forks

Number of forks since last restart. Number of tasks. Each process is represented by one or more        tasks, depending on thread usage.  This display does not repeat.

[root@rahul@linux ~]# vmstat -m Displays slabinfo
Cache                       Num  Total   Size  Pages
nf_conntrack_ffff89e99b09ae00      0      0    304     13

cache: Cache name
num: Number of currently active objects
total: Total number of available objects
size: Size of each object
pages: Number of pages with at least one active object
totpages: Total number of allocated pages
pslab: Number of pages per slab

[root@rahul@linux ~]# vmstat -n   Same as vmstat
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 1  0    692 80950576    464 2569727744    0    0     3   160    0    0  1  0 99  0  0


[root@rahul@linux ~]# vmstat -s        -- Event Counter and  memory statistics
   2774295808  total memory
   2697926400  used memory
    826923712  active memory
   1772232960  inactive memory
     76369280  free memory
          464  buffer memory
   2574770944  swap cache
      4194300  total swap
          692  used swap
      4193608  free swap
    328301314 non-nice user cpu ticks
         3542 nice user cpu ticks
     70364589 system cpu ticks
  32954132294 idle cpu ticks
      4865037 IO-wait cpu ticks
         6250 IRQ cpu ticks
      9848009 softirq cpu ticks
            0 stolen cpu ticks
   1030697686 pages paged in
  53379068841 pages paged out
          104 pages swapped in
          217 pages swapped out
   1162661649 interrupts
   2277229811 CPU context switches
   1476703567 boot time
     31474117 forks

[root@rahul@linux ~]# vmstat -5
usage: vmstat [-V] [-n] [delay [count]]
              -V prints version.
              -n causes the headers not to be reprinted regularly.
              -a print inactive/active page stats.
              -d prints disk statistics
              -D prints disk table
              -p prints disk partition statistics
              -s prints vm table
              -m prints slabinfo
              -t add timestamp to output
              -S unit size
              delay is the delay between updates in seconds.
              unit size k:1000 K:1024 m:1000000 M:1048576 (default is K)
              count is the number of updates.


[root@rahul@linux ~]# vmstat 5
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 1  0    692 75732416    464 2575491584    0    0     3   160    0    0  1  0 99  0  0
 5  0    692 75689440    464 2575514112    0    0     0  5130 17034 21562  2  1 97  0  0
 4  0    692 75722464    464 2575530752    0    0     0 12456 15821 14389  1  1 98  0  0
 4  0    692 75746632    464 2575555584    0    0     0  7586 15094 16254  1  1 99  0  0
 8  0    692 75745712    464 2575565824    0    0     1 19883 16983 16501  2  1 97  0  0

[root@rahul@linux ~]# vmstat -d disk statistics
disk- ------------reads------------ ------------writes----------- -----IO------
       total merged sectors      ms  total merged sectors      ms    cur    sec
ram0       0      0       0       0      0      0       0       0      0      0
ram1       0      0       0       0      0      0       0       0      0      0
ram2       0      0       0       0      0      0       0       0      0      0
ram3       0      0       0       0      0      0       0       0      0      0
ram4       0      0       0       0      0      0       0       0      0      0
ram5       0      0       0       0      0      0       0       0      0      0
ram6       0      0       0       0      0      0       0       0      0      0
ram7       0      0       0       0      0      0       0       0      0      0
ram8       0      0       0       0      0      0       0       0      0      0
ram9       0      0       0       0      0      0       0       0      0      0
ram10      0      0       0       0      0      0       0       0      0      0
ram11      0      0       0       0      0      0       0       0      0      0
ram12      0      0       0       0      0      0       0       0      0      0
ram13      0      0       0       0      0      0       0       0      0      0
ram14      0      0       0       0      0      0       0       0      0      0
ram15      0      0       0       0      0      0       0       0      0      0
loop0      0      0       0       0      0      0       0       0      0      0
loop1      0      0       0       0      0      0       0       0      0      0
loop2      0      0       0       0      0      0       0       0      0      0
loop3      0      0       0       0      0      0       0       0      0      0
loop4      0      0       0       0      0      0       0       0      0      0
loop5      0      0       0       0      0      0       0       0      0      0
loop6      0      0       0       0      0      0       0       0      0      0
loop7      0      0       0       0      0      0       0       0      0      0
zram0      0      0       0       0      0      0       0       0      0      0
fioa    3254      0  312287     674 24372260      0 30035851875 4223132598      0   8400
fiob  6074950      0 1074323673 1057472 455289506      0 11372152579 3526281277      0   3000
fioc  5011713      0 983469690  965440 608269179      0 45632450256 263461364      0  34203
sda    80705    607 1720668   77844 5198429    245 254745602 161652896      0   5797
dm-0   79653      0 1708072   78040 5197923      0 254693648 161865943      0   5780
dm-1     447      0    3576     957    217      0    1736   99496      0     10
dm-2     180      0    2456     188    531      0   50200    3846      0      1
dm-3  11088876      0 2058099023 2130356 1495829218      0 87040454710 4234588663      0  65385
sdb   153548      0 1575199  200552 13143910      0 19489567165 616883680      0  14527
dm-4   20766      6  512943   14624 13143910 3800089 19489567165 625004708      0  14648
dm-5   20721      0  512535   14520 16944001      0 19489567165 837507283      0  14653

Reads
       total: Total reads completed successfully
       merged: grouped reads (resulting in one I/O)
       sectors: Sectors read successfully
       ms: milliseconds spent reading


   Writes
       total: Total writes completed successfully
       merged: grouped writes (resulting in one I/O)
       sectors: Sectors written successfully
       ms: milliseconds spent writing


   IO
       cur: I/O in progress
       s: seconds spent for I/O


[root@rahul@linux ~]# vmstat -p /dev/shm
Partition was not found

[root@rahul@linux ~]# vmstat -S K
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 1  0    692 70379232    464 2579287040    0    0     3   160    0    0  1  0 99  0  0

[root@rahul@linux ~]# vmstat -S k
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 0  0    708 72121024    475 2641190144    0    0     3   160    0    0  1  0 99  0  0

[root@rahul@linux~]# vmstat -S m
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 2  0      0  72135      0 2641250    0    0     3   160    0    0  1  0 99  0  0

[root@rahul@linux ~]# vmstat -S M
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 4  0      0  68850      0 2518896    0    0     3   160    0    0  1  0 99  0  0


[root@rahul@linux ~]# vmstat -V
procps version 3.2.8

Inode in Linux

Let us take a small example where every citizen in European country have a unique personal Identification number. This helps government to track all citizen, helps in documentation. Similar way we have Inodes for all files in linux. During the initial set up of the OS, we define the number of inodes and number of files that is possible in the OS. Inode is a data structure which keeps information of all files like size, user id, group id, permission. But it will not keep information of file name so hard links work fine. We can check inodes with the below command:

[postgres@ip-172-31-28-103 data]$ df -i
Filesystem      Inodes IUsed   IFree IUse% Mounted on
/dev/xvda2     5241840 41618 5200222    1% /
devtmpfs        122059   281  121778    1% /dev
tmpfs           126872     3  126869    1% /dev/shm
tmpfs           126872   337  126535    1% /run
tmpfs           126872    16  126856    1% /sys/fs/cgroup
tmpfs           126872     1  126871    1% /run/user/1000

Soft vs Hard Link in Linux

In linux we have something called links which replicates contents of one file to another with symbolic links. We called them -
      a. Soft Link
      b. Hard Link
Below is a small diagram which explains the full story.

Soft Link

Soft link is symbolic link of one file or directory. We create soft link using the command -

shell> ln -s file_to_be_linked linked_file 

For example, we need to link postgresql-9.3.1 to postgresql. We will issue the below command -

shell> ln -s postgresql-9.3.1 postgresql

In soft link, if we delete/rename the postgresql, the contents are deleted permanently. But if we replace the contents of postgresql-9.3.1, there will be no harm to the link. But if we rename postgresql-9.3.1, then postgresql wont work because soft link was linked with file not the content. If we move postgresql-9.3.1 to different location, postgresql will not be accessible.

We create such links where we want the file name to be present in the directory but content should append in different location.

Hard Link

Hard link are links which contents same information between two files. We create hard link issuing the below command -

shell> ln postgresql-9.3.1 postgresql

If we delete postgresql-9.3.1, still we will be able to access postgresql because the link file points to the same inode. Inode is a data structure which keeps information of all files like size, user id, group id, permission. Hardlinks can only be possible within the same file system as inode is same. Hard links are only possible for files not for directory. Performance will be better here as it will contact the disk pointer.

Is Truncate A Transaction

This is very complex question, which is very difficult to describe. Though all RDBMS are similar but found major difference for transaction in PostgreSQL, MySQL & SQL Server.

PostgreSQL we can have truncate in transaction. As PostgreSQL can run DDL and DML in transaction. If we run truncate in transaction, it can rollback

SQL server can run DML, DDL in transaction but truncate in SQL Server can not be rollback so truncate is not a transaction.

MySQL also don't run truncate in transaction. We can not rollback once we issue truncate command even in transaction.   

Transaction in DBMS

Transaction are set of commands executed in a small program. Transaction in DBMS must follow ACID( Atomicity, Consistency, Isloation & Durability).

Atomicity ensures that transaction is either completed or roll backed. It wont be partially completed. Consistency ensures that the database must be in consistent state after every transaction. Durability ensures that if data are committed, it should be present in DB. Isolation level makes sure, we read committed data.

States of transaction

There are many states within a transaction. It can be either committed, failed or rollback. Lets see the states within a transaction.

Active State

When transaction is being initiated or started.

Partially Committed State

When transaction executed its final statement.

Failed State

A failed transaction can not proceed further. If any statement fails, it moves to failed state. 

Aborted State

When transactions fails and recovery manager rollbacks all contents to original state, that state is called as aborted 

Committed State

When transactions successfully runs all statements, that state is committed state.

Introduction

This is my first blog and I am starting it with the intention to share my knowledge with the whole world. Comments are welcomed  to improve each and every page. 

About Me:

I am open source consultant. Presently with CISCO delivering end to end projects. Have sound knowledge on PostgreSQL, MySQL and Data ware housing tool like Netezza and Amazon Redshift. Though I am Oracle Certified Professional too.

About the Blog:

I am creating this blog to write internal about open source DB and also I will share about Linux and Networking which is part of DB. I will also share my experience to handle various scenarios and also will keep an update on the implementation in simple terms.

Conclusion:

Lets start the journey with good note and have a very good knowledge sharing among all the experts. Thank you for reading my blog. Do comment and share the information.