Category Archives: Linux

sar command (Part I): All you need to know with examples

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Learn System Activity Report sar command with real-world scenario examples. Understand the command’s log files, execution, and different usage.

SAR ! System Activity Report! sar command is the second-best command used to check system performance or utilization after top command. From the man page, ‘The sar command writes to standard output the contents of selected cumulative activity counters in the operating system. The accounting system, based on
the values in the count and interval parameters, writes information the specified number of times spaced at the specified intervals in seconds.’ No doubt this is the best performance monitoring tool to be used for any sysadmin.

Read next part of sar tutorial :

Command log file management:

sar keep collecting system resource utilization and store it in binary files. These files are called datafiles and those are located in /var/log/sa/saXX the path where XX is data in dd format. So this could be one of the locations to check when you are troubleshooting file system utilization.

# ll /var/log/sa
total 29024
-rw-r--r-- 1 root root 494100 Dec  1 23:50 sa01
-rw-r--r-- 1 root root 494100 Dec  2 23:50 sa02
-rw-r--r-- 1 root root 494100 Dec  3 23:50 sa03
-rw-r--r-- 1 root root 494100 Dec  4 23:50 sa04
-rw-r--r-- 1 root root 494100 Dec  5 23:50 sa05
-rw-r--r-- 1 root root 494100 Dec  6 23:50 sa06
-rw-r--r-- 1 root root 494100 Dec  7 23:50 sa07

----- output clipped -----

Log files are binary hence can be read only with sar using -f option. Normal sar command shows your data in real-time when executed. If you need to check historic data you need to use -f option and provide a path of the particular data file.

# sar -u 2 3
Linux 2.6.39-200.24.1.el6uek.x86_64 (testsvr1)         12/19/2016      _x86_64_        (4 CPU)

11:44:29 AM     CPU     %user     %nice   %system   %iowait    %steal     %idle
11:44:31 AM     all     25.37      0.00     10.12      0.00      0.00     64.50
11:44:33 AM     all     25.41      0.00     10.39      0.13      0.00     64.08
11:44:35 AM     all     27.84      0.00     11.36      0.12      0.00     60.67
Average:        all     26.21      0.00     10.62      0.08      0.00     63.08

In the above example, when executed it will run for 23 iterations (we will see what it is, in later part of this post) for 2 seconds each and show you an output which is in real-time. Let’s see -f option :

# sar -u 2 3 -f /var/log/sa/sa15
Linux 2.6.39-200.24.1.el6uek.x86_64 (testsvr1)         12/15/2016      _x86_64_        (4 CPU)

12:00:01 AM     CPU     %user     %nice   %system   %iowait    %steal     %idle
12:10:01 AM     all     10.24      0.00      5.18      0.17      0.00     84.41
12:20:01 AM     all     11.55      0.00      5.02      0.19      0.00     83.24
12:30:01 AM     all     10.79      0.00      4.79      0.17      0.00     84.25
Average:        all     10.86      0.00      5.00      0.17      0.00     83.97

In above example, we ran sar command but on a datafile /var/log/sa/sa15. Hence data is being read from older/historic data files which is not real-time. File’s first entry is always treated as the first iteration and further on data is displayed according to command arguments. Hence you can see the first entry is being of 12AM.

Another beauty of this command for log management is you can save real-time command output in a log file of your choice. Let’s say you need to share the output of a specific time of monitoring then you can save the output in the log file and can share. In this way, you don’t have to share complete day datafile. You have to use -o option along with file path of your choice.

# sar -u 2 3 -o /tmp/logfile
Linux 2.6.39-200.24.1.el6uek.x86_64 (testsvr1)         12/19/2016      _x86_64_        (4 CPU)

11:51:42 AM     CPU     %user     %nice   %system   %iowait    %steal     %idle
11:51:44 AM     all     27.75      0.00      9.88      0.12      0.00     62.25
11:51:46 AM     all     26.00      0.00      9.88      0.12      0.00     64.00
11:51:48 AM     all     25.53      0.00     10.26      0.00      0.00     64.21
Average:        all     26.43      0.00     10.00      0.08      0.00     63.48
# ls -lrt /tmp/logfile
-rw-r--r-- 1 root root 63672 Dec 19 11:51 /tmp/logfile

In the above example, you can see the output is being displayed on the terminal as well as in a file provided in command options. Note that this file is also a binary file only.

Command Intervals and Iterations :

This command takes these two arguments which will define the time factors of output.

Interval is the time in seconds between two iterations of output samples. Normally selected as 2,5,10 seconds. Iteration or count is the number of samples to be taken after an interval of defined seconds. So for a command which says sar 2 5 means 2 interval and 5 iterations i.e. take 5 samples separated by 2 seconds each. i.e. if the command is fired at 12:00:00 then the output will include samples for times 12:00:02, 12:00:04 till 12:00:10. Check any above example and you will figure out how it works.

If the interval parameter is set to zero, the sar command displays the average statistics for the time since the system was started. If the iterations parameter is specified without the count parameter, then reports are generated continuously as shown below.

# sar -u 2
Linux 2.6.39-200.24.1.el6uek.x86_64 (oratest02)         12/19/2016      _x86_64_        (4 CPU)

12:09:28 PM     CPU     %user     %nice   %system   %iowait    %steal     %idle
12:09:30 PM     all      0.75      0.00      0.50      0.25      0.00     98.50
12:09:32 PM     all      0.88      0.00      0.38      0.13      0.00     98.62
12:09:34 PM     all      1.12      0.00      1.75      0.25      0.00     96.88
12:09:36 PM     all      2.38      0.00      1.38      0.12      0.00     96.12
12:09:38 PM     all     14.79      0.00      7.39      0.50      0.00     77.32
------- continuous reports being generated, output clipped -----

We will see useful monitoring example of this command in next post.

How to scan new lun / disk in Linux & HPUX

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Howto guide to scan new disk or LUNs on Linux or HPUX machines. This guide explains steps to scan and then identify new disk device names.

When you add a new disk to the system, you need to scan it so that kernel will be able to identify new hardware and assign a disk name to it. Adding a new disk to the system can be local or from storage. If it’s a local then its an addition of disk in free disk slots attached to server. If its a storage LUN then it’s masking and zoning at storage level to WWN of the server.

Once the disk / LUN is made available/visible to the server, the next step is to scan it. The kernel has a know hardware tree with it. This tree needs to be updated with new disk information. To let the kernel know that a new disk is made available to server disk scanning is required. If the disk is from storage array then there are chances you have storage vendor utilities/scripts available to scan storage on server example: evainfo (for EVA storage), xpinfo (for XP12K storage), powermt (for EMC storage).    If these utilities are not available, you still be able to scan them from OS.

HPUX disk scan :

In HPUX, we have dedicated ioscan command to scan new hardware. You can ask command to scan on hard disks with -C option i.e. class. Before executing this command, keep the output of previous disks (ioscan -funC disk) handy. This output can be compared to new output (command below) to identify new disk.

# ioscan -fnC disk
Class     I  H/W Path        Driver  S/W State   H/W Type     Description
==========================================================================
disk      4  0/0/1/1.0.0     sdisk   CLAIMED     DEVICE       HP 36.4GST373455LC#36
                            /dev/dsk/c1t0d0   /dev/rdsk/c1t0d0
disk      0  0/0/1/1.2.0     sdisk   CLAIMED     DEVICE       HP 36.4GST373455LC#36
                            /dev/dsk/c1t2d0   /dev/rdsk/c1t2d0
disk      1  0/0/2/0.2.0     sdisk   CLAIMED     DEVICE       HP 36.4GST373455LC#36
                            /dev/dsk/c2t2d0   /dev/rdsk/c2t2d0
disk      2  0/0/2/1.2.0     sdisk   CLAIMED     DEVICE       HP      DVD-ROM 305
                            /dev/dsk/c3t2d0   /dev/rdsk/c3t2d0
disk      3  0/10/0/1.0.0.0  sdisk   CLAIMED     DEVICE       I2O     RAID5
                            /dev/dsk/c4t0d0   /dev/rdsk/c4t0d0

Scan output shows you all detected disks on the system and their assigned disk names in CTD format. Sometimes, ioscan unable to install special device files for newly detected disks, in such a situation you can run insf (install special files) command to ensure all detected hardware has device files in place.

# insf -e
insf: Installing special files for btlan instance 0 address 0/0/0/0
insf: Installing special files for stape instance 1 address 0/0/1/0.1.0
insf: Installing special files for sctl instance 0 address 0/0/1/0.7.0
insf: Installing special files for sdisk instance 4 address 0/0/1/1.0.0
insf: Installing special files for sdisk instance 0 address 0/0/1/1.2.0
insf: Installing special files for sctl instance 1 address 0/0/1/1.7.0
----- output clipped ----

New disk even can be identified by comparing directory structure of /dev/disk or /dev/dsk/ before and after the scan. Any new addition during the scan to these directories is your new disk.

Once you identify this new disk, you can use it on the system via volume managers like LVM.

Linux Disk scan:

In Linux, it’s a bit tricky since there is no direct ioscan available. First, you need to get currently available disk details using fdisk command as below :

# fdisk -l |egrep '^Disk' |egrep -v 'dm-'|grep -v identifier
Disk /dev/sda: 74.1 GB, 74088185856 bytes
Disk /dev/sdb: 107.4 GB, 107374182400 bytes
Disk /dev/sdd: 2147 MB, 2147483648 bytes
Disk /dev/sde: 2147 MB, 2147483648 bytes
Disk /dev/sdc: 2147 MB, 2147483648 bytes

Keep this list handy to compare with the list after scan.

Scan SCSI disks

Now, if you have connected disks via SCSI then you need to scan SCSI hosts on the server. Check the current list of hosts on the server as below :

# ls /sys/class/scsi_host/
host0  host1  host2  host3

Now, you have 4 hosts on this server (in the example above). You need to scan all these 4 hosts in order to scan new disks attached to them. This can be done by writing - - - in their respective scan files. See below commands:

echo "- - -" > /sys/class/scsi_host/host0/scan
echo "- - -" > /sys/class/scsi_host/host1/scan
echo "- - -" > /sys/class/scsi_host/host2/scan
echo "- - -" > /sys/class/scsi_host/host3/scan

This completes your scan on SCSI hosts on the server. Now you can again run fdisk command we saw previously and compare the new output with the old one. You will see a new disk being added to the system and its respective device name too.

Scan FC LUNs:

If you have connected disks via FC then you need to scan FC hosts on the server. Check the current list of hosts on the server as below :

# ls /sys/class/fc_host
host0  host1

Now there are 2 FC hosts on the server. Again we need to scan them by writing 1 to their respective issue_lip file along with scan steps from above.

# echo "1" > /sys/class/fc_host/host0/issue_lip
# echo "- - -" > /sys/class/scsi_host/host0/scan
# echo "1" > /sys/class/fc_host/host1/issue_lip
# echo "- - -" > /sys/class/scsi_host/host1/scan

This will scan your FC HBA for new visible disks. Once the command completes (check syslog for completion event), you can use fdisk command to list disks. Compare the output with ‘before scan’ output and get new disk names!

Move disks/LUN from one server to another without losing data

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Howto guide to moving disks or LUN from one server to another without losing any data. This guide is related to disks or LUN which are configured under LVM.

In Unix or Linux infra, it’s pretty common scenarios when you have to move disks or storage LUNs from one server to another server with data on them intact. This is something that is happening in clusters automatically i.e. handled by cluster services. When the primary node goes down, cluster services move disk or luns from primary to the secondary node and make secondary node available for use.

We are going to see how to do this manually using commands. This howto guide gives you an insight into what cluster services do in the background to move data across nodes in case of failures. We will be using LVM (Logical Volume Manager) as our disk manager in this guide since its most widely used volume manager next to VxVM (Veritas Volume Manager).

Steps flow goes like this :

  1. Stop disk access on server1
  2. Remove disk / LUN from server1
  3. Present disk / LUN to server2
  4. Identify new disk / LUN on server2
  5. Import it into LVM
  6. Make it available to use on server2

Let’s see these steps one by one in detail with commands and their outputs. We will be moving mount point /data from server1 to server2. /data is mounted on /dev/vg01/lvol1.

1. Stop disk access on server1

Firstly, you have stopped all user/app access to the related mount points. In our case its /data. You can check if anyone accessing mount point and can kill it using fuser command.

# fuser -cu /data         #TO VIEW USERS
/data:   223412c(user1)
# fuser -cku /data        #TO KILL USERS
# fuser -cu /data

Once you are sure no one is accessing mount point, go ahead and unmount it.

# umount /data

2. Remove disk / LUN from server1

Now, we need to remove disk or LUN from LVM of server1 so that it can be detached from the server gracefully. For this, we will be using vgexport command so that configuration backup can be imported on the destination server.

# vgchange -a n /dev/vg01
Volume group "/dev/vg01" has been successfully changed.
# vgexport -v -m /tmp/vg01.map vg01
Beginning the export process on Volume Group "/dev/vg01". 
vgexport:Volume Group “/dev/vg01” has been successfully removed.

To export VG, you need to de-activate the volume group first and then export VG with a map file. Transfer this map file to server2 with FTP or sftp so that it can be used while importing VG there.

Now, your VG vanishes from server1 i.e. related disk / LUN is no more associated with LVM of server1. Since VG ix exported only, data is intact on disk / LUN physically.

3. Present disk / LUN to server2

Now, you need to physically remove the disk from server1 and physically attach it to server2. If it’s a LUN then remove the mapping of LUN with server1 and map it to server2. You will require to do zoning at the storage level and WWN of both servers.

At this stage, your disk / LUN is removed from server1 and now available/visible to server2. But, it’s not yet known to LVM of server2.

4. Identify new disk / LUN on server2

To identify this newly presented/mapped disk/LUN on server2, you need to scan hardware or FC. Once you get disk number for it (identified in the kernel) proceed with the next steps of LVM.

Read here : Howto scan new lun / disk in Linux & HPUX

5. Import it in LVM

Now, we have disk / LUN identified on server2 along with the VG map file from server1. Using this file and disk name, proceed with importing VG in server2.

# vgimport -v -m /tmp/vg01.map /dev/vg01 list_of_disk
vgimport: Volume group “/dev/vg01” has been successfully created.
Warning: A backup of this volume group may not exist on this machine.
Please remember to take a backup using the vgcfgbackup command after activating the volume group.
# vgchange -a y vg01
Volume group “/dev/vg01” has been successfully changed.
# vgcfgbackup /dev/vg01
Volume Group configuration for /dev/vg01 has been saved in /etc/lvmconf/vg01.conf

First, import VG with vgimport command. In place of the list_of_disk argument in the above example, you have to give your disk name. You can use any VG name here. It’s not mandatory that you have to use VG name same as the first server. After successful import, activate that VG with vgchange.

6. Make it available to use on server2

At this stage, you disk / LUN is available in LVM of server2 with all data on them intact. To make it available for use we need to mount it on the directory. Use mount command:

# mount /dev/vg01/lvol1 /data2

Add entry in /etc/fstab as well to make sure mount point gets mounted at boot too.

cut command and its examples

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Learn how to use the cut command in various scenarios and in scripting with lots of examples. Understand how to extract selective data using cut command.

Everybody in a Linux or Unix environment is familiar with grep command which is used for pattern finding/filtering. This is a kind of selective data extraction from source data. cut is another command which is useful for selective data extraction.

cut command basically extracts a portion of data from each line of a given file or supplied data. Syntax to be used for cut command is –

# cut [options] [file]

Below different options can be used with cut command :

–c To select only this number of character.

Roughly you can say its a column number to be extracted. See below example:

# cat test
sd
teh
dxff
cq2w31q5
# cut -b 4 test


f
w

Here, cut is instructed to select only 4th character. If you look closely, in the output it shows only 4th column letters. Lines which are having less than 4 characters are shown blank in cut output!

This option is useful in scripting when there is a need for single-column extraction from supplied data.

-b To select only this number of bytes.

This option tells the command to select only a given number of a byte from each line of data and show in the output. In most cases, its output is the same as -c option since each character is treated as one byte in a human-readable text file.

-d Use specified delimiter instead of TAB

Normally cut uses tab as a delimiter while filtering data. If your file has different delimiter like a comma , in CSV file or colon : in /etc/passwd file then you can specify this different delimiter to cut command. Command will process data accordingly. This option should always be used with b, c or f options. Only using -d option will result in below error :

# cut -d a test
cut: you must specify a list of bytes, characters, or fields
Try `cut --help' for more information.

-f To select only specified number of fields.

This option is to specify which all fields to be filtered from data. This option if not supplied with delimiter character data (-d option/ no TAB in data) then it will print all lines. For example, if you say -f 2 then it searches for TAB or supplied delimiter values if any in data. If found then it will print 2nd field from delimiter for that line. For lines where it won’t find delimiter, it will print the whole line.

# cat test
this is test file
raw data
asdgfghtr
data
#cut -f1 test
this is test file
raw data
asdgfghtr
# cut -d" " -f2 test
is
data
asdgfghtr
data

Field numbering is as below:

Field one is the left side field from the first occurrence of the delimiter
Field second is the right-side field from the first occurrence of the delimiter
Fields third is the right-side second field from the first occurrence of the delimiter

Observe the above example, where it prints all lines as it is when the delimiter is not specified. When the delimiter is defined as single space ” ” then it prints field two according to numbering explained above.

-s To print only lines with delimiters

Missing delimiter causes cut to print the whole line as it is which may mislead the desired output. This is dangerous when the command is being used in scripts. Hence -s option is available which restricts cut to display the whole line in case of missing delimiter.

# cut -f1 -s test

Using the same test file from the above example when -s is specified then there is no output. This is because the default delimiter TAB does not exist in the file.

Number to be used with -b, -c and -f options:

In all the above examples, we have declared single integers like 2,4 for -b, -c and -f options. But we can also use the below system :

  • x: Single number like we used in all examples
  • x-: From xth bye, character or field till end of line
  • x-y: From xth byte, character or field till yth
  • -y: From first byte, character or field till yth
# cut -d" " -f2- test
is test file
data
asdgfghtr
data
# cut -d" " -f2-3 test
is test
data
asdgfghtr
data
# cut -d" " -f-1 test
this
raw
asdgfghtr
data

Few examples of cut command :

To get a list of users from /etc/passwd file, we will use delimiter as : and cut out the first field.

# cut -d: -f1 /etc/passwd
root
bin
daemon
adm
lp
------ output clipped -----

cut command can be feed with data from the pipe as well. In that case last [file] parameter shouldn’t be defined. Command will read input from pipe and process data accordingly. For example, we are grep ing user with uid 0 and then getting their usernames using cut like below:

# cat /etc/passwd |grep :0: | cut -d: -f1
root
sync
shutdown
halt
operator

Getting userid and group id from /etc/passwd file along with their usernames.

# cat /etc/passwd |cut -d: -f1-4
root:x:0:0
bin:x:1:1
daemon:x:2:2
adm:x:3:4
lp:x:4:7
sync:x:5:0

How to get directory size in Linux

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Learn how to get directory total size in Linux without calculating each and every file’s size within. A very handy command to troubleshoot mount point utilization.

Many times we need to check specific directory size to hunt down the culprit of mount point utilization. There are scenarios where mount points keep getting full and we need to investigate which file or directory is hogging most of the space.

Collectively to check the highest file or directories, I already briefed on post Highest size files in the mount point. Let’s see how we can get directory’s collective size in one go.

We will be using disk usage command du and below of its options :

  • -s: Display the only summary of each element
  • -h: Human readable format for size i.e. KB, MB, GB
  • -c: Produce grand total i.e.  display the total size
# du -sch /dump1/test/mydir
13G     /dump1/test/mydir
13G     total

Here, the specified directory is 13GB. It’s a size of /dump1/test/mydir directory not of /dmp1.

If you want to check the size of every object beneath the specified directory then you can skip -s i.e. summery option from the command.

# du -ch /tmp
4.0K    /tmp/hsperfdata_oracle11
4.0K    /tmp/orbit-root

----- output clipped -----

4.0K    /tmp/keyring-VPiB3D
16K     /tmp/lost+found
652K    /tmp
652K    total

In the above output, you can see each and every object’s size is given and finally at the end total is given which is the final size of the specified directory!

Let me know if you have any queries/questions/feedback in the comments. Also, drop us a message on our contact form.

How to configure NTP client in Linux

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Learn how to configure NTP (Network Time Protocol) on Linux machines to sync time with the NTP server over the network. Also, learn how to manually sync time on NTP.

Nowadays NTP (Network Time Protocol) is one of the essential things in any IT infrastructure. Apart from production even development or test environments also backed with NTP to ensure smooth operations.  Let’s see what is NTP and how to configure it in the Linux machine.

What is NTP

NTP is the protocol used to sync the time of machines with the NTP server (can be an appliance or another Linux machine) over the network. It aims at keeping all the machines clock in sync so that there will be no delays between any two machines in a network. This is very crucial in production environments running finance data. Network time protocol runs on UDP port 123. This should be open between the time server and client in both directions.

What is NTP server

NTP server can be another machine with NTP server-side configuration running or it can be a dedicated NTP appliance. NTP appliance is a small rackmount server looking device that has an antenna attached to it. An antenna can be extended to building rooftops to ensure better signal receiving. These appliances receive signals and hence synchronize their own time with satellites in space. Now their own system time will be set a benchmark to sync other machines with it over the network. The appliance comes with its own configuration which can be done on front display buttons or by connecting to its console. Each vendor has a different set of configs and different methods to set them.

Configure NTP client 

Let’s assume we already have NTP appliance name ntpappliance1.xyz.com with IP 10.10.1.2 in our infra. Now we will see step by step configuring Linux server to sync time with this appliance over the network.

1. Make sure you have ntp package installed. If not install it using steps defined here.

# rpm -q ntp
ntp-4.2.6p5-1.el6.x86_64

2. Edit /etc/ntp.conf file to add appliance or NTP server name into it. Add IP/hostname in the end of the file. If you are supplying hostname here then make a sure relevant entry is present in /etc/hosts file. I have shown both entries in the below example IP and hostname.

# cat /etc/ntp.conf

------- output clipped ------
# Enable writing of statistics records.
#statistics clockstats cryptostats loopstats peerstats
#server 2.sg.pool.ntp.org

server ntpappliance1.xyz.com prefer
server 10.10.1.3

In the above output, 3-time servers are defined and the one with prefer option will always have a preference while syncing time. If it’s not reachable then any one of the remaining servers is chosen by the daemon to sync time with.

3. That’s it! Start your ntp daemon and make sure it’s running.

# /etc/init.d/ntpd start
OR
# service ntpd start
# service ntpd status
ntpd (pid  2261) is running...

4. Check time sync status using command :

# ntpq -p
     remote              refid       st t when poll reach   delay   offset  jitter
==================================================================================
+ntpappliance2.xyz.com 10.10.1.3      3 u   40   64  377  180.764    0.719   0.458
*ntpappliance1.xyz.com 10.10.1.2      3 u   50   64  377  180.851   -0.272   0.149

Here different fields are :

  • remote : Remote time server hostname/IP
  • refid : Association ID
  • st : stratum
  • t : u: unicast, b: broadcast, l: local
  • when : sec/min/hr since last received packet
  • poll : poll interval (log2 s)
  • reach : reach shift register (octal)
  • delay : roundtrip delay
  • offset : offset from server time
  • jitter : Jitter (noise)

Also, the very first value displayed is state i.e. + and * sign. These values can be :

  • + mean Good connectivity and preferred remote server
  • * means currently selected time server for sync
  • - means do not use to sync due to out of tolerance (cluster algorithm)
  • x means do not use to sync due to out of tolerance (intersection algorithm)
  • # means good connectivity but not used to sync yet.

Manually sync time with the server

NTP daemon runs in background and sync time according to polling configuration. But if you want to manually sync time right away with time server then you can do it with below command :

# ntpdate -u ntpappliance2.xyz.com
10 Dec 13:20:05 ntpdate[30337]: adjust time server 10.10.1.3 offset -0.000437 sec

It will update time with a given time server (as an argument in command) right away.

If you are having issues with timeserver connectivity then first troubleshoot at OS and firewall level. You can also view your syslog and grep for the NTP keyword and you will see all NTP related messages logged in Syslog which may help you in troubleshooting.

Understanding /etc/hosts file

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/etc/hosts is a key file for name resolution in any Linux Unix system. Learn fields, formats within /etc/hosts file. Understand the meaning of each field and how it can be set.

This is also one of the important files in the Linux-Unix system like /etc/passwd or /etc/fstab. The name resolution in the Lx-Ux system is being handled by this file. Whenever kernel needs to resolve some hostname to IP, it will search for it in /etc/hosts file. If DNS is configured on the system then it will go for it and then this file doesn’t play much of role in name resolution. Basically this file is a static IP lookup table on the server.

It’s a text file that can be viewed using cat, more, less, etc commands. One can edit this file using text editors like vi. Sample /etc/hosts file is shown below :

# cat /etc/hosts
127.0.0.1   localhost localhost.localdomain localhost4 
::1         localhost localhost.localdomain localhost6
10.10.1.64  server34 #This server

# Test servers 
10.10.1.12 test01
10.10.1.121 test02

# NTP server
10.10.1.85 ntpsvr1.kerneltalks.com  ntpsrv1
10.10.1.86 ntpsvr2.kerneltalks.com     #standby server

The format being followed is <IP> <FQDN> <alias> where both fields are separated by space or tab and one IP per line. Comments can be added with lines starting with the # symbol. Comments can be added on the same line of IP entry too. Any text following the # symbol will be ignored until the end of the line. These lines will be ignored by the kernel/shell/program when it reads this file. Those are just comments added for the understanding of the user (human). Like in the above example NTP server and standby server both are comments.

The hostname can contain only alphanumeric characters, minus sign - and period . It should always start with the alphabet and ends with an alphanumeric character.

There is also the 3rd field in each row which is optional. This field is for aliases. These are short names, alternate names, etc for the same IP. In the above example, ntpsrv1 is an alias to IP 10.10.1.85

You will see a couple of entries in all /etc/hosts file on your environment. Most of them are loopback address i.e. 127.0.0.1 which will be pointing to localhost and another is that server’s own IP & hostname entry.

Understanding /etc/fstab file

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/etc/fstab is a key file for file systems in any Linux Unix system. Learn fields, formats within /etc/fstab file. Understand the meaning of each field and how it can be set.

/etc/fstab is one of the key files in running a Linux or UNIX system. File system mounting can be controlled using this file. This is one of the files being used at boot to validate and mount file systems on the machine.  This file is human-readable and can be edited with a text editor like vi.

This file contains 6 parameters per row. Each row represents one file system details. They are as below :

  1. Volume
  2. Mount point
  3. File system type
  4. Options
  5. Dump
  6. Pass

Let’s see one by one –

1. Volume

This is a disk or logical volume which is the source to be mounted on the mount point specified in the second field. See the below example of fstab from the Linux system.

# cat /etc/fstab


# /etc/fstab
# Created by anaconda on Thu Dec  5 15:47:52 2013
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
/dev/mapper/vg_00-lv_root /                       ext4    defaults        1 1
UUID=f2918ad9-f5ce-485d-81ae-e874f57f6f57 /boot                   ext4    defaults        1 2
/dev/mapper/vg_00-lv_home /home                   ext4    defaults        1 2
/dev/mapper/vg_00-lv_tmp /tmp                    ext4    defaults        1 2
/dev/mapper/vg_00-lv_usr /usr                    ext4    defaults        1 2
/dev/mapper/vg_00-lv_var /var                    ext4    defaults        1 2
/dev/mapper/vg_00-lv_swap swap                    swap    defaults        0 0
tmpfs                   /dev/shm                tmpfs   defaults        0 0
devpts                  /dev/pts                devpts  gid=5,mode=620  0 0
sysfs                   /sys                    sysfs   defaults        0 0
proc                    /proc                   proc    defaults        0 0
/dev/sdb                /app                    ext3    defaults        1 2
10.10.2.3:/my_share     /tmp/nfs_share          nfs      defaults       0 0

In the above example, you can see volume is specified by UUID or logical volume name or disk name or IP:/directory.

/boot entry is specified by UUID. UUID is a universally unique ID assigned to each disk when it’s formatted in the system. The disk can be identified by UUID or disk name in the kernel.  Since its unique number, it’s ideal to use UUID in fstab for important file systems!

/var, /tmp, etc entries are defined using volume as a logical volume name. They are logical volumes part of the volume group vg00. See LVM legends to get familiarize with naming conventions.

/dev/shm is defined by tmpfs volume. Its a temporary file system volume created and identified by the kernel on the root disk. devpts, sysfsare part of such system-defined file systems.

Second Last entry,  you can see disk sdb is also defined as a volume for /app entry.

Lastly, the NFS share is mounted on /tmp/nfs_share directory. There IP address of the NFS server and its exported share volume name combination is defined as a volume.

This is the first argument to be supplied in mount command while mounting any filesystem.

Normally HPUX uses LVM as a partition manager hence only logical volumes are found as a volume entry in fstab. See below the example of fstab from the HPUX system.

$ cat /etc/fstab

# System /etc/fstab file.  Static information about the file systems
# See fstab(4) and sam(1M) for further details on configuring devices.
/dev/vg00/lvol3 / vxfs delaylog 0 1
/dev/vg00/lvol1 /stand vxfs tranflush 0 1
/dev/vg00/lvol4 /home vxfs delaylog 0 2
/dev/vg00/lvol5 /opt vxfs delaylog 0 2
/dev/vg00/lvol6 /tmp vxfs delaylog 0 2
/dev/vg00/lvol7 /usr vxfs delaylog 0 2
/dev/vg00/lvol8 /var vxfs delaylog 0 2
/dev/vg00/lvol10 /var/adm/sw vxfs delaylog 0 2
/dev/vg00/lvol11 /admin vxfs delaylog 0 2
10.10.2.3:/my_share /tmp/nfs_share nfs defaults 0 0

2. Mount point

Its second field in an entry of fstab. This is the name of the directory on which volume should be mounted. It should always be an absolute path (i.e. starts with/and has all directory hierarchy till last expected directory) in this field.

Directories like /var, /boot, /tmp, /stand, /usr, /home, /proc, /sys are (and should be) reserved for system mount points. In HPUX even logical volume numbers of root VG are reserved for system mount points like lvol1 should always be /stand. 2 for swap, 3 for root, etc.

This is the second argument to be supplied to mount command when mounting any file system.

3. File system type

This is FS type to be considered while mounting the given volume on the specified mount point. Different file system types have different functions and advantages to offer. You need to specify the same FS type which was used at the time of formatting respective volume. ext3, ext4 (Linux FS), vxfs (veritas FS), NFS (Network FS), swap (SWAP FS) are a few types.

This can be supplied to mount command with -F option.

4. Options

Those are file system options that will enhance the user experience of the mount point. They also impact on the performance of the file system and impact in recovery in case of failures. Value defaults in the above example instructs mount command to use parameters defined inbuilt. They can be seen in the man page :

defaults
              Use default options: rw, suid, dev, exec, auto, nouser, async, and relatime.

All available options can be summarized as below :

Option
Description
sync All I/O to the filesystem should be done synchronously.
async All I/O to the filesystem should be done asynchronously.
atime inode access time is controlled by kernel defaults.
noatime Do not update inode access times on this filesystem
auto Mount it when -a used (mount -a)
noauto Dont ‘auto’
dev Interpret character or block special devices on the filesystem
nodev Dont ‘dev’
diratime Update directory inode access times on this filesystem.
nodiratime Dont ‘diratime’
dirsync All directory updates within the filesystem should be done synchronously.
exec Permit execution of binaries
noexecDont ‘exec’
group Allow normal group users to mount
mand Allow mandatory locks on this filesystem.
relatime Update inode access times relative to modify or change time.
norealtime Dont ‘realtime’
delaylog Affect how vxfs maintains journals which impacts performance and ability to recover the file system
nomand Dont ‘mand’
suid Allow set-user-identifier or set-group-identifier bits to take effect.
nosuid Dont ‘suid’
remount Attempt to remount an already-mounted filesystem.
rw Read write mode
ro
 Read only mode
owner Allow non-root user to mount if he is owner of device
user Allow an ordinary user to mount the filesystem.
nouser Dont ‘user’
largefiles Allow file size more than 2TB
transflush Performance related

These options can be supplied to mount command using -o.

5. Dump

This is an old fashioned backup option in case the server goes down. If this is set to 1 then FS dump will happen when the system goes down due to some issue. Setting this 0 will nullify this option.

6. Pass

This tells kernel about file system check priority or sequence. fsck is a facility that checks the file system for its consistency. During boot, if fsck is invoked then it looks for this file. If set to 0, fsck will be skipped for that mount point. If set to 1 then that mount points will be first in sequence to be fscked.

NFS configuration in Linux and HPUX

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Learn how to configure the network file system (NFS) in Linux and HPUX servers. Howto export NFS, start/stop NFS services, and control access on it.

NFS configurations

The network file system is one of the essential things in today’s IT infrastructure. One server’s file system can be exported as NFS over network and control access over it. Other servers can mount these exported mount points locally as an NFS mount. This enables the same file system making available to many systems thus many users. Let’s see NFS configurations in Linux and HPUX.

NFS Configuration file in Linux

We assume the NFS daemon is installed on a server and running in the background. If not check package installation steps and how to start service on Linux. One can check if NFS is running on the server with service or ps -ef command. For NFS server i.e. server exporting directory should have portmap service running.

Make sure you have TCP and UDP port 2049, 111 on firewalls between client and server. It can be in OS firewall, iptables, network firewalls, or security groups in the cloud.

root@kerneltalks # ps -ef |grep -i nfs
root      1904     2  0  2015 ?        00:00:08 [nfsd4]
root      1905     2  0  2015 ?        00:00:00 [nfsd4_callbacks]
root      1906     2  0  2015 ?        00:01:33 [nfsd]
root      1907     2  0  2015 ?        00:01:32 [nfsd]
root      1908     2  0  2015 ?        00:01:33 [nfsd]
root      1909     2  0  2015 ?        00:01:37 [nfsd]
root      1910     2  0  2015 ?        00:01:24 [nfsd]

root@kerneltalks # service nfs status
rpc.svcgssd is stopped
rpc.mountd (pid 1897) is running...
nfsd (pid 1913 1912 1911 1910 1909 1908 1907 1906) is running...
rpc.rquotad (pid 1892) is running...

root@kerneltalks # rpcinfo -p localhost
   program vers proto   port  service
    100000    4   tcp    111  portmapper
    100000    3   tcp    111  portmapper
    100000    2   tcp    111  portmapper
    100000    4   udp    111  portmapper
    100000    3   udp    111  portmapper
    100000    2   udp    111  portmapper
----- output clipped -----

/etc/exports is the configuration file which has all exported volume details along with their respective permissions. /etc/exports follows the format as below :

<export> <host> (options)

where –

  • export is filesystem/directory to be exported
  • the host is hostname/IP to which export is accessible where wile cards are acceptable
  • options are permissions which are ro, rw, sync, async.

Refer below chart which can be used to decide your entry in this file.

NFS config file parameters

Parameter
Example
/my_share server3 (rw, sync) Export /my_share directory for server3 with read write access in sync mode
/my_share * (ro, sync) Export /my_share for any host with read only permission and sync mdoe
/my_share 10.10.2.3 (rw,async) Export /my_share for IP 10.10.2.3 with rw in async.
/my_share server2 (ro, sync) server3 (rw, sync) Exporting to two diff servers with diff permissions 
root@kerneltalks # cat /etc/exports
/my_share       10.10.15.2(rw,sync)
/new_share       10.10.1.40(rw,sync)

/etc/exports file can be edited using vi editor or using /usr/sbin/exportfs command.

How to start-stop NFS service in Linux

Once you made the changes in the file you need to restart NFS daemon to take all these changes in effect. This can be done using the service NFS restart command. If your NFS is already running and you just need to take a new configuration in action you can reload config using service NFS reload. To stop NFS you can run service nfs stop command.

root@kerneltalks # service nfs status
rpc.svcgssd is stopped
rpc.mountd (pid 1897) is running...
nfsd (pid 1913 1912 1911 1910 1909 1908 1907 1906) is running...
rpc.rquotad (pid 1892) is running...

How to re-export NFS shares after editing the configuration file

In running the NFS environment, where multiple clients already mounted NFS shares from the NFS server and you need to edit NFS share configuration. You can edit the NFS configuration file and re-export NFS shares using exportfs command.

Make sure only additions happened to config file while reloading config otherwise it may affect already connected NFS shares. 

root@kerneltalks # exportfs -ra

How to mount NFS share

At the destination, where export needs to be mounted should have NFS daemon running too. Mounting a share is a very easy two-step procedure.

  1. Create a directory to mount the share
  2. mount share using the mount command.

To make this permanent i.e. mounting share at boot time, make an entry to /etc/fstab like below so that manually mounting after the reboot of server can be avoided.

10.10.2.3:/my_share /tmp/nfs_share nfs defaults 0 0

NFS configuration in HPUX

This part is the same as Linux. In some versions, you need to edit /etc/dfs/dfstab file. This file takes share commands as a per line entry. It can be filled like below :

share -F nfs -o root=server2:server3 /my_share

Above line indicates exporting /my_share directory for server2 and server3 with root account access.

Also, we need to specify NFS_SERVER=1 parameter in /etc/rc.config.d/nfsconf on the NFS server. By default, it is set to 0 i.e. server acts as NFS client. Along with this NFS_CORE and START_MOUNTD needs to be marked to value 1 as well.

How to start-stop NFS service in HPUX

We have covered it here: NFS server start/stop on HPUX

For reloading config file in HPUX, you can run shareall command.

Mounting share

This part is the same as Linux

Errors seen in Linux

If you did not prepare client properly then you might see below error :

mount: wrong fs type, bad option, bad superblock on 10.10.2.3:/mys_hare,
       missing codepage or helper program, or other error
       (for several filesystems (e.g. nfs, cifs) you might
       need a /sbin/mount.<type> helper program)

       In some cases useful info is found in syslog - try
       dmesg | tail or so.

Install nfs-utils, nfs-common packages and you should be able to mount NFS filesystem without any issues.

How to rename volume group

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Learn how to rename the volume group in Linux or Unix. Understand what happens in the background when you change the volume group name of existing VG.

A volume group can be renamed with easy vgrename command Linux. But first, we will see how it can be done without vgrename command so that step by step you will understand what actually happens in the background while VG name changes.

We have seen how to create VG in the past and how to export/import VG. We are going to use these commands to rename VG. Below steps needs to be followed –

  1. Stop all user/app access to all mount points within VG using fuser
  2. Un-mount all LV using umount
  3. Deactivate VG using vgchange
  4. Export VG using vgexport
  5. Create a new name folder and group file using mknod
  6. Import VG with a new name in command options using vgimport
  7. Activate VG using vgchange
  8. Mount all LV using mount
  9. Edit related entries in /etc/fstab with a new name

See below output for the above-mentioned steps (HPUX console).

# fuser -cku /data
/data:   223412c(user1)
# umount /data
# vgchange -a n /dev/vg01
Volume group "/dev/vg01" has been successfully changed.
# vgexport -v -m /tmp/vg01.map vg01
Beginning the export process on Volume Group "/dev/vg01". 
/dev/dsk/c0t1d0 vgexport:Volume Group “/dev/vg01” has been successfully removed.
# mkdir /dev/testvg
# mknod /dev/testvg/group c major 0xminor
# vgimport -v -m /tmp/vg01.map /dev/testvg list_of_disk
vgimport: Volume group “/dev/testvg” has been successfully created.
Warning: A backup of this volume group may not exist on this machine.
Please remember to take a backup using the vgcfgbackup command after activating the volume group
# vgchange -a y testvg
Volume group “/dev/testvg” has been successfully changed.
# mount /dev/testvg/lvol1 /data

In the above step by step process, you can see how VG changes its name. We are changing its VG related file and directory and then we import it using old configuration but the new name.

In Linux, we have one command which does all this stuff in the background for you. vgrename is a command which used to rename VG in Linux. You have to supply the old VG name and required a new name.

# vgrename /dev/vg01 /dev/testvg
Volume group "/dev/vg01" successfully renamed to "/dev/testvg"
OR
# vgrename vg01 testvg
Volume group "vg01" successfully renamed to "testvg"

Keep in mind, this command also requires de-activated VG to work. So this is not an online process. It supports the below options :

  • -f Forcefully rename
  • -v Verbose mode