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How To Resize /dev/shm Filesystem In Linux?

Walker — Mon, 03 May 2010 17:28:53 +0000

By default installation, RHEL and most Linux distributions mount tmpfs (a RAM-based temporarily filesystem) to /dev/shm directory and this temporarily filesystem size is always set to be half of the installed memory.

If that default size is not something expected, you can increase or reduce the /dev/shm filesystem size at wish.

To be even extreme, you may drop or disable this temporarily RAM-based filesystem entirely, to prevent it from auto-mount during system boots up, if none of the application in that Linux server relying on shared memory function or explicitly using tmpfs (as a quick-and-dirty trick to improve execution performance).

To increase or decrease /dev/shm filesystem size

1) Open /etc/fstab with vi or any text editor of your choice,

2) Locate the line of /dev/shm and use the tmpfs size option to specify your expected size,

e.g. 512MB:

tmpfs      /dev/shm      tmpfs   defaults,size=512m   0   0

e.g. 2GB:

tmpfs      /dev/shm      tmpfs   defaults,size=2g   0   0

The /etc/fstab content format is documented in man fstab and the tmpfs filesystem options can be found in man mount

3) To make change effective immediately, run this mount command to remount the /dev/shm filesystem:

mount -o remount /dev/shm

To disable the /dev/shm or tmpfs filesystem

Actually, Linux allocates the memory for this tmpfs on demand basis, up to the maximum size that shown in df -h command output. If none of the application is using the /dev/shm, this tmpfs in fact does not consume any memory space. So, why have to disable it?

To disable /dev/shm immediately once only (but enable it on subsequent reboots), just execute the umount command:

umount /dev/shm

To prevent tmpfs from auto-mount each time the Linux boots up, just remark or delete that line from /etc/fstab.

Red Hat Enterprise Linux Swap Size Recommendation

Walker — Sun, 02 May 2010 15:22:34 +0000

Are you looking for the recommended Linux swap partition size for a server with huge amount of RAM installed? Well, I have just stumbled upon one of such recommendation from RHEL documentation while I was actually looking for reference of previous topic.

Some of you, especially the veterans, might know there is this good old formula that suggest swap space size should be at least 1.5 times (some even said 2 times) of the RAM installed.

Nowadays, most of the brand new servers easily come with few tens to hundred gigabytes of RAM installed. Apparently, the required Linux swap file system size will be darn huge if to apply this formula and you wonder is this really needed and justified.

With reference to RHEL 5.5 installation guide, the new study suggests that the amount of swap space needed by a system is much depends on the memory workload running on that system.

This sound reasonable but not practical, as it is not easy to gauge memory workload before the system is running. Thus, the RHEL engineers suggest their own recommended swap space:

Min. 2GB of swap space for system installed with 4GB of RAM or less
Min. 4GB of swap space for system installed with 4GB to 16GB of RAM
Min. 8GB of swap space for system installed with 16GB to 64GB of RAM
Min. 16GB of swap space for system installed with 64GB to 256GB of RAM
Min. 32GB of swap space for system installed with 256GB to 512GB of RAM

In addition, the swap space should be spanned across multiple disk drives for the performance’s sake.

Although the reference is made for RHEL 5.5 running on x86, AMD64, and Intel64 system, I think it is worth for reference or applied with little adjustment if you’re not working with RHEL. But for CentOS users, you probably know that this reference should works too :-)

How To Find The Lost RHEL5 Installation Number?

Walker — Sun, 02 May 2010 10:51:18 +0000

Let’s say a brand new Dell server preinstalled RHEL 5.5 with a valid subscription number (a.k.a. installation number or registration number) has just been delivered to SA team.

Then, the developers (users of that new machine) require Red Hat Enterprise Linux to be reinstalled (for whatever reasons).

You, as one of the Linux system administrator, found out that the RHEL 5 installation number is missing or misplaced at somewhere. Oh no, God knows they (the developers, and maybe your teammate) will have all fingers pointing to you.

The trick to recover the lost RHEL installation number

This trick MAY or may NOT work. Since this DELL server is known to have preinstalled RHEL 5.5 with a valid subscription number, the chance to recover RHEL installation number is high:

1) Login to the system as root

2) Change directory to /etc/sysconfig/rhn and uses cat command to display the install-num file. Alternatively, you can just cat the file as this:

cat /etc/sysconfig/rhn/install-num

According to RHEL 5 Release Notes, the installation number is saved in /etc/sysconfig/rhn/install-num, if entered during RHEL installation. This install-num file is referred by rhn_register when registering with Red Hat Network.

How To Use ps Command To Find Top CPU And Memory Usage Of Processes?

Walker — Tue, 13 Apr 2010 17:54:34 +0000

The Linux system is slow because of high CPU utilization and memory is running low? Now, how to use the ps command to find out what are the running processes with top CPU and memory usage?

Well, there are no short of command line tools and GUI programs to find these out. But I think the ps command is one of the top 10 Linux commands and thus believe that most of UNIX/Linux users have came across ps command in the first day of UNIX/Linux introduction class.

But, how many users bother to type man ps and vet through the lengthy manual page for the available option switches?

Customize the ps command output for the subject

I have tested this in RHEL5 Linux command prompt:

ps -eo SFS --sort SFS

where the SFS (short for Standard Format Specifiers) can be one or more of the following keywords. Take note that not all of these SFS are available for sorting, e.g. time.

pid to display the process ID of the process.
start to display the time of command was started.
time is an alias of cputime and is better alternative to bsdtime; to display the cumulative CPU time (user and system) in [dd-]hh:mm:ss format.
rss to display the process’s Resident Set Size (RSS) in KB. RSS is regarded as non-swapped physical memory taken by a process.
vsz to display virtual memory size of the process in KB, excluding the device mappings.
cmd to display the running command or process and its specified arguments.
comm to display the running command or process but exclude its specified arguments.

The ps command example

To find out top processes in term of physical memory usage in Linux system, type this:

ps -eo pid,rss,comm --sort rss

As the standard format specifiers time is not available for sorting, it is a bit troublesome if uses ps command to find top CPU usage of processes. E.g. to find Linux processes that use more than 5 minutes of CPU time:

ps -eo pid,time,comm | grep -v 00:0[0-5]

Other than the 7 standard format specifiers mentioned above for used with -o option switch of ps command, there are many more (e.g. cputime, bsdtime, etc) covered in man ps that might be interesting and useful to you.

How To Change Linux Command Prompt Color?

Walker — Tue, 23 Feb 2010 17:09:06 +0000

The trick to customize Linux command prompt color and message is kinda useful for system administrator or developer who has access to live and testing servers. The difference of Linux prompt color and message between production server and staging server could alert users which system they are working on.

The tips to customize Linux command prompt color

With reference to IBM Tip: Prompt magic and the nice color chart, I have applied the following Linux system prompt customization to /etc/profile on all Linux production servers (hopefully someone who do remote access to any one of these live systems will be vividly alerted that this is not a staging or testing server):

export PS1="\[[\e[1;33m\]\u@\[\e[1;36;41m\]\H\[\e[0m\] \W] "

If you are not happy with the Linux system prompt color I choose, just change the number in red to other color code, with reference to the color chart above, where 30-37 is foreground color code (text color) and 40-47 is background color code.

The number in blue can be either 0 or 1, where 0 means to reset the Linux system prompt settings to default value (e.g. foreground, background, and boldness) and 1 means to print text with bold font type.

How To Display Date And Time In Linux History Command Output?

Walker — Tue, 23 Feb 2010 13:44:12 +0000

There is a trick to display timestamp in Linux history command output. With this feature turned on, one can easily understand or review what commands have been executed, on which date at what time, in the Linux system in question.

By default, however, the history command is not enabled to display date and time of each command executed in Red Hat, as with most Linux distributions.

Anyway, it is not complicated to turn on the chronological feature of history command in Linux; even a novice can do it in no time.

The trick to display timestamp in Linux history command output

To include standard timestamp format in history command output, as shown in above screenshot, all you have to do is login as root to open /etc/profile with vi editor (or any Linux text file editor you’re comfortable with) and add this following line:

export HISTTIMEFORMAT='%F %T >> '

This single magic command line exports the shell environment variable HISTTIMEFORMAT globally, so that every Linux users will able to see their respective command execution history with date and time information.

If you are not system administrator or not a root user in Linux system but would like to see timestamp in history command output, just add that magic line in your own account profile, for example the $HOME/.bash_profile if you’re using Bash Shell. You should able to see the timestamp in history command output after logout and login (or if you execute export HISTTIMEFORMAT='%F %T >> ' in command line).

The value of HISTTIMEFORMAT is a format string used by strftime. If you like to further customize the date/time format in history command output, just run man 3 strftime to get detail of timestamp string format that is valid for HISTTIMEFORMAT.

Use TRK To Reset Or Recover Windows 7 Account Password

Walker — Mon, 11 Jan 2010 17:52:01 +0000

I have just tested the latest Trinity Rescue Kit 3.3 build 334 and confirm this freeware stills able to recover or reset Windows 7 administrator account password. It works just nice as expected on my Dell Latitude with vanilla Windows 7 Ultimate installation.

In case you forget the Windows admin account password and find no other way to recover it (i.e. no password reset disk, no clue from the password hint, etc), the TRK freeware might able to take you out of this nightmare:

1) Download TRK ISO file and verify its MD5 checksum against the published file checksum value before burning the CD image to CD/DVD-ROM.

For Netbook without optical drive, you have to boot the TRK CD/DVD-ROM from another computer with CD/DVD-ROM drive and refer to the guide of create a bootable TRK USB flash drive.

2) Update the computer boot drive priority in BIOS setup to make sure it boots up from USB flash drive or CD/DVD-ROM drive, whichever applicable.

Some BIOS might have a hotkey which if pressed during start-up could allow user to choose a different boot drive as needed. For example, Dell Latitude uses F12, latest AMI BIOS uses F8, etc.

3) Boots the computer up from Trinity Rescue Kit media. At the (Linux) command prompt, type the following command and press ENTER:

winpass -u Administrator

CAUTION

TRK is actually a customized Linux distribution. Thus the command is case-sensitive, i.e. “a” is not equal to “A”
The Windows 7 real, built-in administrator account is not enabled by default and thus you have to enable this built-in administrator account with TRK first. Otherwise, replace Administrator account name with the user-defined administrator account. If this is called Walker, make sure you replace “Administrator” with “Walker” and not “walker”.

4) At the “user edit menu” prompt, type 1 and press ENTER key to effectively reset the said Windows account password to blank (empty) password! Having said that, you will able to log in with this account again to Windows 7, even if you have forgotten the password.