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udev

After cloning a VMWare image I had noticed that my CentOS Linux server renamed eth0 as eth1, eth1 as eth2 and so on. This is due to wrong or duplicate mac address. To fix this problem login to your CentOS Linux server using console and type the following commands:
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Linux udev tip: Assign Static SCSI Device Name

So how do you assign static names for SCSI device using udev under Linux? Simply use the scsi_id and other udev tools such as :

[a] scsi_id - retrieve and generate a unique SCSI identifier.
[b] udev_start - script to initialize /dev by using udev
[c] udevtest - simulate a udev run and print the action to the console.

What the hell is udev?

udev is the device manager for the Linux 2.6 kernel series. Its primary function is managing device nodes in /dev. Old UNIX system creates device in the /dev with static files. udev dynamically provides only the nodes for the devices actually present on a system.

Note following configuration is only tested on Gentoo / RHEL 5.x / CentOS 5.x Linux server systems but it should work with other distros with udev support.

Step # 1: Get WWID of the SCSI device

To get the WWID of /dev/sdd, type:
# scsi_id -g -u -s /block/sdd
Where,

  • -g : Treat the device as white listed
  • -u : Reformat the output
  • -s /bock/sdd : Generate an id for the /block/sdd (/dev/sdd). The mount point must not be included. For example, use /block/sdd, not /sys/block/sdd.

Step # 2: Create /etc/udev/rules.d/25-names.rules file

Open /etc/udev/rules.d/25-names.rules and append following text (replace WWID with actual id obtained from above command):

KERNEL=="sd*", BUS=="scsi", PROGRAM=="/sbin/scsi_id -g -u -s %p", RESULT=="WWID", SYMLINK+="scsiharddisk%n"

Above rule will create /dev/scsiharddisk (a symlink - select any other meaningful name as per your proecjet ) for the SCSI device with a given WWID. Please note that partitions on this device will be assigned the device names like /dev/scsiharddisk1, /dev/scsiharddisk2 and so on. Next, verify that everything is correct order, enter:
# udevtest
Start initialize /dev for new devices:
# start_udev
Verify symbolic links are created, enter:
# ls -l /dev/scsiharddisk*

Read the man pages for more information:
man scsi_id
man udevtest
man 7 udev

What Is /dev/shm And Its Practical Usage

/dev/shm is nothing but implementation of traditional shared memory concept. It is an efficient means of passing data between programs. One program will create a memory portion, which other processes (if permitted) can access. This will result into speeding up things on Linux.
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