rootninja

Figure out exactly how your devices and partitions are mapped

Look in the directory /dev/disk/by-id to see how your attached storage devices are mapped in /dev. If they are not already identified by UUID or Label, they will just get the next available letter in the alphabet. sda, sdb, then sdc, etc. The reason why you want to see them “by-id” is because if you plug in multiple devices in a different order one day, you may find what was “sdb3″ yesterday is “sdc3″ today.

# ls -l /dev/disk/by-id/
total 0
lrwxrwxrwx 1 root root 03-19 ata-SAMSUNG_HD80 -> ../../sda
lrwxrwxrwx 1 root root 03-19 ata-SAMSUNG_HD80-part1 -> ../../sda1
lrwxrwxrwx 1 root root 03-19 ata-SAMSUNG_HD80-part2 -> ../../sda2
lrwxrwxrwx 1 root root 03-19 ata-SAMSUNG_HD80-part3 -> ../../sda3
lrwxrwxrwx 1 root root 03-19 ata-SAMSUNG_HD80-part4 -> ../../sda4
lrwxrwxrwx 1 root root 03-19 ata-SAMSUNG_HD80-part5 -> ../../sda5
lrwxrwxrwx 1 root root 03-19 scsi-36255a8ab200 -> ../../sdb
lrwxrwxrwx 1 root root 03-19 scsi-36255a8ab200-part1 -> ../../sdb1
lrwxrwxrwx 1 root root 03-19 scsi-36255a8ab201 -> ../../sdc
lrwxrwxrwx 1 root root 03-19 scsi-36255a8ab202 -> ../../sdd
lrwxrwxrwx 1 root root 03-19 scsi-36255a8ab203 -> ../../sde
lrwxrwxrwx 1 root root 03-19 scsi-36255a8ab204 -> ../../sdf

Lets say sdb1 is the device you want to restrict

* Lookup the Universally Unique Identifier using the blkid command (part of e2fsprogs)
* The identifiers are generated when partitions are created. Non partitioned devices will not have these identifiers.
* Use of UUIDs are preferred over Labels since Labels are not unique. (labels such as “home”, “root”, “swap”, etc. are frequently used)

# blkid
/dev/sda5: UUID="82176f36-a3cf-34c1-ae90-aa348e34733e" SEC_TYPE="ext2" TYPE="ext3"
/dev/sda2: UUID="ab77293b-7f27-47bf-b74c-8374b27fb57b" SEC_TYPE="ext2" TYPE="ext3"
/dev/sda1: UUID="f67c2297-44f5-4723-a279-b0740fcc8bf5" TYPE="ext2"
/dev/sda3: TYPE="swap" UUID="a8b829a5-0bf1-4baf-adc7-d77adb7a7a46"
/dev/sdb1: UUID="d75a72c8-ab7b-4f55-8757-6d912cca387a" SEC_TYPE="ext2" TYPE="ext3"

Specify the device using the UUID in /etc/fstab

* include the noauto option to avoid auto-mounting this partition and ro for read-only just incase it is explicitly mounted

# grep d75a72c8-ab7b-4f55-8757-6d912cca387a /etc/fstab

UUID=d75a72c8-ab7b-4f55-8757-6d912cca387a /bad ext3 noauto,ro 0 0

Edit the fstab on other hosts where the storage may be visible

* The UUID, as it is visible from any host, will not change. Once you know the identifier for the partition, you can modify the fstab on all hosts to avoid auto-mounting.

# ssh host2 echo UUID=d75a72c8-ab7b-4f55-8757-6d912cca387a /bad ext3 noauto,ro 0 0 >> /etc/fstab

Alternative to blkid

Once you know which character device you’re looking for, you can also look in /dev/disk/by-uuid/ and find the uuid linked there.

# ls -l /dev/disk/by-uuid/
total 0
lrwxrwxrwx 1 root root 03-19 82176f36-a3cf-34c1-ae90-aa348e34733e -> ../../sda5
lrwxrwxrwx 1 root root 03-19 ab77293b-7f27-47bf-b74c-8374b27fb57b -> ../../sda2
lrwxrwxrwx 1 root root 03-19 f67c2297-44f5-4723-a279-b0740fcc8bf5 -> ../../sda3
lrwxrwxrwx 1 root root 03-19 a8b829a5-0bf1-4baf-adc7-d77adb7a7a46 -> ../../sda1
lrwxrwxrwx 1 root root 03-19 d75a72c8-ab7b-4f55-8757-6d912cca387a -> ../../sdb1

The drawbacks of UUIDs

If you copy partitions around using rsync, tar, or dd, then you will quickly find you have to edit /boot/grub/menu.lst and /etc/fstab if you’re using UUIDs everywhere.

In this case, the duplication of generic Labels is a good thing. I usually use the same layout of “BOOT”, “SWAP”, “ROOT”, and “HOME” labels for my 4 main partitions. Using label names with a partitions intended function is a good thing. I doubt you’ll ever want to mount SWAP in the kernel line of menu.lst and you’ll probably never find yourself using HOME as a label for swapon.

In these cases, labels are reusable even when the UUID may or may not have been changed.

# swapon LABEL=SWAP

title Fedora (2.6.26.6-49.fc8)
        root (hd0,0)
        kernel /vmlinuz-2.6.26.6-49.fc8 ro root=LABEL=ROOT vga=791
        initrd /initrd-2.6.26.6-49.fc8.img