Corraling Linux Hard Disk Names
More Udev Fun
Carla Schroder
Thursday, April 3, 2008 09:05:57 AM
Just like in the olden days, udev may change your device names. While udev has a lot of advantages, it often means spending time figuring out how to nail down device names and mountpoints. Most Linux distributions ship with default udev configurations that create persistent names for hard drives. Look for something like /etc/udev/rules.d/65-persistent-storage.rules to see how your system is configured. However, /etc/fstab can still get bollixed, or you might want a removable USB storage device to have a persistent name and mountpoint, so your best tactic is to use UUIDs instead of /dev names. An example entry looks like this, except in real life the UUID line is unbroken:
# /dev/sda1
UUID=b099f554-db0b-45d4-843e-0d6a1c43ba44
/home ext3 defaults 0 2
Yes, UUIDs are long and unattractive. But they are unique, so no matter what udev or anything else on your system tries to do with them, they will always be the same. How do you know what the UUID is?
# vol_id -u /dev/sda1
b099f554-db0b-45d4-843e-0d6a1c43ba445
On Fedora it's /lib/udev/vol_id.
Another option is to use filesystem labels. Red Hat, Fedora, and all of their extended family like to use them. Labels are quick and easy to create or change. An entry in /etc/fstab looks like this:
LABEL=/1 / ext3 defaults 1 1
vol_id --export [device name] displays complete information, including labels. Where does this label come from? Fedora creates it at installation. To create or change filesystem labels, you need to use a command specific to your filesystem. e2label is for Ext3. For ReiserFS, use reiserfstune, and you must unmount the filesystem first. On XFS use xfs_admin, and for JFS you need jfs_tune. For FAT filesystems use mlabel, which is part of mtools.
With libata, how do you know which of your PATA drives are masters or slaves, and on which IDE controller? Just look at the output of dmesg|grep ata, and then use this table to figure out what's what:
ata1.00 primary master
ata1.01 primary slave
ata2.00 secondary master
ata2.01 secondary slave
dmesg also tells you if the kernel sees your PATA and ATAPI drives as hd or sd. Run dmesg|grep '[s|h]d[a-z]' to find out.
Resources
Carla Schroder is the author of the Linux Cookbook and the newly-released Linux Networking Cookbook, and is a regular contributor to LinuxPlanet.
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