[aklug] Re: UUIDs

Arthur Corliss wrote:
> On Mon, 6 Apr 2009, Thomison, Lee wrote:
>
>> Be careful using UUIDs. I'm finding that's 'here be dragons' territory. U=
>> buntu (which admittedly I'm still learning) likes to use UUIDs, but there a=
>> re circumstances that they seem to be dynamic in, such as mounting a raid5 =
>> on a mountpoint in a volgroup (in my case). And sometimes they just flat d=
>> on't work sometimes, and other times (no apparent change) they do just fine=
>> . Phase of the moon, I guess.
>
> Can you elaborate on this? While I don't use UUIDs for mounting purposes,
> mdadm uses them by default for RAID members, and I've never had a problem.
> Of course, all my filesystems are LVs, so all the RAIDs are just PVs, so
> that may be where our configs differ.
>
Agreed, UUID are essential to building scalable operating systems since
it helps define devices based on a signature that is, for the most part,
unique to the system that generated it if it's careful and UUID ends up
being used for physical volumes/raid volumes, raid devices themselves,
filesystems on top of those devices and of course swap devices and hey..
how about files themselves for some filesystems! UUID simply being a
universally (anything) unique identifier. So assigning a raid volumes
UUID to a mount in /etc/fstab and so forth won't do a lick of good.
Sorry if this was the case.

/etc/blktab (generated by blkid) and the blkid utility are great for
peeking at block devices to see the UUID of the filesystem they may be
carrying.

Also, in a previous post on duplicating a drive, make sure you don't
clone filesystems you expect to mount later if both block devices will
stay in your system. Two or more identical filesystem UUID equates to
failure. The tools that generate and tune filesystems can usually
change the filesystems UUID if this is the case.

For hardened devices I often issue an LVM snapshot of the root
filesystem and mount the snapshot vs the root filesystem by default on
boot. I write back changes whenever I need to as it helps me maintain a
very clean and stable environment for telco related stuffs to operate
in. In this situation I can't afford to use UUID based mounting since
making a snapshot itself means multiple block devices point to the same
filesystem, essentially, and will return the same UUID when queried.

Anyhoot, have a good day peeps.

Shane

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Received on Mon Apr 6 09:49:02 2009