Bcache: Caching beyond just RAM [LWN.net]

Bcache: Caching beyond just RAM

Posted Jul 8, 2010 14:05 UTC (Thu) by cesarb (subscriber, #6266)
Parent article: Bcache: Caching beyond just RAM

> as long as the filesystem has an embedded UUID

This sounds problematic to me. There is no guarantee that the UUID is unique. It might have been unique when it was *created*, but as soon as you duplicate a partition (with dd to another disk, a RAID 1 which is later split, or anything else), it ceases being unique. You could change the UUID of the copy, but who will remember to do that?

Bcache: Caching beyond just RAM

Posted Jul 8, 2010 14:57 UTC (Thu) by faramir (subscriber, #2327) [Link]

Given that many distributions now use UUIDs rather then device names in /etc/fstab, not having unique UUIDs is already a recipe for disaster.

Unique UUIDs

Posted Jul 8, 2010 16:19 UTC (Thu) by wstearns (guest, #4102) [Link]

You're correct; at least in the case of RAID 1 mdadm, the md device and the underlying partitions all share the exact same UUID. When I use bcache on a raid array, I don't cache the underlying partitions, I cache the raid device itself. That lets the raid code handle a missing component partition, raid resync, and any other raid issues internally.
If you copy a filesystem with dd or raid split as you mention, you still have to tell bcache both what UUID and what block device to cache, letting you specify the original partition or the new copy, whichever you want cached. So bcache shouldn't introduce new problems; a non-bcache linux system will have problems already if you have 2 different filesystems with the same UUID as others have mentioned.