[lug] LVM and disk failure

Daniel Webb lists at danielwebb.us
Sun Jan 8 12:00:29 MST 2006


On Sun, Jan 08, 2006 at 09:39:11AM -0700, Dan Ferris wrote:

> LVM doesn't increase or decrease robustness.  It has nothing to do with 
> robustness.

Everyone is saying this to me, but I think we're meaning two different things.
What you mean is "LVM doesn't increase or decrease robustness of a given
physical volume".  I agree.  What I mean is "LVM decreases the robustness of a
logical volume relative to a similar size physical volume".  In other words, a
300G logical volume spanning three 100G physical volumes is 1/3 as robust as a
logical volume spanning a single 300G physical volume.  That's assuming that
each physical drive has the same chance of failures as any other.

If your filesystem spans multiple physical volumes, it absolutely decreases
robustness, unless that filesystem deals well with getting a large piece
chopped out of it.

For my 1000-disk example, I'd be a little worried even with RAID.  RAID
doesn't protect against corruption, and you could still have a RAID controller
go haywire and scramble one of the RAID sets.  Not likely, but it would only
take 1 in 1000 to cause a disaster.  Then, like you say, backups.

In the case I have in mind, I'm thinking of user accounts that should be
resizable.  I think a better way will be to create a new logical volume and
filesystem for each user, instead of putting all users on the same filesystem.
Then at worst those users whose logical volumes cross a physical boundary are
twice as likely to lose their filesystem, but that's not nearly as bad as the
risk of doing it all on one filesystem across many physical drives.




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