[linux-lvm] snapshots on RAID5 blow up machine ("switching cache buffer size")

Jason H. Smith jhs at oes.co.th
Fri Jul 4 08:28:01 UTC 2003

On Wednesday 02 July 2003 02:37 pm, Scott Mcdermott wrote:
> I've searched the archives and see that this is related to
> the filesystem using 4k blocks whereas snapshot IO uses 1k
> blocks, so RAID5 code gets confused, but I don't understand
> the internals of the filesystem to be able to say this is
> expected behavior.
> My questions are these:
>     - Is this a RAID5 problem or an LVM problem, or both?

I ran into this a while ago and I eventually decided that, currently, 
software RAID 5 and LVM are, to put it nicely, "incompatible" if you want 
long-term snapshots.  I'm no expert, but IMO, these days, the RAID5 
subsystem should not assume i/o of equal chunk sizes, so it is md's 
problem.  IIRC, the 2.6 kernel is supposed to address this; but there's 
not much you can do at the moment.

Somebody please correct me if I'm wrong.

>     - Is this problem nonexistent on RAID1 backed or
>       RAID10 backed VGs (especially the latter since I am
>       contemplating a switch thereto)?

I believe that's correct, but please don't quote me.  Testing this should 
be simple enough.

Jason Smith
Open Enterprise Systems
Bangkok, Thailand
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