[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.
Open Enterprise Systems
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