[linux-lvm] pvmove painfully slow on parity RAID

[Stuart D. Gathman]

On Fri, 31 Dec 2010, Spelic wrote:

> Ok never mind, I found the problem:
> LVM probably uses O_DIRECT, right?
> Well it's absymally slow on MD parity raid (I checked with dd on the bare MD
> device just now) and I don't know why it's so slow. It's not because of the
> rmw because it's slow even the second time I try, when it does not read
> anything anymore because all reads are in cache already.

The point of O_DIRECT is to *not* use the cache.  Although a write-through
cache would seem to be OK, you have to make sure that ALL writes write-through
the cache, or the data on parity raid will be corrupted.

The R/M/W problem afflicts every level of parity raid in subtle ways.
That's why I don't like it.

-- 
	      Stuart D. Gathman <stuart at bmsi.com>
    Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
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