[linux-lvm] pvmove painfully slow on parity RAID

[Stuart D. Gathman]

On Wed, 29 Dec 2010, Spelic wrote:

> I tried multiple times for every device with consistent results, so I'm pretty
> sure these are actual numbers.
> What's happening?
> Apart from the amazing difference of parity raid vs nonparity raid, with
> parity raid it seems to vary randomly with the number of devices and the
> chunksize..?

This is pretty much my experience with parity raid all around.  Which
is why I stick with raid1 and raid10.

That said, the sequential writes of pvmove should be fast for raid5 *if*
the chunks are aligned so that there is no read/modify/write cycle.

1) Perhaps your test targets are not properly aligned?

2) Perhaps the raid5 implementation (hardware? linux md? 
   experimental lvm raid5?) does a read modify write even when it 
   doesn't have to.

Your numbers sure look like read/modify/write is happening for some reason.

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