Re: [linux-lvm] pvmove painfully slow on parity RAID

["Stuart D. Gathman" <stuart@bmsi.com>]

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.

-- 
	      Stuart D. Gathman <stuart@bmsi.com>
    Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
"Confutatis maledictis, flammis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.