Re: [linux-lvm] advice for curing terrible snapshot performance?

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

On Mon, 15 Nov 2010, chris (fool) mccraw wrote:

> > BTW, rewriting that 1G file would be normal speed, since
> > the modified chunks have already been copied to the snapshot.
> 
> i'd think that, and you'd think that, but it is not the case.  most of
> my tests were done by rewriting the file 4x, and while the snap %used
> (monitored with the 'lvs' command) doesn't keep going up, performance
> stays the same.

Are you writing to the snapshot or the origin?  If writing to the
snapshot, and if your snap% is stable, then you are getting the addition seek
time to jump over to the COW for those sectors.

Once the COW has the copy of the original data for a chunk, then reads/writes
to that chunk on the origin should be identical to reads/writes without the
snapshot, except for some minor CPU overhead.

Another possibility is that while the snap% is not visibly increasing, you
are in fact updating new areas with each test.

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