[linux-lvm] advice for curing terrible snapshot performance?
Stuart D. Gathman
stuart at bmsi.com
Mon Nov 15 23:51:37 UTC 2010
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 at 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.
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