On Mon, Nov 15, 2010 at 15:51, Stuart D. Gathman <stuart@bmsi.com> wrote: > 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. I've never written to the snapshot, or even mounted it except to prove that i could... > 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. i don't doubt that, but i don't even want to write to a snapshot. i want to write to the primary, and only occasionally even read from the snapshot. it is the writes to the primary that i have been documenting. > Another possibility is that while the snap% is not visibly increasing, you > are in fact updating new areas with each test. and it's also possible i've been misremembering about the snap% not growing. in fact in a quick test, it did grow with each rewrite. perhaps i am remembering a test on an ext3 parent, whereas i am now using xfs. _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
