Reducing chunksize may help a bit but you may end up having performance problems when you read snapshot data later. Thanks, Malahal. Gabriel Barazer [gabriel@oxeva.fr] wrote: > On 10/18/2007 4:01:40 PM +0200, "Zurell, Falko" > <Falko.Zurell@idmedia.com> wrote: > >Ok, i understand the point with the swap file of vi. But why is this > >with snapshots activated that much slower than without? > >Why can the file be saved in seconds without cow instead of about 2 > >minutes with cow. Needing twice the time for saving would be reasonable. > > I think this is because the re-writing operation on a standard block > device is done using a comparison between the data to write, and the > data already written (which is nearly all the time read from the OS file > cache in your test case, instead of the block device itself, because the > file written was entirely read just before) whereas the LVM snapshot > target redirect any write to the COW space without this comparison step. > Maybe there is a possible optimization of the snapshot dm target which > could compare block before deciding to write them on the COW space. > Obvisouly, such optimization process add a slight overhead an can > possibly have a negative impact on the performance. > > Gabriel > > _______________________________________________ > 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/ _______________________________________________ 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/
