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Re: AW: Re: [linux-lvm] very slow fw performance when snapshots active



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


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