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

[malahal at us.ibm.com]

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 at oxeva.fr] wrote:
> On 10/18/2007 4:01:40 PM +0200, "Zurell, Falko" 
> <Falko.Zurell at 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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