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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