[linux-lvm] advice for curing terrible snapshot performance?

[chris (fool) mccraw]

On Mon, Nov 15, 2010 at 10:08, Joe Pruett <joey at q7.com> 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.
>>
> i did similar tests and i saw that even though i was rewriting the file,
> it appeared that the ext3 layer was reallocating new blocks.  the
> snapshot usage would increase to show 3x the number of blocks i had
> thought it should have touched, and then it seemed to stay stable.  this
> is one of the problems with a block level (as opposed to file level)
> snapshot.

interestingly, xfs does not show the usage increase as sharply
(IIRC--i stopped checking that number after a few runs), but neither
is the performance better.