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

[Joe Pruett]

>> 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.