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

[chris (fool) mccraw]

On Mon, Nov 15, 2010 at 15:51, Stuart D. Gathman <stuart at bmsi.com> wrote:
> On Mon, 15 Nov 2010, chris (fool) mccraw 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.
>
> Are you writing to the snapshot or the origin?  If writing to the
> snapshot, and if your snap% is stable, then you are getting the addition seek
> time to jump over to the COW for those sectors.

I've never written to the snapshot, or even mounted it except to prove
that i could...


> Once the COW has the copy of the original data for a chunk, then reads/writes
> to that chunk on the origin should be identical to reads/writes without the
> snapshot, except for some minor CPU overhead.

i don't doubt that, but i don't even want to write to a snapshot.  i
want to write to the primary, and only occasionally even read from the
snapshot.  it is the writes to the primary that i have been
documenting.


> Another possibility is that while the snap% is not visibly increasing, you
> are in fact updating new areas with each test.

and it's also possible i've been misremembering about the snap% not
growing.  in fact in a quick test, it did grow with each rewrite.
perhaps i am remembering a test on an ext3 parent, whereas i am now
using xfs.