On 01/04/11 12:59, Hugo Mills wrote:
On Fri, Apr 01, 2011 at 12:14:50PM +0100, Struan Bartlett wrote:
My company is testing btrfs (kernel 2.6.38) on a slave MySQL
database server with a 195Gb filesystem (of which about 123Gb is
used). So far, we're quite impressed with the performance. Our
database loads are high, and if filesystem performance wasn't good,
MySQL replication wouldn't be able to keep up and the slave latency
would begin to climb. This though, is generally not happening, which
is good.
However, we recently tried running 'btrfs fi balance' on the
filesystem, and found this deteriorated performance significantly,
and the MySQL replication latency did begin to climb. Several hours
later, with the btrfs-cleaner thread apparently still busy, and our
replication latency running to a couple of hours, and no sign of the
balancing operation finishing, we decided we needed to terminate the
balancing operation, which we did by rebooting the server.
That, however, is suboptimal in a production environment, and so
I've some questions.
1) Is the balancing operation expected to take many hours (or days?)
on a filesystem such as this? Or are there known issues with the
algorithm that are yet to be addressed?
A balance rewrites all the data on the filesystem, so it can take a
very long time (I think the longest reported time I've seen from
anyone was 48 hours, on several terabytes of data). However, this will
be highly dependent on the amount of I/O bandwidth available to the
FS, and on the size of the data to be written.
2) Is it supposed to be desirable to run balancing operations
periodically anyway? Our server is running on hardware mirrored
disks, so our btrfs filesystem is simply created in spare space on
the LVM volume group, using a single LV block device. Does balancing
help improve performance/optimise free space in this setup anyway?
Not that I'm aware of, particularly in the light of the recent
patch that frees up unused block groups. Others here may have a more
informed take on this, though.
3) If there's an ioctl for launching a balancing operation, would it
be an idea to add one for pausing a balancing operation? If
balancing may take 'significant' lengths of time, and if it's
intended that balancing be done periodically, it might be helpful if
one could start balancing when loads are lower, and make sure one
can stop them when resources are needed (in our case, when slave
latency exceeds acceptable limits).
There's patches for a cancel operation on the mailing list.
Further, I've got (as yet) unreleased patches for various forms of
partial balance, at least one of which would allow a balance to be
restarted after it was cancelled. The only reason I've not released
them is because I want to do a final check of what I send to the list
to ensure that I'm not making an idiot of myself (and wasting people's
time) with malformed patches. I hope to have time for this on Sunday.
Hugo.
Hugo - thanks very much for your thorough reply. I look forward to being
able to cancel a balancing operation, but in the meantime we simply
won't bother setting any going, and see how things go. So far, our btrfs
slave database has been running two weeks, with a rolling history of
snapshots taken every ten minutes, without any other apparent issues.
Struan
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