Robert Milkowski wrote:
Hello Mark,
Monday, September 11, 2006, 4:25:40 PM, you wrote:
MM> Jeremy Teo wrote:
Hello,
how are writes distributed as the free space within a pool reaches a
very small percentage?
I understand that when free space is available, ZFS will batch writes
and then issue them in sequential order, maximising write bandwidth.
When free space reaches a minimum, what happens?
Thanks! :)
MM> Just what you would expect to happen:
MM> As contiguous write space becomes unavailable, writes will be come
MM> scattered and performance will degrade. More importantly: at this
MM> point ZFS will begin to heavily write-throttle applications in order
MM> to ensure that there is sufficient space on disk for the writes to
MM> complete. This means that there will be less writes to batch up
MM> in each transaction group for contiguous IO anyway.
MM> As with any file system, performance will tend to degrade at the
MM> limits. ZFS keeps a small overhead reserve (much like other file
MM> systems) to help mitigate this, but you will definitely see an
MM> impact.
I hope it won't be a problem if space is getting low i a file system
with quota set however in a pool the file system is in there's plenty
of space, right?
If you are running close to your quota, there will be a little bit of
performance degradation, but not to the same degree as when running low
on free space in the pool. The reason performance degrades when you're
near your quota is that we aren't exactly sure how much space will be
used until we actually get around to writing it out (due to compression,
snapshots, etc). So we have to write things out in smaller batches (ie.
flush out transaction groups more frequently than is optimal).
--matt
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