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?
As Matt mentioned, there is some small impact in this scenario: the
file system that is near its quota can only place a small amount of
data in each transaction group; so it will try to "hurry" the
transaction groups along by asking that they be "closed early". This
may result in instances where a transaction group will be smaller
than it otherwise would be.
In general, the filesystem that is near its quota *will* see slower
performance, and the rest of the pool *may* sometimes see slightly lower
performance.
-Mark
_______________________________________________
zfs-discuss mailing list
zfs-discuss@opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
