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! :)
Just what you would expect to happen:
As contiguous write space becomes unavailable, writes will be come
scattered and performance will degrade. More importantly: at this
point ZFS will begin to heavily write-throttle applications in order
to ensure that there is sufficient space on disk for the writes to
complete. This means that there will be less writes to batch up
in each transaction group for contiguous IO anyway.
As with any file system, performance will tend to degrade at the
limits. ZFS keeps a small overhead reserve (much like other file
systems) to help mitigate this, but you will definitely see an
impact.
-Mark
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