I should also have mentioned that if the pool has a separate log device
then this shouldn't happen.Assuming the slog is big enough then it
it should have enough blocks to not be forced into using main pool
device blocks.
Neil.
On 09/09/10 10:36, Neil Perrin wrote:
Arne,
NFS often demands it's transactions are stable before returning.
This forces ZFS to do the system call synchronously. Usually the
ZIL (code) allocates and writes a new block in the intent log chain to
achieve this.
If ever it fails to allocate a block (of the size requested) it it forced
to close the txg containing the system call. Yes this can be extremely
slow but there is no other option for the ZIL. I'm surprised the wait
is 30 seconds.
I would expect mush less, but finding room for the rest of the txg
data and metadata
would also be a challenge.
Most (maybe all?) file systems perform badly when out of space. I
believe we give a recommended
free size and I thought it was 90%.
Neil.
On 09/09/10 09:00, Arne Jansen wrote:
Hi,
currently I'm trying to debug a very strange phenomenon on a nearly full
pool (96%). Here are the symptoms: over NFS, a find on the pool takes
a very long time, up to 30s (!) for each file. Locally, the performance
is quite normal.
What I found out so far: It seems that every nfs write (rfs3_write)
blocks
until the txg is flushed. This means a write takes up to 30 seconds.
During
this time, the nfs calls block, occupying all NFS server threads.
With all
server threads blocked, all other OPs (LOOKUP, GETATTR, ...) have to
wait
until the writes finish, bringing the performance of the server
effectively
down to zero.
It may be that the trigger for this behavior is around 95%. I managed
to bring
the pool down to 95%, now the writes get served continuously as it
should be.
What is the explanation for this behaviour? Is it intentional and can
the
threshold be tuned? I experienced this on Sol10 U8.
Thanks,
Arne
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