On 09/04/09 09:54, Scott Meilicke wrote:
Roch Bourbonnais Wrote:
""100% random writes produce around 200 IOPS with a 4-6 second pause
around every 10 seconds. "
This indicates that the bandwidth you're able to transfer
through the protocol is about 50% greater than the bandwidth
the pool can offer to ZFS. Since, this is is not sustainable, you
see here ZFS trying to balance the 2 numbers."
When I have tested using 50% reads, 60% random using iometer over NFS,
I can see the data going straight to disk due to the sync nature of NFS.
But I also see writes coming to a stand still every 10 seconds or so,
which I have attributed to the ZIL dumping to disk. Therefore I conclude
that it is the process of dumping the ZIL to disk that (mostly?) blocks
writes during the dumping.
The ZIL does does not work like that. It is not a journal.
Under a typical write load write transactions are batched and
written out in a group transaction (txg). This txg sync occurs
every 30s under light load but more frequently or continuously
under heavy load.
When writing synchronous data (eg NFS) the transactions get written immediately
to the intent log and are made stable. When the txg later commits the
intent log blocks containing those committed transactions can be
freed. So as you can see there is no periodic dumping of
the ZIL to disk. What you are probably observing is the periodic txg
commit.
Hope that helps: Neil.
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