On Thu, 27 Aug 2009, David Bond wrote:
I just noticed that if the server hasnt hit its target arc size, the
pauses are for maybe .5 seconds, but as soon as it hits its arc
target, the iops drop to around 50% of what it was and then there
are the longer pauses around 4-5 seconds. and then after every pause
the performance slows even more. So it appears it is definately
server side.
This is known behavior of zfs for asynchronous writes. Recent zfs
defers/aggregates writes up to one of these limits:
* 7/8ths of available RAM
* 5 seconds worth of write I/O (full speed write)
* 30 seconds aggregation time
Notice the 5 seconds. This 5 seconds results in the 4-6 second pause
and it seems that the aggregation time is 10 seconds on your system
with this write load. Systems with large amounts of RAM encounter
this issue more than systems with limited RAM.
I encountered the same problem so I put this in /etc/system:
* Set ZFS maximum TXG group size to 3932160000
set zfs:zfs_write_limit_override = 0xea600000
By limiting the TXG group size, the size of the data burst is limited,
but since zfs still writes the TXG as fast as it can, other I/O will
cease during that time.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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