lonny wrote:
On May 11, 2007, at 9:09 AM, Bob Netherton wrote:
**On Fri, 2007-05-11 at 09:00 -0700, lonny wrote:
**I've noticed a similar behavior in my writes. ZFS seems to write in bursts of
** around 5 seconds. I assume it's just something to do with caching?
^Yep - the ZFS equivalent of fsflush. Runs more often so the pipes don't
^get as clogged. We've had lots of rain here recently, so I'm sort of
^sensitive to stories of clogged pipes.
^
**Is this behavior ok? seems it would be better to have the disks writing
** the whole time instead of in bursts.
^
^Perhaps - although not in all cases (probably not in most cases).
^Wouldn't it be cool to actually do some nice sequential writes to
^the sweet spot of the disk bandwidth curve, but not depend on it
^so much that a single random I/O here and there throws you for
^a loop ?
^
^Human analogy - it's often more wise to work smarter than harder :-)
^
^Directly to your question - are you seeing any anomalies in file
^system read or write performance (bandwidth or latency) ?
^Bob
No performance problems so far, the thumper and zfs seem to handle everything
we throw at them. On the T2000 internal disks we were seeing a bottleneck when
using a single disk for our apps but moving to a 3 disk raidz alleviated that.
The only issue is when using iostat commands the bursts make it a little harder
to gauge performance. Is it safe to assume that if those bursts were to reach
the upper performance limit that it would spread the writes out a bit more?
The burst of activity every 5 seconds is when the transaction group is
committed.
Batching up the writes in this way can lead to a number of efficiencies (as Bob
hinted).
With heavier activity the writes will not get spread out, but will just takes
longer.
Another way to look at the gaps of IO inactivity is that they indicate
underutilisation.
Neil.
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