On Wed, 2007-09-26 at 07:22 -0400, Jonathan Edwards wrote:
> the bottom line is that there's 2 competing cache
> strategies that aren't very complimentary.
To put it differently, technologies like ZFS change the optimal way to
build systems.
The ARC exists to speed up reads, and needs to be large, low latency,
and can be (and usually is) stored in volatile memory.
A separate intent log exists to speed up synchronous writes, must be
nonvolatile, doesn't need to be all that large, and provides some
performance benefit if it's faster than the typical disk in the pool.
I wouldn't buy something like a 3510-raid new specifically to use as a
dedicated intent log device, but I had some surplus equipment fall in my
lap (including a 3510 jbod chassis with no disks in it, and 3510-raid
and 3510-jbods with varying sizes of disks) and looked at the optimal
way to use the pile of parts I had on hand.
With the particular pile of parts I have on hand, dedicating a
partly-populated 3510 to intent log storage "wastes" no more than about
7% of capacity in return for what looks like a 30-40% reduction in the
wall-clock elapsed time of some NFS-write intensive jobs.
Your mileage will vary.
- Bill
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