2011-06-19 3:47, Richard Elling пишет:
On Jun 16, 2011, at 8:05 PM, Daniel Carosone wrote:
On Thu, Jun 16, 2011 at 10:40:25PM -0400, Edward Ned Harvey wrote:
From: Daniel Carosone [mailto:d...@geek.com.au]
Sent: Thursday, June 16, 2011 10:27 PM
Is it still the case, as it once was, that allocating anything other
than whole disks as vdevs forces NCQ / write cache off on the drive
(either or both, forget which, guess write cache)?
I will only say, that regardless of whether or not that is or ever was true,
I believe it's entirely irrelevant. Because your system performs read and
write caching and buffering in ram, the tiny little ram on the disk can't
possibly contribute anything.
I disagree. It can vastly help improve the IOPS of the disk and keep
the channel open for more transactions while one is in progress.
Otherwise, the channel is idle, blocked on command completion, while
the heads seek.
Actually, all of the data I've gathered recently shows that the number of
IOPS does not significantly increase for HDDs running random workloads.
However the response time does :-( My data is leading me to want to restrict
the queue depth to 1 or 2 for HDDs.
SDDs are another story, they scale much better in the response time and
IOPS vs queue depth analysis.
Now, is there going to be a tunable which would allow us to set
queue depths per-device? Or tunables are so evil that you'd
"rather poke an eye your with a stick"? (C) Richard Elling ;)
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