On Friday, January 25, 2013 06:18 AM, Edward Ned Harvey (openindiana) wrote:
When you're doing async writes (or sync writes with ZIL disabled, which are in
effect, async writes) then the HBA write-back actually hurts your performance a
little bit. Like 5-10% or so.
The conclusion to draw is:
(a) The absolute fastest thing you can do is use a dumb controller, and
sync=disabled. This maximizes performance for both sync & async writes. But
only if it's safe for you to run in that configuration. Otherwise ...
(b) The second fastest thing you can do is use a dumb controller, and a dedicated
log device. This again maximizes performance for both sync & async, but now
it's safe for whatever your usage patterns.
(c) The third fastest thing is to have a smart HBA with NV write-back. This hurts
async write performance slightly, but the sync write performance is far better than
the dumb & naked bare-bones solution.
(d) The dumb & naked bare-bones solution is to use a dumb controller, with ZIL
enabled (which is default), and no special log device. Your async writes are
optimized, but your sync writes are a dog, which tend to also drag your async
writes into the gutter with them. Believe it or not, dedicating a HDD slog to a
HDD pool actually improves performance signfiicantly just because you're able to
isolate the sync writes to a device while the async can still hit the pool without
interference. But this is kind of a dumb configuration, so we don't talk about it
much except for entertainment purposes. ;-)
So basically, no matter what you do, you're not going to significantly impact
the async write performance. The sync performance is the only thing you have
to think about optimizing.
The performance delta from (d) to (c) is approximately proportional to (c) vs
(b), which is approximately proportional to (b) vs (a). A reasonable ballpark
is to say approx 1.75x to 2x jump on each of those deltas.
Long story short: This was all a response to the intended-to-be-tangential comment "Who uses
hardware raid anymore now that we have zfs?" And the answer is, "Anybody who has an HBA,
doesn't have an SSD, and can't or won't disable the ZIL." This is a surprisingly high number
of people, because people previously bought (and continue to buy) servers that have HBA's, which
later get provisioned to run zfs. And then lots of times, they don't know or can't spring for the
slog solution.
:-D I'm here to entertain since I have not been able to spring for a ssd
for use as a slog. :-D
Thanks Edward.
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