Date: Wed, 19 Aug 2015 12:10:39 -0400 From: PK1048 <paul@pk1048.com> To: FreeBSD Filesystems <freebsd-fs@freebsd.org> Cc: javocado <javocado@gmail.com>, "Schweiss, Chip" <chip@innovates.com> Subject: Re: Optimizing performance with SLOG/L2ARC Message-ID: <3FE10173-656C-4744-AB2D-32148A34CB46@pk1048.com> In-Reply-To: <CALeZrrRex43B6N7GkTtaP36v5q7mQF8U9V1bU2W1%2B2uh8fhhiQ@mail.gmail.com> References: <CAP1HOmTidC3%2BG4XfhvkQxieo%2BSYMq-JWiXF9Cs4FSW2VqkktWA@mail.gmail.com> <023F881D-CCC5-4FCA-B09D-EB92C3BFBC03@pk1048.com> <CALeZrrRex43B6N7GkTtaP36v5q7mQF8U9V1bU2W1%2B2uh8fhhiQ@mail.gmail.com>
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On Aug 19, 2015, at 12:02, Schweiss, Chip <chip@innovates.com> wrote: > On Wed, Aug 19, 2015 at 10:29 AM, PK1048 <paul@pk1048.com> wrote: >=20 >>=20 >> Please note that, depending on your workload, an SSD may _not_ be any >> faster than a HDD. I am in the process of rebuilding a file server = that >> exhibited poor NFS SYNC write performance. Yet it had a mirrored pair = of >> SSDs. Unfortunately, those SSDs had _worse_ write performance than an = HDD >> for small (4 KB) writes. Based on recommendations from the OpenZFS = list I >> have a pair of Intel 3710 SSDs coming in to try, they are supposed to = have >> much better write performance (at all block sizes) and much better >> reliability long term (10x full disk writes per day for 5 years). = I=92ll know >> more once they arrive and I can test with them. >>=20 >=20 > Pure SSD pools still need a log device. Sorry I was unclear, I was NOT suggesting a pure SSD pool. > ZFS doesn't play well with the > ZIL on the pool with SSDs. Even an SSD of the same type as the pool > devices as the log device will fix the latency problem and throughput > problems. If your load is sync writes then you decidedly want a LOG device, even = if it is the same type as the devices in the pool. For the reasons = others have posted. > It seems counter-intuitive but a very real problem, there is a long = thread > about this on the Illumos ZFS list. If you don't believe it, turn off = sync > on your SSD pool and performance will skyrocket. But remember to turn it back on after you test so that you don=92t break = posix sync behavior and raise the possibility of loosing writes in = flight. I am horrified at the number of posts on the Internet that tell = you to simply disable sync to fix sync performance issues (VM images = accessed via NFS being a very common one).
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