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Subject: Re: Optimizing performance with SLOG/L2ARC
From: Daniel Kalchev <daniel@digsys.bg>
In-Reply-To: <CAP1HOmTidC3+G4XfhvkQxieo+SYMq-JWiXF9Cs4FSW2VqkktWA@mail.gmail.com>
Date: Wed, 19 Aug 2015 15:11:49 +0300
Cc: FreeBSD Filesystems <freebsd-fs@freebsd.org>
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> On 19.08.2015 =D0=B3., at 3:28, javocado <javocado@gmail.com> wrote:
>=20
>=20
> 3. I know it's a good idea to mirror the SLOG (and I have). Do I =
understand
> correctly that I do not need to mirror the L2ARC since it's just a =
read
> cache, nothing to lose if the SSD goes down?
>=20

There is a little known and grossly underestimated benefit of using =
SLOG, even if it is not on SSD: less fragmentation in the pool.

This is because, without SLOG ZFS will allocate the intent log records =
from blocks in the pool. then free those blocks. These are all small =
writes and leave behind a lot of holes.

SLOG should be on it=E2=80=99s own drive anyway, normally, it=E2=80=99s =
write-only and besides latency, a normal HDD should do as well.

Of course, everything needs to be tested against the expected workload.

Daniel