Optimizing performance with SLOG/L2ARC

[Daniel Kalchev]

> On 19.08.2015 г., at 3:28, javocado <javocado at gmail.com> wrote:
> 
> 
> 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?
> 

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’s own drive anyway, normally, it’s write-only and besides latency, a normal HDD should do as well.

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

Daniel