Re: Postgresql 9.4 and ZFS?

On 10/1/15 8:50 AM, Joseph Kregloh wrote:
> In my testing with pgbench I actually saw a decrease in performance with
> a ZIL enabled. I ended up just keeping the L2ARC and dropping the. ZIL
> will not provide you with any speed boost as a database. On a NAS with
> NFS shared for example, a ZIL would work well. ZIL is more for data
> protection than anything.
>
> I run in Production FreeBSD 10.1 with an NVMe mirror for L2ARC, the rest
> of the storage is spinning drives. With a combination of filesystem
> compressions. For example, archival tablespaces and the log folder are
> on gzip compression on an external array. Faster stuff like the xlog are
> lz4 and on an internal array.

I'm not a ZFS expert, but my understanding is that a ZIL *that has lower
latency than main storage* can be a performance win. This is similar to
the idea of giving pg_xlog it's own dedicated volume so that it's not
competing with all the other IO traffic every time you do a COMMIT.

Recent versions of Postgres go to a lot of trouble to make fsync as
painless as possible, so a ZIL might not help much in many cases. Where
it could still help is if you're running synchronous_commit = true and
you consistently get lower latency on the ZIL than on the vdev's; that
will make every COMMIT run faster.

(BTW, this is all based on the assumption that ZFS treats fsync as a
synchronous request.)
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com