Optimizing performance with SLOG/L2ARC

[Bob Friesenhahn]

On Tue, 18 Aug 2015, javocado wrote:

> I've been trying to optimize and enhance my ZFS filesystem performance
> (running FreeBSD 8.3amd) which has been sluggish at times. Thus far I have
> added RAM (256GB) and I've added an SLOG (SSD mirror). The RAM seems to
> have helped a bit, but not sure if the SLOG was of much help. My vdev is
> decently busy, with writes and reads averaging at 100 per second with
> spikes as high as 500.

Lots of interesting questions.  You have not told us the use case for 
your system, or the zpool layout.

Your comment about 'vdev is decently busy' causes me to think that 
perhaps you have just one and that more vdevs will (at least) linearly 
improve over-all performance.

> 1. based on the output above, I believe a larger ARC may not necessarily
> benefit me at this point. True?

It looks like your ARC is doing well.

> 2. Is more (L2)ARC always better?

No.  If (L2)ARC ends up empty, then it is wasted.

> 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?

That is my understanding.  Everything in the (L2)ARC is also on the 
pool disks.

Look into re-architecting your pool.  It is not clear what type of 
reads/writes are taking place but if these are random access to pool 
disks and you are using a raidzN organization, then you may be 
bottlenecked on disk write I/Os.  This is the important thing to 
determine.  If you were bottlenecked on async disk write I/Os and your 
slog is relatively idle, then you may benefit most from more vdevs 
and possibly more disks.  Decreasing read I/Os will help, but can only 
go so far.

Use mirrors if you can afford it.

Bob
-- 
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/