Areca vs. ZFS performance testing.

[Simun Mikecin]

Jeremy Chadwick wrote:
> Well, my understanding (which is probably wrong) is that the memory
> used for the ARC is somehow separate from that of the kmap.  I was
> under the impression the kmap was used by ZFS for other things, and
> did not include ARC.

kmem is used by ARC. You can check your total kmem usage by ZFS using 'vmstat -m' under the line that says 'solaris'.

> People have advocated increasing arc_min and arc_max in the past, citing
> large performance gains as arc_max gets larger; you might see people
> mentioning that they see great performance increases when increasing
> arc_max from 64M to 128M.  My understanding is that increasing the ARC
> provides more actual cached data that ZFS can reference (vs. pulling it
> off disk).  Again, if I'm incorrect, please state so.

You are correct about the benefits of increasing arc_max. I don't know of any benefits of tuning arc_min. Maybe someone else can answer this.

By default on 7-STABLE arc_max will be 3/4 of kmem_size. So if you are using 1536M for kmem_size, arc_max will be 1152M by default. But some people will maybe need to lower it to avoid panic during heavy I/O since in those scenarios ARC cache size could for short periods of time be larger than arc_max and reach kmem limit.