Date: Fri, 31 Oct 2008 04:57:18 -0700 (PDT) From: Simun Mikecin <numisemis@yahoo.com> To: Jeremy Chadwick <koitsu@FreeBSD.org> Cc: freebsd-fs@freebsd.org, Danny Carroll <fbsd@dannysplace.net>, freebsd-hardware@freebsd.org Subject: Re: Areca vs. ZFS performance testing. Message-ID: <174490.95560.qm@web36607.mail.mud.yahoo.com>
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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.
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