RAM and zfs and multiple disks

[Tom Evans]

On Thu, Oct 10, 2013 at 6:07 PM, John <freebsd-lists at potato.growveg.org> wrote:
> Hello list,
>
> I'd like to have zfs on my freebsd desktop. However,
> this motherboard can take 8GB RAM, max.
>
> I'd like to get 2 x4Tb drives. Realistically, do I
> need another motherboard? The primary reason to have ZFS
> is to guard against bitrot.
>

My home ZFS file server is a Core2 Duo with 8GB of RAM and has 12 x
1.5TB arranged in 2 x 6 disk raidz, and before the second set of disks
were added it ran with only 2 GB just fine.

The only ZFS setting I "had" to tweak was to limit ARC to 4 GB, in
order to have plenty of free RAM:
   vfs.zfs.arc_max="4294967296"

I say "had to", because I didn't really, I just prefer not having the
ARC taking all free RAM and releasing as necessary.

Depending upon your usage, you may find that the ARC is not terribly
useful anyway, eg in my case there is 4 GB of ARC covering 14336 GB of
data, and since my data is mainly archived binary data, I'm unlikely
to re-use the same data twice in one day. Where the ARC really comes
in handy for me is prefetch, loading large files into the ARC as I
start reading them. Prefetching is disabled if you have less than 4 GB
of RAM.

Cheers

Tom