to gmirror or to ZFS

Warren Block wblock at
Tue Jul 16 18:42:15 UTC 2013

On Tue, 16 Jul 2013, aurfalien wrote:
> On Jul 16, 2013, at 2:41 AM, Shane Ambler wrote:
>> I doubt that you would save any ram having the os on a non-zfs drive as
>> you will already be using zfs chances are that non-zfs drives would only
>> increase ram usage by adding a second cache. zfs uses it's own cache
>> system and isn't going to share it's cache with other system managed
>> drives. I'm not actually certain if the system cache still sits above
>> zfs cache or not, I think I read it bypasses the traditional drive cache.
>> For zfs cache you can set the max usage by adjusting vfs.zfs.arc_max
>> that is a system wide setting and isn't going to increase if you have
>> two zpools.
>> Tip: set the arc_max value - by default zfs will use all physical ram
>> for cache, set it to be sure you have enough ram left for any services
>> you want running.
>> Have you considered using one or both SSD drives with zfs? They can be
>> added as cache or log devices to help performance.
>> See man zpool under Intent Log and Cache Devices.
> This is a very interesting point.
> In terms if SSDs for cache, I was planning on using a pair of Samsung Pro 512GB SSDs for this purpose (which I haven't bought yet).
> But I tire of buying stuff, so I have a pair of 40GB Intel SSDs for use as sys disks and several Intel 160GB SSDs lying around that I can combine with the existing 256GB SSDs for a cache.
> Then use my 36x3TB for the beasty NAS.

Agreed that 256G mirrored SSDs are kind of wasted as system drives.  The 
40G mirror sounds ideal.

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