to gmirror or to ZFS

aurfalien aurfalien at
Tue Jul 16 17:49:47 UTC 2013

On Jul 16, 2013, at 2:41 AM, Shane Ambler wrote:

> On 16/07/2013 14:41, aurfalien wrote:
>> On Jul 15, 2013, at 9:23 PM, Warren Block wrote:
>>> On Mon, 15 Jul 2013, aurfalien wrote:
>>>> ... thats the question :)
>>>> At any rate, I'm building a rather large 100+TB NAS using ZFS.
>>>> However for my OS, should I also ZFS or simply gmirror as I've a
>>>> dedicated pair of 256GB SSD drives for it.  I didn't ask for SSD
>>>> sys drives, this system just came with em.
>>>> This is more of a best practices q.
>>> ZFS has data integrity checking, gmirror has low RAM overhead.
>>> gmirror is, at present, restricted to MBR partitioning due to
>>> metadata conflicts with GPT, so 2TB is the maximum size.
>>> Best practices... depends on your use.  gmirror for the system
>>> leaves more RAM for ZFS.
>> Perfect, thanks Warren.
>> Just what I was looking for.
> 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.

- aurf

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