to gmirror or to ZFS

Frank Leonhardt frank2 at
Tue Jul 16 10:34:07 UTC 2013

On 16/07/2013 10:41, 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.
I agree with the sentiment of using the SSD as ZFS cache - it's possibly 
the only logical use for them.

I guess that with 100Tb worth of Winchesters you're not on a very tight 
budget, and not too tight on RAM for the OS either. If I was going to do 
this I'd stick with the OS on UFS and a gmirror because I simply don't 
trust ZFS. This is based on pure prejudice and inexperience.

I know how to arrange disks on a UNIX file system for performance - what 
to use for swap, where tmp files should go and so on. I also know where 
every file will be, physically, in the event of trouble. And here's the 
clincher: If the machine blows up I can simply take one of the mirrored 
drives, slap it in to some new hardware and I've got a very reasonable 
chance that it'll boot. Can I do this with ZFS? I get the feeling that 
the answer is an emphatic "maybe".

So all things considered, I'd need a good reason not to stick with what 
I know works reliably and can be recovered in the event of a disaster 
(UFS), but I'm happy to watch and learn from everyone else's experience!

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