zfs on gmirror slice

[Mark Stapper]

Emil Mikulic wrote:
> On Wed, Sep 02, 2009 at 09:20:21AM +0200, Mark Stapper wrote:
>   
>> updating a zfs filesystem which you are running from is next to
>> impossible.
>>     
>
> [citation needed]  :)
>   
Well, to update your zfs filesystem version, the filesystem is first
unmounted, then updated, and then mounted again.
citation coming up!
# umount /
umount: unmount of / failed: Invalid argument
>   
>> So, i would recommend setting up gmirror to mirror your whole disks,
>> install the base system(boot and "world") on a small UFS slice, and use
>> the rest of the disc as zfs slice.
>>     
>
> As Thomas Backman pointed out, this means you won't get self-healing.
>   
self-healing sounds very nice, but with mirrorring you have data on two
discs, so in that case there no "healing" involved, it's just
checksumming and reading the non-corrupted copy.
From the gmirror manpage: "All operations like failure detection, stale
component detection, rebuild of stale components, etc. are also done
automatically."
This would indicate the same functionality, with a much less fancy name.
However, i have not tested it the way they demonstrate zfs's
"self-healing" property.
I might, if I get the time to run it in a virtual machine one of these
days..
> I don't know if a ZFS mirror performs smarter disk access scheduling
> than gmirror.  Someone oughta measure.  ;)
>   
NCQ should help here, but still, very interesting.
gmirror is fast though. I've even heard it's faster than software
RAID1/ataraid. (not confirmed).
Even if this is true, running zfs on top of gmirror probably isn't
faster then running zfs with a mirrored pool.

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