ZFS l2arc and HAST ? newbie question

[Pawel Jakub Dawidek]

On Tue, Jun 15, 2010 at 03:21:45PM +0200, Thomas Steen Rasmussen wrote:
> Hello list,
> 
> I am playing with HAST in order to build some redundant storage
> for a mailserver, using ZFS as the filesystem.
> I have the following zpool layout before stating the HAST experiments:
> 
>         NAME              STATE     READ WRITE CKSUM
>         tank              ONLINE       0     0     0
>           raidz2          ONLINE       0     0     0
>             label/hd4     ONLINE       0     0     0
>             label/hd5     ONLINE       0     0     0
>             label/hd6     ONLINE       0     0     0
>             label/hd7     ONLINE       0     0     0
>         logs              ONLINE       0     0     0
>           mirror          ONLINE       0     0     0
>             label/ssd0s1  ONLINE       0     0     0
>             label/ssd1s1  ONLINE       0     0     0
>         cache
>           label/ssd0s2    ONLINE       0     0     0
>           label/ssd1s2    ONLINE       0     0     0
> 
> As I understand it, to accomplish this with HAST I will need to make a
> HAST resource for each physical disk, like so:
> 
>         NAME              STATE     READ WRITE CKSUM
>         tank              ONLINE       0     0     0
>           raidz2          ONLINE       0     0     0
>             hast/hahd4    ONLINE       0     0     0
>             hast/hahd5    ONLINE       0     0     0
>             hast/hahd6    ONLINE       0     0     0
>             hast/hahd7    ONLINE       0     0     0
> 
> But what about slog and cache devices, currently on SSD disks for
> performance reasons ? It doesn't really make sense to synchronize
> a cache disk over the network, does it ?

No, it doesn't. Cache is forgotten on import anyway, so don't bother.
You have to be careful, though, because you probably need to remove old
cache device from the pool after import on secondary and add local disk.

> Could I build the zpool with the SSD disks directly (without
> HAST) and would ZFS survive an export/import on the other host,
> when the cache disks are suddently different ? I am thinking cache
> only here, not slog.

It simply won't find cache disks, you will need to do what I described
above.

> Do SSD l2arc / slog even make any sense when I am "deliberately"
> slowing down the filsystem with network redundancy anyway ?

Forget about HAST for L2ARC. In case of SLOG it can still be faster over
the network than pool with local SATA disks without SLOG. As usual the
best way to verify this is to test it for your workload:)

> Oh, and is there any problems using labels for HAST devices ? My
> controller likes to give new device names to disks now and then,
> and it has been a blessing to use labels instead of device names,
> so I'd like to continue doing that when using HAST.

Use labeled providers in hast.conf, but there is no need to label HAST
providers (/dev/hast/<name>).

-- 
Pawel Jakub Dawidek                       http://www.wheelsystems.com
pjd at FreeBSD.org                           http://www.FreeBSD.org
FreeBSD committer                         Am I Evil? Yes, I Am!
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