ZFS-inly server and dedicated ZIL

[Charles Sprickman]

On Apr 10, 2013, at 9:23 AM, Dmitry Morozovsky wrote:

> Dear colleagues,
> 
> I'm planning to make new PostgreSQL server using zaid10-like ZFS with two SSDs 
> splitted into mirrored ZIL and striped arc2.

This might seem like an odd suggestion, but if you're putting the pool on SSDs (is that correct?), I'd totally skip the separate ZIL device and ARC.  I think you'll find the SSDs will need zero help from another log device and L2ARC is probably just not that helpful for DB loads.

> However, it seems current 
> ZFS implementation does not support this:
> 
> ./lib/libzfs/common/libzfs_pool.c-		case EDOM:
> ./lib/libzfs/common/libzfs_pool.c-			zfs_error_aux(hdl, dgettext(TEXT_DOMAIN,
> ./lib/libzfs/common/libzfs_pool.c:			    "root pool can not have multiple vdevs"
> ./lib/libzfs/common/libzfs_pool.c-			    " or separate logs"));
> ./lib/libzfs/common/libzfs_pool.c-			(void) zfs_error(hdl, EZFS_POOL_NOTSUP, msg);
> 
> Am I right, or did I missed something obvious?

I've asked about this on this very list some time ago, but no one really had any answers:

http://lists.freebsd.org/pipermail/freebsd-fs/2012-September/015142.html

The last post in that thread brings up an interesting point that was not answered, which is can our zfs boot loader handle the ZIL playback on boot?  I would assume so (regardless of where the ZIL device lives), but who knows?  Yet another ZFS mystery. :)

To summarize, yes, you can work around the root pool restriction, which is supposedly a Solaris thing that got carried over.  You do this by unsetting the "bootfs" property on the pool (ie: "zpool set bootfs='' poolname"), adding your log devices to the pool, and then setting the bootfs property again.  Works for me.

Someone noted this only works for mirrors, but I've done it on raidz pools as well.

It would be great to have someone weigh-in on whether this is valid or not.

The blog post most people reference regarding this issue is here:

http://astralblue.livejournal.com/371755.html

Charles

> 
> Ok, if so: In this situation, I see two possibilities:
> - make system boot from internal USB stick (only /bootdisk with /boot and 
> /rescue) with the rest of ZFS-on-root
> - use dedicated pair of disks for ZFS pool without ZIL for system.
> 
> what would you recommend?
> 
> Thanks!
> 
> -- 
> Sincerely,
> D.Marck                                     [DM5020, MCK-RIPE, DM3-RIPN]
> [ FreeBSD committer:                                 marck at FreeBSD.org ]
> ------------------------------------------------------------------------
> *** Dmitry Morozovsky --- D.Marck --- Wild Woozle --- marck at rinet.ru ***
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