freebsd stable - Feb 2014 - Separate ZIL on Root

Hi,
I have a FreeBSD 9.2 system with 3 disks in a mirror and I am looking at adding
an SSD as log & cache to improve performance.

However I have found that it will refuse to add a log dev due to..
       /*
        * A root pool with concatenated devices is not supported.
        * Thus, can not add a device to a root pool.
        *
        * Intent log device can not be added to a rootpool because
        * during mountroot, zil is replayed, a seperated log device
        * can not be accessed during the mountroot time.
        *
        * l2cache and spare devices are ok to be added to a rootpool.
        */
       if (spa_bootfs(spa) != 0 && nl2cache == 0 && nspares ==
0) {
               nvlist_free(config);
               spa_close(spa, FTAG);
               return (SET_ERROR(EDOM));
       }

in /usr/src/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ioctl.c

However I don't think this is true in FreeBSD, certainly there are people
working around it, eg..
http://iamsto.wordpress.com/2013/10/04/howto-freebsd-zfs-add-ssd-as-logzil-read-cache-to-root-pool/

I understand that if I lose ZIL on root then I will have a fun time recovering,
although I expect this would entail a site visit and boot off a USB key and then
do  'zpool import -m ...'.

So, does anyone know if there is a real problem or a hold over from Solaris?

Thanks.
--
Daniel O'Connor software and network engineer
for Genesis Software - http://www.gsoft.com.au
"The nice thing about standards is that there
are so many of them to choose from."
  -- Andrew Tanenbaum
GPG Fingerprint - 5596 B766 97C0 0E94 4347 295E E593 DC20 7B3F CE8C






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On Wed, Feb 19, 2014, at 1:19, Daniel O'Connor
wrote:
> 
> So, does anyone know if there is a real problem or a hold over from
> Solaris?
> 
It works just fine but it might not be "recommended". I run it on a
few
boxes, Glen on many. Just remove the bootfs flag from the root zpool,
add the zil, and then add the boot flag back. I think this was more
dangerous when the loss of a zil was a serious matter, but isn't such a
concern anymore. Someone else will probably fill in details or correct
me on these points :-)