>>>>> "mb" == Matt Beebe <[EMAIL PROTECTED]> writes:
mb> Anyone know of a SATA and/or SAS HBA with battery backed write
mb> cache?
I've never heard of a battery that's used for anything but RAID
features. It's an interesting question, if you use the controller in
``JBOD mode'' will it use the write cache or not? I would guess not,
but it might. And if it doesn't, can you force it, even by doing
sneaky things like making 2-disk mirrors where 1 disk happens to be
missing thus wasting half the ports you bought, but turning on the
damned write cache? I don't know.
The alternative is to get a battery-backed SATA slog like the gigabyte
iram. However, beware, because once you add a slog to a pool, you can
never remove it. You can't improt the pool without the slog, not even
DEGRADED, not even if you want ZFS to pretend the slog is empty, not
even if the slog actually was empty. IIRC (might be confused) Ross
found the pool will mount at boot without the slog if it's listed in
zpool.cache (why? don't know, but I think he said it does), but once
you export the pool there is no way to get it back into zpool.cache
since zpool.cache is a secret binary config file. Can you substitute
any empty device for the missing slog? nope---the slog has secret
binary header label on it.
I'm guessing one of the reasons you wanted a non-RAID controller with
a write cache was so that if the controller failed, and the exact same
model wasn't available to replace it, most of your pool would still be
readable with any random controller, modulo risk of corruption from
the lost write cache. so...with the slog, you don't have that,
because there are magic irreplaceable bits stored on the slog without
which your whole pool is useless.
bash-3.00# zpool import -d /usr/vdev
pool: slogtest
id: 11808644862621052048
state: ONLINE
action: The pool can be imported using its name or numeric identifier.
config:
slogtest ONLINE
mirror ONLINE
/usr/vdev/d0 ONLINE
/usr/vdev/d1 ONLINE
logs
slogtest ONLINE
/usr/vdev/slog ONLINE
bash-3.00# mv vdev/slog .
bash-3.00# zpool import -d /usr/vdev
pool: slogtest
id: 11808644862621052048
state: FAULTED
status: One or more devices are missing from the system.
action: The pool cannot be imported. Attach the missing
devices and try again.
see: http://www.sun.com/msg/ZFS-8000-6X
config:
slogtest UNAVAIL missing device
mirror ONLINE
/usr/vdev/d0 ONLINE
/usr/vdev/d1 ONLINE
Additional devices are known to be part of this pool, though their
exact configuration cannot be determined.
bash-3.00#
damn. ``no user-serviceable parts inside.'' however, if you were
sneaky enough to save a backup copy of your empty slog to get around
Solaris's obtinence, maybe you can proceed:
bash-3.00# gzip slog <-- save a copy of the exported
empty slog
bash-3.00# ls -l slog.gz
-rw-r--r-- 1 root root 106209 Sep 3 16:17 slog.gz
bash-3.00# gunzip < slog.gz > vdev/slog
bash-3.00# zpool import -d /usr/vdev
pool: slogtest
id: 11808644862621052048
state: ONLINE
action: The pool can be imported using its name or numeric identifier.
config:
slogtest ONLINE
mirror ONLINE
/usr/vdev/d0 ONLINE
/usr/vdev/d1 ONLINE
logs
slogtest ONLINE
/usr/vdev/slog ONLINE
bash-3.00# zpool import -d /usr/vdev slogtest
bash-3.00# pax -rwpe /usr/sfw/bin /slogtest
^C
bash-3.00# zpool export slogtest
bash-3.00# gunzip < slog.gz > vdev/slog <-- wipe the slog
bash-3.00# zpool import -d /usr/vdev slogtest
bash-3.00# zfs list -r slogtest
NAME USED AVAIL REFER MOUNTPOINT
slogtest 18.1M 25.4M 17.9M /slogtest
bash-3.00# zpool scrub slogtest
bash-3.00# zpool status slogtest
pool: slogtest
state: ONLINE
scrub: scrub completed with 0 errors on Wed Sep 3 16:23:44 2008
config:
NAME STATE READ WRITE CKSUM
slogtest ONLINE 0 0 0
mirror ONLINE 0 0 0
/usr/vdev/d0 ONLINE 0 0 0
/usr/vdev/d1 ONLINE 0 0 0
logs ONLINE 0 0 0
/usr/vdev/slog ONLINE 0 0 0
errors: No known data errors
bash-3.00#
I'm not sure this will always work, because there probably wasn't
anything in the slog when I wiped it. But I guess it's better than
``restore your pool from backup'' because of the pedantry of some
wallpaper tool and brittle windows-registry-style binary config files.
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