Erik Trimble wrote:
Since the best way to get this is to use a Mirror or RAIDZ vdev, I'm
assuming that the proper way to get benefits from both ZFS and HW RAID
is the following:
(1) ZFS mirror of HW stripes, i.e. "zpool create tank mirror
hwStripe1 hwStripe2"
(2) ZFS RAIDZ of HW mirrors, i.e. "zpool create tank raidz hwMirror1,
hwMirror2"
(3) ZFS RAIDZ of HW stripes, i.e. "zpool create tank raidz hwStripe1,
hwStripe2"
mirrors of mirrors and raidz of raid5 is also possible, but I'm pretty
sure they're considerably less useful than the 3 above.
Personally, I can't think of a good reason to use ZFS with HW RAID5;
case (3) above seems to me to provide better performance with roughly
the same amount of redundancy (not quite true, but close).
I almost regret extending this thread more :-) but I havent seen anyone
spell out one thing in simple language, so i'll attempt to do that now.
#2 is incredibly wasteful of space, so I'm not going to address it. it is
highly redundant, that's great. if you need it, do it. I'm more concerned
with the concept of
zfs of two hardware raid boxes that have internal disk redundancy
vs
zfs of two hardware raid boxes that are pure stripes (raid 0)
(doesnt matter if using zfs mirror vs raidz to me, for this aspect of things)
The point that I think people should remember, is that if you lose a drive
in a pure raid0 configuration... your time to recover that hwraid unit and
bring it back to full operation in the filesystem.. is HUGE.
It will most likely be unacceptibly long. hours if not days, for a decent
sized raid box.
So, you can choose to throw away half your disk space in that hwraid box for
redundancy, or use raid5.
raid5 IS useful in zfs+hwraid boxes, for "Mean Time To Recover" purposes.
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