>>>>> "np" == Niall Power <niall.po...@sun.com> writes:
np> So I'd like to ask if this is an appropriate use of ZFS mirror
np> functionality?
I like it a lot.
I tried to set up something like that ad-hoc using a firewire disk on
an Ultra10 at first, and then, just as you thought, tried using one
firewire disk and one iSCSI disk to make the mirror. It was before
ZFS boot, so I mirrored /usr and /var only with ZFS, and / with SVM
(internal 2GB scsi to firewire). I was trying to get around the 128GB
PATA limitation in the Ultra 10. It was a lot of sillyness, but it
was still useful even though I ran into a lot of bugs that have been
fixed since I was trying it. The stuff you successfully tested
explores a lot of the problem areas I had---hangs on disconnecting,
incomplete resilvering, both sound fixed---but iSCSI still does not
work well because the system will ``patiently wait'' forever during
boot for an absent iSCSI target. On SPARC neither firewire nor iscsi
was bootable back then, so you're in a much better spot there too than
I was with only a single bootable SVM component and a lot of painful
manual rescue work to do if that failed.
From reading the list you might be able to do something similar with
the storagetek AVS/ii/geo-cluster stuff, but I haven't tried it and
remember some problem with running it on localhost---I think you need
two machines, just because of UI limitations. It might resilver
faster than ZFS though, and it's always a Plan B if you run into a
show-stopper. Also (if it worked at all) it solves the
slower-performacne-while-connected problem.
In the long run some USB stick problems may surface because the wear
leveling is done in 16MB sections, and you could blow your stick if
you have a 16MB region which is ``hot''. I wonder if parts of a zpool
are hotter than others? With AVS the dirty bitmap might be hot.
I guess you are not erally imagining sticks though, just testing with
them. You're imagining something more like the time capsule, where
the external drive is bigger than the internal one, that it'll be used
more on laptops. At home you keep a large, heavy disk which holds a
mirror of your laptop ZFS root on one slice, plus an unredundant
scratch pool made of the extra free space.
Finally, I still don't understand the ZFS quorum rules. What happens
if you:
(1) boot the internal disk, change some stuff, shut down.
(2) Then boot the USB-stick/big-home-disk, change some stuff, shut down.
(3) Then boot with both disks.
corruption or successful scrub? Which changes survive? because
people WILL do that. some will not even remember that they did it,
will even lie and deny it.
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