On 09/23/10 04:40 PM, Frank Middleton wrote:
Bumping this because no one responded. Could this be because
it's such a stupid question no one wants to stoop to answering it,
or because no one knows the answer? Trying to picture, say, what
could happen in /var (say /var/adm/messages), let alone a swap
zvol, is giving me a headache...
On 07/09/10 17:00, Frank Middleton wrote:
This is a hypothetical question that could actually happen:
Suppose a root pool is a mirror of c0t0d0s0 and c0t1d0s0
and for some reason c0t0d0s0 goes off line, but comes back
on line after a shutdown. The primary boot disk would then
be c0t0d0s0 which would have much older data than c0t1d0s0.
Under normal circumstances ZFS would know that c0t0d0s0
needs to be resilvered. But in this case c0t0d0s0 is the boot
disk. Would ZFS still be able to correctly resilver the correct
disk under these circumstances? I suppose it might depend
on which files, if any, had actually changed...
Booting from the out-of-date disk will fail with the message:
"The boot device is out of date. Please try booting from '%s'"
where a different device is suggested.
The reason for this is that once the kernel has gotten far enough to
import the root pool and read all the labels (and thereby determine that
the device being booted does not have the latest contents), it's too
late to switch over to a different disk for booting. So the next best
action is to fail and let the user explicitly boot from another disk in
the pool (using whatever means the BIOS provides for this, or by
selecting a different disk using the OBP interface on sparc platforms).
Once the system is up, the out-of-date disk will be resilvered.
Ideally, the boot loader should examine all the disks in the root pool
and boot off one with the most recent contents. I believe that there is
a bug filed requesting that this be implemented for sparc platforms.
It's more difficult with x86 platforms (there are issues with accessing
more than one disk from within the current boot loader), and I don't
know what the prospects are for implementing that logic. But in any
case, the boot logic prevents you from unwittingly booting off an
out-of-date disk when a more up-to-date disk is online.
Lori
Thanks -- Frank
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