Booting to root on gmirror with disk failure,
is it even possible?
tobi at casino.uni-stuttgart.de
Wed Sep 5 04:56:51 PDT 2007
> Before I invest significantly more time into my current gmirror
> issues, I have but two simple questions for anyone out there:
> 1. Has anyone used gmirror for the root partition and been able to
> successfully boot with one failed (or un-plugged) disk? It's the
> latter part of the question that is the real issue for me. I'm just
> looking for a confirmed "it's possible".
Yes, it is possible. IBM xSeries 346, FreeBSD 6.2-RELEASE, amd64. U360
hard drives. More specs are available from IBM. Using gmirror because we
only have an Adaptec "HostRAID" (aka "FakeRAID") controller and not a
real "ServerRaid", i.e. our SCSI controller basically has no useful RAID
capabilities built in.
My test case is to unplug any one disk while the system is running.
(Don't do this with your system unless your hardware is specified for
hot plugging!). FreeBSD detects a bus reset, marks the gmirror as
degraded and continues operating normally, and I can also reboot the
degraded gmirror without any problems.
The more conservative test case is to power down the system, unplug any
one disk, and restart the system. No problems with that either.
In fact, the absolutely robust behaviour of gmirror was one of my key
arguments for switching from Linux to FreeBSD :-).
Of course there are a zillion ways to fail your hard disk, and there
could be cases where one hard disk might start behaving erratically, and
gmirror might not be able to detect all such cases and might try to
continue using the failed disk. This could theoretically lead to some
nasty data integrity issues in the worst case. But this is true for any
RAID, even when implemented in hardware IMO.
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