Date: Tue, 19 Oct 2010 15:52:52 +0100 From: Karl Pielorz <kpielorz_lst@tdx.co.uk> To: freebsd-fs@freebsd.org Subject: ZFS 'read-only' device / pool scan / import? Message-ID: <AE519076FDEA1259C5DEA689@HexaDeca64.dmpriest.net.uk>
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Hi,
On FreeBSD if I bring the system up 'single user' - the first time I do
(for example):
"
zpool status
"
There's a pause, a flurry of disk activity - and the system appears to
import any pools it finds, and the status appears.
I'd guess at this point - some data is written to the disks? - Is there any
way of avoiding that, i.e. a kind of "If you were to import the
pools/display a status, what would you show, without actually writing any
data?" (or importing it) - i.e. to ensure the devices are opened read only?
The reason I ask is we recently had a pool that had a mishap. The backing
RAID controller dropped a drive (we were using them in JBOD). This happened
cleanly. The system got shutdown, and I think the wrong drive was replaced.
When it came up the RAID controller 'collapsed' the device list (so there
was no gap where the old drive was) - that, plus the wrong drive being
replaced meant when we looked at the system we had:
"
pool: vol
state: UNAVAIL
status: One or more devices could not be used because the label is missing
or invalid. There are insufficient replicas for the pool to
continue functioning.
"
A number of devices were listed as 'corrupted data' - some devices were
listed twice as members of the pool - i.e. pretty screwed up.
'undoing' the damage and restarting the server - just threw up the same
status.
I'm wondering if through the action of having the pool imported/mounted
etc. - ZFS has actually *written* to the drives that were available that
other drives aren't available / corrupt - and basically, because that info
was written, and check-summed correctly - it takes that as gospel now,
rather than actually 're-checking' the drives (or is simply unable to
re-check the drives - because the meta data has been changed from the
previous boot).
If you see what I mean :)
In the end, we're fortunate - we have backups (and they're currently
restoring now) - but I was just interested in if you 'attempt' to
mount/import a messed up pool - it could potentially screwup any chances of
mounting that pool cleanly again, even if you were to 'undo' the hardware
changes.
I have a feeling that a zpool import or 'initial' zpool status has to be a
read/write operation (i.e. would fail anyway if you could magically make
the underlying devices read-only?)
-Kp
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