zfs discuss - Jun 2007 - Importing pool from readonly device? (as in ZFS fun!)

Hi Mario,

On Thu, 2007-06-14 at 15:23 -0100, Mario Goebbels wrote:
> root at bigmclargehuge:~/LargeFiles > zpool import -o readonly testpool
> internal error: Read-only file system
> Abort (core dumped)
> root at bigmclargehuge:~/LargeFiles > 
Interesting, I''ve just filed 6569720 for this behaviour - thanks for
spotting this! Regardless of whether ZFS supports this, we definitely
shouldn''t core dump.
> Does ZFS still write to the zpool when mounted with readonly? 
Yep it does - it needs to mark the pool as being in use and no longer
exported. I think this is done by changing the pool state nvpair in
the vdev_label (if I''m not mistaken - section 1.3.3 in the on-disk
format document covers this.)

	cheers,
			tim

[1]
http://www.opensolaris.org/os/community/zfs/docs/ondiskformat0822.pdf

-- 
Tim Foster, Sun Microsystems Inc, Solaris Engineering Ops
http://blogs.sun.com/timf

Trying some funky experiments, based on hearing about this readonly ZFS
in MacOSX, I''m kidding around with creating file based pools and then
burning them to a CDROM. When running zpool import, it does find the
pool on the CD, but then warns about a read-only device, followed by a
core dump.

root at bigmclargehuge:~/LargeFiles > zpool import -o readonly testpool
internal error: Read-only file system
Abort (core dumped)
root at bigmclargehuge:~/LargeFiles > 

Does ZFS still write to the zpool when mounted with readonly? If not,
why would a readonly device pose a problem? Apart from being gimmicky
(Hey, escalator sorting on a CDROM).

Now, if I had three DVD burners and some DVD-RWs, I would try a
DVD-RAID. :V

Thanks.
-mg


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> > root at bigmclargehuge:~/LargeFiles > zpool import -o readonly
testpool
> > internal error: Read-only file system
> > Abort (core dumped)
> > root at bigmclargehuge:~/LargeFiles > 
> 
> Interesting, I''ve just filed 6569720 for this behaviour - thanks
for
> spotting this! Regardless of whether ZFS supports this, we definitely
> shouldn''t core dump.
I just remember those weirdo RAID5 controllers we had way back in a
different company. It was a pretty expensive thing, with hardware XOR,
battery, cache and all that. Going by that, it wasn''t a toy. Yet, when
a
disk failed, it dropped into readonly mode.

I guess if someone were to have a concoction like this, ZFS would break
in various ways. That is if someone insists on using hardware RAID.
Can''t remember the brand, though.
> > Does ZFS still write to the zpool when mounted with readonly? 
> 
> Yep it does - it needs to mark the pool as being in use and no longer
> exported. I think this is done by changing the pool state nvpair in
> the vdev_label (if I''m not mistaken - section 1.3.3 in the on-disk
> format document covers this.)
Awwww :(

Solaris doesn''t by chance ship with a tool to mount devices as
readonly,
but writes modifications into a (temporary) file? Kind of like various
emulators do?

Regards,
-mg
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