EBS snapshot backups from a FreeBSD zfs file system: zpool freeze?

[Will Andrews]

On Wed, Jul 3, 2013 at 6:13 PM, Steven Hartland <killing at multiplay.co.uk> wrote:
> Not been following the thread really so excuse if this has already
> been mentioned ;-)
>
> There is a zpool freeze <pool> which stops spa_sync() from doing
> anything, so that the only way to record changes is on the ZIL.
>
> The comment in the zpool_main is: "'freeze' is a vile debugging
> abomination" so it's evil but might be what you want if you're up to
> writing some code.

zpool freeze is a debugging-only command, as the comment suggests.  It
is not really of much use outside of testing changes to ZIL code.
Once run, the only thing you can do to get normal I/O running again is
to export the pool and import it again.

The point of the command is to ensure that ZIL blocks exist on a pool
when it is exported, so they are guaranteed to have to be replayed on
import.  It is used in the STF test suite for the express purpose of
testing the ZIL replay.  They write some stuff, freeze the pool, write
some more stuff, export the pool, use zdb to check for ZIL blocks,
then import it and check again, both to see that the changes were
applied, and to see that the ZIL blocks are gone.

Most likely, doing something more like Berend wants requires a
slightly different approach.

--Will.