Re: [dm-devel] [PATCH 2/2] dmcache: Implement a flush message

[Joe Thornber <thornber@redhat.com>]

On Thu, May 09, 2013 at 01:47:51PM -0700, Darrick J. Wong wrote:
> Create a new 'flush' message that causes the dmcache to write all of its
> metadata out to disk.  This enables us to ensure that the disk reflects
> whatever's in memory without having to tear down the cache device.  This helps
> me in the case where I have a cached ro fs that I can't umount and therefore
> can't tear down the cache device, but want to save the cache metadata anyway.
> The command syntax is as follows:
> 
> # dmsetup message mycache 0 flush now

Nack.

[Ignoring the ugly 'now' parameter.]

I think you're in danger of hiding the real issue.  Which is if the
target's destructor and post suspend is not being called then, as far
as dm-cache is concerned this is a crash.  Any open transactions will
be lost as it automatically rolls back.

We need to understand more why this is happening.  It's actually
harmless atm for dm-cache, because we're forced to commit before using
a new migration.  But for dm-thin you can lose writes.  Why are you
never tearing down your dm devices?

- Joe