Re: [Qemu-devel] [RFC] qcow2 journalling draft

[Benoît Canet]

Le Thursday 05 Sep 2013 à 11:24:40 (+0200), Stefan Hajnoczi a écrit :
> On Wed, Sep 04, 2013 at 11:55:23AM +0200, Benoît Canet wrote:
> > > > I'm not sure if multiple journals will work in practice.  Doesn't this
> > > > re-introduce the need to order update steps and flush between them?
> > > 
> > > This is a question for Benoît, who made this requirement. I asked him
> > > the same a while ago and apparently his explanation made some sense to
> > > me, or I would have remembered that I don't want it. ;-)
> > 
> > The reason behind the multiple journal requirement is that if a block get
> > created and deleted in a cyclic way it can generate cyclic 
> > insertions/deletions
> > journal entries.
> > The journal could easilly be filled if this pathological corner case happen.
> > When it happen the dedup code repack the journal by writting only the non
> > redundant information into a new journal and then use the new one.
> > It would not be easy to do so if non dedup journal entries are present in 
> > the
> > journal hence the multiple journal requirement.
> > 
> > The deduplication also need two journals because when the first one is 
> > frozen it
> > take some time to write the hash table to disk and anyway new entries must 
> > be
> > stored somewhere at the same time. The code cannot block.
> > 
> > > It might have something to do with the fact that deduplication uses the
> > > journal more as a kind of cache for hash values that can be dropped and
> > > rebuilt after a crash.
> > 
> > For dedupe the journal is more a "resume after exit" tool.
> 
> I'm not sure anymore if dedupe needs the same kind of "journal" as a
> metadata journal for qcow2.
> 
> Since you have a dirty flag to discard the "journal" on crash, the
> journal is not used for data integrity.
> 
> That makes me wonder if the metadata journal is the right structure for
> dedupe?  Maybe your original proposal was fine for dedupe and we just
> misinterpreted it because we thought this needs to be a safe journal.

Kevin what do you think of this ?
I could strip down the dedupe journal code to specialize it.

Best regards

Benoît