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

[Benoît Canet]

> > 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.

Best regards

Benoît