Re: [Qemu-devel] [RFC V8 01/24] qcow2: Add journal specification.

["Benoît Canet" <benoit.canet@irqsave.net>]

> Simple is good.  Even for deduplication alone, I think data integrity is
> critical - otherwise we risk stale dedup metadata pointing to clusters
> that are unallocated or do not contain the right data.  So the journal
> will probably need to follow techniques for commits/checksums.

I agree that checksums are missing for the dedup.
Maybe we could even use some kind of error correcting code instead of a checksum.

Concerning data integrity the events that the deduplication code cannot loose
are hash deletions because they mark a previously inserted hash as obsolete.

The problem with a commit/flush mechanism on hash deletion is that it will slow
down the store insertion speed and also create some extra SSD wear out.

To solve this I considered the fact that the dedup metadata as a whole is
disposable.

So I implemented a "dedup dirty" bit.

When QEMU stop the journal is flushed and the dirty bit is cleared.
When QEMU start and the dirty bit is set a crash is detected and _all_ the
deduplication metadata is dropped.
The QCOW2 data integrity won't suffer only the dedup ratio will be lower.

As you said once on irc crashes don't happen often.

Benoît