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

[Benoît Canet]

> > 2. Byte-granularity means that read-modify-write is necessary to append
> >    entries to the journal.  Therefore a failure could destroy previously
> >    committed entries.
> > 
> >    Any ideas how existing journals handle this?
> 
> You commit only whole blocks. So in this case we can consider a block
> only committed as soon as a TYPE_END entry has been written (and after
> that we won't touch it any more until the journalled changes have been
> flushed to disk).
> 
> There's one "interesting" case: cache=writethrough. I'm not entirely
> sure yet what to do with it, but it's slow anyway, so using one block
> per entry and therefore flushing the journal very often might actually
> be not totally unreasonable.

This sure would finish to kill the performance because this would be an io
per metadata written to disk.

> 
> Another thing I'm not sure about is whether a fixed 4k block is good or
> if we should leave it configurable. I don't think making it an option
> would hurt (not necessarily modifyable with qemu-img, but as a field
> in the file format).

I agree.
I also think about make the number of block to be flushed at once configurable.

Benoît