Re: [Qemu-devel] [PATCH v5] block:add-cow file format

[Marcelo Tosatti]

On Tue, Dec 06, 2011 at 04:20:55PM +0100, Kevin Wolf wrote:
> Am 06.12.2011 16:06, schrieb Marcelo Tosatti:
> > On Tue, Dec 06, 2011 at 12:53:16PM -0200, Marcelo Tosatti wrote:
> >> On Tue, Dec 06, 2011 at 01:59:48PM +0100, Kevin Wolf wrote:
> >>>>> +
> >>>>> +    ret = bdrv_pread(bs->file, sizeof(header), state->bitmap,
> >>>>> +            state->bitmap_size);
> >>>>> +    if (ret != state->bitmap_size) {
> >>>>> +        goto fail;
> >>>>> +    }
> >>>>
> >>>> Reading the entire bitmap in memory is not acceptable, it may be huge.
> >>>> Better mmap it and use msync(MS_SYNC) when writing it back. This way the
> >>>> host can free memory easily upon pressure.
> >>>
> >>> You can't use mmap in block drivers. It would only work with raw-posix
> >>> backends, if at all.
> >>>
> >>> Kevin
> >>
> >> This is just the bitmap, a plain file. Why would you want to use
> >> anything other than a plain file to use as storage for the bitmap?
> > 
> > Well, mmap'ing would make life much simpler, but it has limitations such 
> > as portability.
> > 
> > Then what is necessary is a cache similar to qcow2's metadata cache.
> 
> Right, we can probably generalise the qcow2 code and make it available
> for other drivers as well.

Hum, generalising sounds overly complicated (and there is a time
constraint to this). IMHO a cache internal to add-cow.c just to avoid
reading the entire bitmap would do the trick.