Re: [Qemu-devel] [PATCH v2 RFC 0/8] block: persistent dirty bitmaps

[Stefan Hajnoczi]

On Fri, Jun 12, 2015 at 12:58:35PM +0300, Denis V. Lunev wrote:
> On 11/06/15 23:06, Stefan Hajnoczi wrote:
> >The load/store API is not scalable when bitmaps are 1 MB or larger.
> >
> >For example, a 500 GB disk image with 64 KB granularity requires a 1 MB
> >bitmap.  If a guest has several disk images of this size, then multiple
> >megabytes must be read to start the guest and written out to shut down
> >the guest.
> >
> >By comparison, the L1 table for the 500 GB disk image is less than 8 KB.
> >
> >I think something like qcow2-cache.c or metabitmaps should be used to
> >lazily read/write persistent bitmaps.  That way only small portions need
> >to be read/written at a time.
> >
> >Stefan
> for the first iteration we could open the image, start tracking,
> read bitmap as one entity in the background and or read
> and collected data.
> 
> partial read could be done in the next step

Making bitmap load/store fully lazy will require changes to the
load/store API, so it's worth thinking about a little upfront.
Otherwise there will be a lot of code churn when the fully lazy patches
are posted.  As a reviewer it's in my interest to only spend time
reviewing the final version instead of code that gets thrown out :-),
but I understand.

If you can make the read lazy to some extent that's a good start.

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