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

[Stefan Hajnoczi]

On Mon, Jun 08, 2015 at 06:21:18PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> QCow2 header is extended by fields 'nb_dirty_bitmaps' and
> 'dirty_bitmaps_offset' like with snapshots.
> 
> Proposed command line syntax is the following:
> 
> -dirty-bitmap [option1=val1][,option2=val2]...

Two questions:

1. How does this code ensure that the dirty bitmap is consistent after
crash/power failure?

At the minimum, enabled dirty bitmaps must be discarded after
crash/power failure if we cannot guarantee they are up-to-date.  It's
worse to rely on an outdated dirty bitmap than to detect failure and
start afresh.

2. How do persistent dirty bitmaps work with live migration?  Remember
there are two storage cases for live migration: shared storage (NAS or
SAN) and non-shared storage (disk images must be copied over).

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