Incremental Sync Mode for blockdev-mirror

[Ali Gholami Rudi]

Hi,

We are using blockdev-mirror to synchronize Qemu block devices with
backup images to protect user VM images from disk failures.  The
problem is that when Qemu restarts (after it has been quit or killed),
the none sync mode does not synchronize blocks that have been missed
in the end of the last Qemu session.

If a persistent dirty bitmap is kept for blockdev-mirror,
synchronization can probably continue after Qemu restarts without
problem.  Looking at the code, blockdev-mirror incremental sync mode
is not supported right now.  However, supporting it by making
MirrorBlockJob.dirty_bitmap persistent does not seem very difficult.

I wonder why it has not been implemented in mirror.c.  Are there good
reasons for not supporting it?

Thanks,
Ali

PS We also tried quorum devices to implement RAID-1 arrays.  However
in our experiments blockdev-mirror was much faster.  Are there better
alternatives?