On Fri, Nov 21, 2014 at 01:27:40PM +0300, Vladimir Sementsov-Ogievskiy wrote:
There is a constraint if we want to get live migration for free: The
bitmap contents must be accessible with bdrv_read() and
bdrv_get_block_status() to skip zero regions.
Hm. I'm afraid, it still will not be free. If bitmap is active, it's actual
version is in memory. To migrate bitmap file like a disk image, we should
start syncing it with every write to corresponding disk, doubling number of
io.
It would be possible to drive-mirror the persistent dirty bitmap and
then flush it like all drives when the guest vCPUs are paused for
migration.
After thinking more about it though, this approach places more I/O into
the critical guest downtime phase. In other words, slow disk I/O could
lead to long guest downtimes while QEMU tries to write out the dirty
bitmap.
Moreover, we have normal dirty bitmaps, which have no name/file, do we
migrate them? If, for example, the migration occurs when backup in progress?
Active bitmaps should be migrated in the same way for
persistent/named/normal bitmaps. I can't find in qemu source, is there
bitmap migration?
bs->dirty_bitmaps is not migrated, in fact none of BlockDriverState is
migrated.
QEMU only migrates emulated device state (e.g. the hardware registers
and associated state). It does not emulate host state that the guest
cannot see like the dirty bitmap.
Or you are saying about migrating disabled bitmaps? Hm. We should sync
bitmap file on bitmap_disable. Disabled persistent bitmap is just a static
file ~30mb, we can easily migrate it without common procedure with cow or
something like this..
Active dirty bitmaps should migrate too. I'm thinking now that the
appropriate thing is to add live migration of dirty bitmaps to QEMU
(regardless of whether they are active or not).
Stefan