Re: [Qemu-devel] [PATCH v5 00/12] Dirty bitmaps migration

[Denis V. Lunev]

On 21/05/15 16:51, Vladimir Sementsov-Ogievskiy wrote:
> Hi all.
>
> Hmm. There is an interesting suggestion from Denis Lunev (in CC) about 
> how to drop meta bitmaps and make things easer.
>
> method:
>
> > start migration
> disk and memory are migrated, but not dirty bitmaps.
> > stop vm
> create all necessary bitmaps in destination vm (empty, but with same 
> names and granularities and enabled flag)
> > start destination vm
> empty bitmaps are tracking now
> > start migrating dirty bitmaps. merge them to corresponding bitmaps 
> in destination
> while bitmaps are migrating, they should be in some kind of 
> 'inconsistent' state.
> so, we can't start backup or other migration while bitmaps are 
> migrating, but vm is already _running_ on destination.
>
> what do you think about it?

the description is a bit incorrect

- start migration process, perform memory and disk migration
   as usual. VM is still executed at source
- start VM on target. VM on source should be on pause as usual,
   do not finish migration process. Running VM on target "writes"
   normally setting dirty bits as usual
- copy active dirty bitmaps from source to target. This is safe
   as VM on source is not running
- "OR" copied bitmaps with ones running on target
- finish migration process (stop source VM).

Downtime will not be increased due to dirty bitmaps with this
approach, migration process is very simple - plain data copy.

Regards,
    Den

> On 13.05.2015 18:29, Vladimir Sementsov-Ogievskiy wrote:
> > These patches provide dirty bitmap migration feature. Only named dirty
> > bitmaps are to be migrated. Migration may be enabled using migration
> > capabilities.
> >
> > v5:
> >      - rebase on master
> >      - drop [PATCH RFC v4 10/13] iotests: add event_wait to VM class
> >      - remove rfc, as incremental backup series by John Snow are in
> >        upstream
> >
> > v4 significant changes:
> >   0001: tiny bugfix: out[i] -> out[i-start], same for 'in'
> >   0007: set chunk size to 1kb, disable live iteration for
> >         migrating data < 1mb size.
> >
> >   tests: only one with md5 sum is here. used function event_wait
> >          by John Snow. (I hope, you don't mind me just adding this
> >          function with your 'Signed-off-by')
> >
> >   rfc: This patch set is based on v13 of
> >        "block: incremental backup series" by John Snow, which are
> >        not pushed yet.
> >
> > v3:
> >   based on v13 of "block: incremental backup series" by John Snow.
> >
> >   changes from v2:
> >   removed patch for adding dirty parameter (migration capablities used
> >   instead).
> >     0001: printf's dropped, qapi used
> >   0002: part0 -> zeroes
> >   0003: part0 -> zeroes
> >   0005: dirty_dirty -> meta
> >         add comments about meta bitmap
> >           0006: the format is changed, nodes used instead of devices.
> >
> >   other patches are new.
> >
> >   rfc: there are two tests. They are the same but using different
> >   interfaces: md5 checksum of the bitmap last layer in query-block or
> >   separate query-block-dirty-bitmap with dirty bitmap regions.
> >   The second form is more appropriate for debugging, the first is more
> >   appropriate for simple regression control. Which should go to
> >   upstream?
> >
> > v2:
> >   1. bug-fixes, that are already in upstream, and renaming of function
> >   bdrv_reset_dirty_bitmap (which is already in Snow's series) are
> >   dropped
> >   2. bitmap store/restore: the concept renamed to serialization, added
> >   function hbitmap_deserialize_part0, to not transfer zero blocks
> >   3. migration dirty parameter: added description comment
> >   4. Other patches are new.
> >
> > v2.rfc:
> > Actually, in this version of the series I'm trying not use
> > migration/block.c at all. Instead a separate migration unit is added
> > in the new file migration/dirty-bitmap.c. Now bitmaps are migrated
> > like blocks in block migration, they have their "dirty-dirty" bitmaps,
> > for tracking set/unset changes during migration.
> >
> > The advantages are:
> >    - no complications of migration/block.c
> >    - separate dirty-dirty bitmaps provide handling of "unset's"
> >    - more effective meta-data/data ratio - no tiny bitmap-blocks.
> >
> >
> >
> > v1:
> > These patches provide dirty bitmap migration feature. Only named dirty
> > bitmaps are to be migrated. Migration is made as a part of block
> > migration in block-migration.c.
> >
> > Dirty bitmap migration may be enabled by "dirty" parameter for qmp 
> > migrate
> > command. If "blk" and "inc" parameters are false when "dirty" is true
> > block migration is actually skipped: no allocatoions, no bdrv_read's,
> > no bdrv_write's, only bitmaps are migrated.
> >
> > The patch set includes two my previous bug fixes, which are necessary
> > for it. The patch set is based on Incremental backup series by John
> > Snow.
> > Vladimir Sementsov-Ogievskiy (12):
> >    hbitmap: serialization
> >    block: BdrvDirtyBitmap serialization interface
> >    block: tiny refactoring: minimize hbitmap_(set/reset) usage
> >    block: add meta bitmaps
> >    block: add bdrv_next_dirty_bitmap()
> >    qapi: add dirty-bitmaps migration capability
> >    migration: add migration/block-dirty-bitmap.c
> >    iotests: maintain several vms in test
> >    iotests: add add_incoming_migration to VM class
> >    qapi: add md5 checksum of last dirty bitmap level to query-block
> >    iotests: add dirty bitmap migration test
> >    migration/qemu-file: make functions qemu_(get/put)_string public
> >
> >   block.c                        |  98 +++++-
> >   include/block/block.h          |  22 ++
> >   include/migration/block.h      |   1 +
> >   include/migration/migration.h  |   1 +
> >   include/migration/qemu-file.h  |  17 +
> >   include/qemu/hbitmap.h         |  67 ++++
> >   migration/Makefile.objs        |   2 +-
> >   migration/block-dirty-bitmap.c | 693 
> > +++++++++++++++++++++++++++++++++++++++++
> >   migration/migration.c          |   9 +
> >   migration/qemu-file.c          |  18 ++
> >   qapi-schema.json               |   4 +-
> >   qapi/block-core.json           |   4 +-
> >   tests/qemu-iotests/117         |  84 +++++
> >   tests/qemu-iotests/117.out     |   5 +
> >   tests/qemu-iotests/group       |   1 +
> >   tests/qemu-iotests/iotests.py  |  14 +-
> >   util/hbitmap.c                 | 106 +++++++
> >   vl.c                           |   1 +
> >   18 files changed, 1137 insertions(+), 10 deletions(-)
> >   create mode 100644 migration/block-dirty-bitmap.c
> >   create mode 100755 tests/qemu-iotests/117
> >   create mode 100644 tests/qemu-iotests/117.out