On Mon, Sep 18, 2017 at 06:26:51PM +0200, Max Reitz wrote: > On 2017-09-18 12:06, Stefan Hajnoczi wrote: > > On Sat, Sep 16, 2017 at 03:58:01PM +0200, Max Reitz wrote: > >> On 2017-09-14 17:57, Stefan Hajnoczi wrote: > >>> On Wed, Sep 13, 2017 at 08:19:07PM +0200, Max Reitz wrote: > >>>> This patch implements active synchronous mirroring. In active mode, the > >>>> passive mechanism will still be in place and is used to copy all > >>>> initially dirty clusters off the source disk; but every write request > >>>> will write data both to the source and the target disk, so the source > >>>> cannot be dirtied faster than data is mirrored to the target. Also, > >>>> once the block job has converged (BLOCK_JOB_READY sent), source and > >>>> target are guaranteed to stay in sync (unless an error occurs). > >>>> > >>>> Optionally, dirty data can be copied to the target disk on read > >>>> operations, too. > >>>> > >>>> Active mode is completely optional and currently disabled at runtime. A > >>>> later patch will add a way for users to enable it. > >>>> > >>>> Signed-off-by: Max Reitz <mre...@redhat.com> > >>>> --- > >>>> qapi/block-core.json | 23 +++++++ > >>>> block/mirror.c | 187 > >>>> +++++++++++++++++++++++++++++++++++++++++++++++++-- > >>>> 2 files changed, 205 insertions(+), 5 deletions(-) > >>>> > >>>> diff --git a/qapi/block-core.json b/qapi/block-core.json > >>>> index bb11815608..e072cfa67c 100644 > >>>> --- a/qapi/block-core.json > >>>> +++ b/qapi/block-core.json > >>>> @@ -938,6 +938,29 @@ > >>>> 'data': ['top', 'full', 'none', 'incremental'] } > >>>> > >>>> ## > >>>> +# @MirrorCopyMode: > >>>> +# > >>>> +# An enumeration whose values tell the mirror block job when to > >>>> +# trigger writes to the target. > >>>> +# > >>>> +# @passive: copy data in background only. > >>>> +# > >>>> +# @active-write: when data is written to the source, write it > >>>> +# (synchronously) to the target as well. In addition, > >>>> +# data is copied in background just like in @passive > >>>> +# mode. > >>>> +# > >>>> +# @active-read-write: write data to the target (synchronously) both > >>>> +# when it is read from and written to the source. > >>>> +# In addition, data is copied in background just > >>>> +# like in @passive mode. > >>> > >>> I'm not sure the terms "active"/"passive" are helpful. "Active commit" > >>> means committing the top-most BDS while the guest is accessing it. The > >>> "passive" mirror block still works on the top-most BDS while the guest > >>> is accessing it. > >>> > >>> Calling it "asynchronous" and "synchronous" is clearer to me. It's also > >>> the terminology used in disk replication (e.g. DRBD). > >> > >> I'd be OK with that, too, but I think I remember that in the past at > >> least Kevin made a clear distinction between active/passive and > >> sync/async when it comes to mirroring. > >> > >>> Ideally the user wouldn't have to worry about async vs sync because QEMU > >>> would switch modes as appropriate in order to converge. That way > >>> libvirt also doesn't have to worry about this. > >> > >> So here you mean async/sync in the way I meant it, i.e., whether the > >> mirror operations themselves are async/sync? > > > > The meaning I had in mind is: > > > > Sync mirroring means a guest write waits until the target write > > completes. > > I.e. active-sync, ... > > > Async mirroring means guest writes completes independently of target > > writes. > > ... i.e. passive or active-async in the future. > > So you really want qemu to decide whether to use active or passive mode > depending on what's enough to let the block job converge and not > introduce any switch for the user? > > I'm not sure whether I like this too much, mostly because "libvirt > doesn't have to worry" doesn't feel quite right to me. If we don't make > libvirt worry about this, then qemu has to worry. I'm not sure whether > that's any better. > > I think this really does get into policy territory. Just switching to > active mode the instant target writes are slower than source writes may > not be what the user wants: Maybe it's OK for a short duration because > they don't care about hard convergence too much. Maybe they want to > switch to active mode already when "only" twice as much is written to > the target as to the source. > > I think this is a decision the management layer (or the user) has to make.
Eric: Does libvirt want to be involved with converging the mirror job (i.e. if the guest is writing to disk faster than QEMU can copy data to the target)? Stefan