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. Async mirroring means guest writes completes independently of target writes.