docs/interop/live-block-operations.rst | 50 ++++++++++++++++++++++------------ qapi/block-core.json | 8 ++++++ 2 files changed, 40 insertions(+), 18 deletions(-)
When you cancel an in-progress live block operation with QMP
`block-job-cancel`, it emits the event: BLOCK_JOB_CANCELLED. However,
when `block-job-cancel` is issued after `drive-mirror` has indicated (by
emitting the event BLOCK_JOB_READY) that the source and destination
remain synchronized:
[...] # Snip `drive-mirror` invocation & outputs
{
"execute":"block-job-cancel",
"arguments":{
"device":"virtio0"
}
}
{"return": {}}
It (`block-job-cancel`) will counterintuitively emit the event
'BLOCK_JOB_COMPLETED':
{
"timestamp":{
"seconds":1510678024,
"microseconds":526240
},
"event":"BLOCK_JOB_COMPLETED",
"data":{
"device":"virtio0",
"len":41126400,
"offset":41126400,
"speed":0,
"type":"mirror"
}
}
But this is expected behaviour, where the _COMPLETED event indicates
that synchronization has successfully ended (and the destination has a
point-in-time copy, which is at the time of cancel).
So add a small note to this effect in 'block-core.json'. While at it,
also update the "Live disk synchronization -- drive-mirror and
blockdev-mirror" section in 'live-block-operations.rst'.
(Thanks: Max Reitz for reminding me of this caveat on IRC.)
Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
---
Changes in v3:
- Fix / rewrite the section "Live disk synchronization -- drive-mirror
and blockdev-mirror" to note this gotcha [John Snow]
Changes in v2:
- "Note:" seems to be a special construct in Patchew, my usage caused a
build failure. So do: s/Note:/Note that/
- Add the missing 'Signed-off-by'
---
docs/interop/live-block-operations.rst | 50 ++++++++++++++++++++++------------
qapi/block-core.json | 8 ++++++
2 files changed, 40 insertions(+), 18 deletions(-)
diff --git a/docs/interop/live-block-operations.rst b/docs/interop/live-block-operations.rst
index 5f01797..3cb64f6 100644
--- a/docs/interop/live-block-operations.rst
+++ b/docs/interop/live-block-operations.rst
@@ -506,26 +506,40 @@ Again, given our familiar disk image chain::
[A] <-- [B] <-- [C] <-- [D]
-The ``drive-mirror`` (and its newer equivalent ``blockdev-mirror``) allows
-you to copy data from the entire chain into a single target image (which
-can be located on a different host).
-
-Once a 'mirror' job has started, there are two possible actions while a
-``drive-mirror`` job is active:
-
-(1) Issuing the command ``block-job-cancel`` after it emits the event
- ``BLOCK_JOB_CANCELLED``: will (after completing synchronization of
- the content from the disk image chain to the target image, [E])
- create a point-in-time (which is at the time of *triggering* the
- cancel command) copy, contained in image [E], of the the entire disk
+The ``drive-mirror`` (and its newer equivalent ``blockdev-mirror``)
+allows you to copy data from the entire chain into a single target image
+(which can be located on a different host), [E].
+
+.. note::
+
+ When you cancel an in-progress 'mirror' job *before* the source and
+ target are synchronized, ``block-job-cancel`` will emit the event
+ ``BLOCK_JOB_CANCELLED``. However, note that if you cancel a
+ 'mirror' job *after* it has indicated (by emitting the event
+ ``BLOCK_JOB_READY``) that the source and target now remain
+ synchronized, then ``block-job-cancel`` will emit the event
+ ``BLOCK_JOB_COMPLETED``.
+
+ Besides the 'mirror' job, the "active ``block-commit``" is the only
+ other block device job that emits the event ``BLOCK_JOB_READY``.
+ The rest of the block device jobs ('stream', "non-active
+ ``block-commit``", and 'backup') end automatically.
+
+So there are two possible actions, after a 'mirror' job has
+emitted the event ``BLOCK_JOB_READY``, indicating that source and target
+now remain synchronized:
+
+(1) Issuing the command ``block-job-cancel`` (after it emits the event
+ ``BLOCK_JOB_COMPLETED``) will create a point-in-time (which is at
+ the time of *triggering* the cancel command) copy of the entire disk
image chain (or only the top-most image, depending on the ``sync``
- mode).
+ mode), contained in the target image [E].
-(2) Issuing the command ``block-job-complete`` after it emits the event
- ``BLOCK_JOB_COMPLETED``: will, after completing synchronization of
- the content, adjust the guest device (i.e. live QEMU) to point to
- the target image, and, causing all the new writes from this point on
- to happen there. One use case for this is live storage migration.
+(2) Issuing the command ``block-job-complete`` (after it emits the event
+ ``BLOCK_JOB_COMPLETED``) will adjust the guest device (i.e. live
+ QEMU) to point to the target image, [E], causing all the new writes
+ from this point on to happen there. One use case for this is live
+ storage migration.
About synchronization modes: The synchronization mode determines
*which* part of the disk image chain will be copied to the target.
diff --git a/qapi/block-core.json b/qapi/block-core.json
index ab96e34..4ecfd1f 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -2065,6 +2065,14 @@
# BLOCK_JOB_CANCELLED event. Before that happens the job is still visible when
# enumerated using query-block-jobs.
#
+# Note that the 'block-job-cancel' command will emit the event
+# BLOCK_JOB_COMPLETED if you issue it ('block-job-cancel') after 'drive-mirror'
+# has indicated (by emitting the event BLOCK_JOB_READY) that the source and
+# destination remain synchronized. In this case, the BLOCK_JOB_COMPLETED
+# event indicates that synchronization (from 'drive-mirror') has successfully
+# ended and the destination now has a point-in-time copy, which is at the time
+# of cancel.
+#
# For streaming, the image file retains its backing file unless the streaming
# operation happens to complete just as it is being cancelled. A new streaming
# operation can be started at a later time to finish copying all data from the
--
2.9.5
On 11/17/2017 10:25 AM, Kashyap Chamarthy wrote: > When you cancel an in-progress live block operation with QMP > `block-job-cancel`, it emits the event: BLOCK_JOB_CANCELLED. However, > when `block-job-cancel` is issued after `drive-mirror` has indicated (by > emitting the event BLOCK_JOB_READY) that the source and destination > remain synchronized: > > But this is expected behaviour, where the _COMPLETED event indicates > that synchronization has successfully ended (and the destination has a > point-in-time copy, which is at the time of cancel). > > So add a small note to this effect in 'block-core.json'. While at it, > also update the "Live disk synchronization -- drive-mirror and > blockdev-mirror" section in 'live-block-operations.rst'. > > (Thanks: Max Reitz for reminding me of this caveat on IRC.) Sorry for not wordsmithing on earlier versions: > > Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com> > --- > +.. note:: > + > + When you cancel an in-progress 'mirror' job *before* the source and > + target are synchronized, ``block-job-cancel`` will emit the event > + ``BLOCK_JOB_CANCELLED``. However, note that if you cancel a > + 'mirror' job *after* it has indicated (by emitting the event > + ``BLOCK_JOB_READY``) that the source and target now remain > + synchronized, then ``block-job-cancel`` will emit the event Might read slightly nicer as: the source and target have reached synchronization, > +++ b/qapi/block-core.json > @@ -2065,6 +2065,14 @@ > # BLOCK_JOB_CANCELLED event. Before that happens the job is still visible when > # enumerated using query-block-jobs. > # > +# Note that the 'block-job-cancel' command will emit the event > +# BLOCK_JOB_COMPLETED if you issue it ('block-job-cancel') after 'drive-mirror' > +# has indicated (by emitting the event BLOCK_JOB_READY) that the source and > +# destination remain synchronized. In this case, the BLOCK_JOB_COMPLETED > +# event indicates that synchronization (from 'drive-mirror') has successfully > +# ended and the destination now has a point-in-time copy, which is at the time > +# of cancel. Accurate, but a bit hard to follow the flow of the sentence. Might read nicer as: Note that if you issue 'block-job-cancel' after 'drive-mirror' has indicated (via the event BLOCK_JOB_READY) that the source and destination are synchronized, then the event triggered by this command changes to BLOCK_JOB_COMPLETED, to indicate that the mirroring has ended and the destination now has a point-in-time copy tied to the time of the cancellation. Documentation is worth of inclusion in 2.11. Whether you keep your wording, or incorporate mine in a v4, you can add: Reviewed-by: Eric Blake <eblake@redhat.com> -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
On Fri, Nov 17, 2017 at 11:07:41AM -0600, Eric Blake wrote: > On 11/17/2017 10:25 AM, Kashyap Chamarthy wrote: [...] > Sorry for not wordsmithing on earlier versions: That's fine. Always appreciate your wordsmithing :-) > > + 'mirror' job *after* it has indicated (by emitting the event > > + ``BLOCK_JOB_READY``) that the source and target now remain > > + synchronized, then ``block-job-cancel`` will emit the event > > Might read slightly nicer as: > > the source and target have reached synchronization, Ah, that sounds nicer indeed. Will fix. [...] > > +# Note that the 'block-job-cancel' command will emit the event > > +# BLOCK_JOB_COMPLETED if you issue it ('block-job-cancel') after 'drive-mirror' > > +# has indicated (by emitting the event BLOCK_JOB_READY) that the source and > > +# destination remain synchronized. In this case, the BLOCK_JOB_COMPLETED > > +# event indicates that synchronization (from 'drive-mirror') has successfully > > +# ended and the destination now has a point-in-time copy, which is at the time > > +# of cancel. > > > Accurate, but a bit hard to follow the flow of the sentence. Yeah, I wasn't satisfied with my phrasing too. I sat on it for nearly half an hour, and still it came out super clunky. > Might read nicer as: > > Note that if you issue 'block-job-cancel' after 'drive-mirror' has > indicated (via the event BLOCK_JOB_READY) that the source and > destination are synchronized, then the event triggered by this command > changes to BLOCK_JOB_COMPLETED, to indicate that the mirroring has > ended and the destination now has a point-in-time copy tied to the > time of the cancellation. Thanks, yours reads much nicer. > Documentation is worth of inclusion in 2.11. Whether you keep your > wording, or incorporate mine in a v4, you can add: > Reviewed-by: Eric Blake <eblake@redhat.com> I'll definitely incorporate in v4. I'll also make similar wording adjustment in live-block-operations.rst. Thanks for the quick review. -- /kashyap
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