qemu-block
[Top][All Lists]
Advanced

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[Qemu-block] [PATCH v3] QAPI & interop: Clarify events emitted by 'block


From: Kashyap Chamarthy
Subject: [Qemu-block] [PATCH v3] QAPI & interop: Clarify events emitted by 'block-job-cancel'
Date: Fri, 17 Nov 2017 17:25:36 +0100

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 <address@hidden>
---
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




reply via email to

[Prev in Thread] Current Thread [Next in Thread]