[Qemu-devel] [PATCH] block: extend BLOCK_IO_ERROR event with nospace indicator

[Luiz Capitulino <lcapitulino@redhat.com>]

Management software, such as RHEV's vdsm, want to be able to allocate
disk space on demand. The basic use case is to start a VM with a small
disk and then the disk is enlarged when QEMU hits a ENOSPC condition.

To this end, the management software has to be notified when QEMU
encounters ENOSPC. The solution implemented by this commit is simple:
it extends the BLOCK_IO_ERROR with a 'nospace' key, which is true
when QEMU is stopped due to ENOSPC.

Note that support for querying this event is already present in
query-block by means of the 'io-status' key. Also, the new 'nospace'
BLOCK_IO_ERROR field shares the same semantics with 'io-status',
which basically means that werror= has to be set to either
'stop' or 'enospc' to enable 'nospace'.

Finally, this commit also updates the 'io-status' key doc in the
schema with a list of supported device models.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
---

Three important observations:

 1. We've talked with oVirt and OpenStack folks. oVirt folks say that
    this implementation is enough for their use-case. OpenStack don't
    need this feature

 2. While testing this with a raw image on a (smaller) ext2 file mounted
    via the loopback device, I get half "Invalid argument" I/O errors and
    half  "No space" errors". This means that half of the BLOCK_IO_ERROR
    events that are emitted for this test-case will have nospace=false
    and the other half nospace=true. I don't know why I'm getting those
    "Invalid argument" errors, can anyone of the block layer comment
    on this? I don't get that with a qcow2 image (I get nospace=true for
    all events)

 3. I think this should go via block tree

 block.c              | 22 ++++++++++++++--------
 qapi/block-core.json |  8 +++++++-
 2 files changed, 21 insertions(+), 9 deletions(-)

diff --git a/block.c b/block.c
index 1df13ac..b334e35 100644
--- a/block.c
+++ b/block.c
@@ -3632,6 +3632,18 @@ BlockErrorAction bdrv_get_error_action(BlockDriverState *bs, bool is_read, int e
     }
 }
 
+static void send_qmp_error_event(BlockDriverState *bs,
+                                 BlockErrorAction action,
+                                 bool is_read, int error)
+{
+    BlockErrorAction ac;
+
+    ac = is_read ? IO_OPERATION_TYPE_READ : IO_OPERATION_TYPE_WRITE;
+    qapi_event_send_block_io_error(bdrv_get_device_name(bs), ac, action,
+                                   bdrv_iostatus_is_enabled(bs),
+                                   error == ENOSPC, &error_abort);
+}
+
 /* This is done by device models because, while the block layer knows
  * about the error, it does not know whether an operation comes from
  * the device or the block layer (from a job, for example).
@@ -3657,16 +3669,10 @@ void bdrv_error_action(BlockDriverState *bs, BlockErrorAction action,
          * also ensures that the STOP/RESUME pair of events is emitted.
          */
         qemu_system_vmstop_request_prepare();
-        qapi_event_send_block_io_error(bdrv_get_device_name(bs),
-                                       is_read ? IO_OPERATION_TYPE_READ :
-                                       IO_OPERATION_TYPE_WRITE,
-                                       action, &error_abort);
+        send_qmp_error_event(bs, action, is_read, error);
         qemu_system_vmstop_request(RUN_STATE_IO_ERROR);
     } else {
-        qapi_event_send_block_io_error(bdrv_get_device_name(bs),
-                                       is_read ? IO_OPERATION_TYPE_READ :
-                                       IO_OPERATION_TYPE_WRITE,
-                                       action, &error_abort);
+        send_qmp_error_event(bs, action, is_read, error);
     }
 }
 
diff --git a/qapi/block-core.json b/qapi/block-core.json
index fb74c56..567e0a6 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -336,6 +336,7 @@
 #
 # @io-status: #optional @BlockDeviceIoStatus. Only present if the device
 #             supports it and the VM is configured to stop on errors
+#             (supported device models: virtio-blk, ide, scsi-disk)
 #
 # @inserted: #optional @BlockDeviceInfo describing the device if media is
 #            present
@@ -1569,6 +1570,11 @@
 #
 # @action: action that has been taken
 #
+# @nospace: #optional true if I/O error was caused due to a no-space
+#           condition. This key is only present if query-block's
+#           io-status is present, please see query-block documentation
+#           for more information (since: 2.2)
+#
 # Note: If action is "stop", a STOP event will eventually follow the
 # BLOCK_IO_ERROR event
 #
@@ -1576,7 +1582,7 @@
 ##
 { 'event': 'BLOCK_IO_ERROR',
   'data': { 'device': 'str', 'operation': 'IoOperationType',
-            'action': 'BlockErrorAction' } }
+            'action': 'BlockErrorAction', '*nospace': 'bool' } }
 
 ##
 # @BLOCK_JOB_COMPLETED
-- 
1.9.3