block: add event when disk usage exceeds threshold

Managing applications, like oVirt (http://www.ovirt.org), make extensive
use of thin-provisioned disk images.
To let the guest run smoothly and be not unnecessarily paused, oVirt sets
a disk usage threshold (so called 'high water mark') based on the occupation
of the device,  and automatically extends the image once the threshold
is reached or exceeded.

In order to detect the crossing of the threshold, oVirt has no choice but
aggressively polling the QEMU monitor using the query-blockstats command.
This lead to unnecessary system load, and is made even worse under scale:
deployments with hundreds of VMs are no longer rare.

To fix this, this patch adds:
* A new monitor command `block-set-write-threshold', to set a mark for
  a given block device.
* A new event `BLOCK_WRITE_THRESHOLD', to report if a block device
  usage exceeds the threshold.
* A new `write_threshold' field into the `BlockDeviceInfo' structure,
  to report the configured threshold.

This will allow the managing application to use smarter and more
efficient monitoring, greatly reducing the need of polling.

[Updated qemu-iotests 067 output to add the new 'write_threshold'
property. --Stefan]
[Changed g_assert_false() to !g_assert() to fix the build on older glib
versions. --Kevin]

Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1421068273-692-1-git-send-email-fromani@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 50 insertions, 1 deletions

diff --git a/qapi/block-core.json b/qapi/block-core.json
index b7d9772..a3fdaf0 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -257,6 +257,9 @@
 #
 # @cache: the cache mode used for the block device (since: 2.3)
 #
+# @write_threshold: configured write threshold for the device.
+#                   0 if disabled. (Since 2.3)
+#
 # Since: 0.14.0
 #
 ##
@@ -271,7 +274,8 @@
             '*bps_max': 'int', '*bps_rd_max': 'int',
             '*bps_wr_max': 'int', '*iops_max': 'int',
             '*iops_rd_max': 'int', '*iops_wr_max': 'int',
-            '*iops_size': 'int', 'cache': 'BlockdevCacheInfo' } }
+            '*iops_size': 'int', 'cache': 'BlockdevCacheInfo',
+            'write_threshold': 'int' } }
 
 ##
 # @BlockDeviceIoStatus:
@@ -1917,3 +1921,48 @@
 ##
 { 'enum': 'PreallocMode',
   'data': [ 'off', 'metadata', 'falloc', 'full' ] }
+
+##
+# @BLOCK_WRITE_THRESHOLD
+#
+# Emitted when writes on block device reaches or exceeds the
+# configured write threshold. For thin-provisioned devices, this
+# means the device should be extended to avoid pausing for
+# disk exhaustion.
+# The event is one shot. Once triggered, it needs to be
+# re-registered with another block-set-threshold command.
+#
+# @node-name: graph node name on which the threshold was exceeded.
+#
+# @amount-exceeded: amount of data which exceeded the threshold, in bytes.
+#
+# @write-threshold: last configured threshold, in bytes.
+#
+# Since: 2.3
+##
+{ 'event': 'BLOCK_WRITE_THRESHOLD',
+  'data': { 'node-name': 'str',
+            'amount-exceeded': 'uint64',
+            'write-threshold': 'uint64' } }
+
+##
+# @block-set-write-threshold
+#
+# Change the write threshold for a block drive. An event will be delivered
+# if a write to this block drive crosses the configured threshold.
+# This is useful to transparently resize thin-provisioned drives without
+# the guest OS noticing.
+#
+# @node-name: graph node name on which the threshold must be set.
+#
+# @write-threshold: configured threshold for the block device, bytes.
+#                   Use 0 to disable the threshold.
+#
+# Returns: Nothing on success
+#          If @node name is not found on the block device graph,
+#          DeviceNotFound
+#
+# Since: 2.3
+##
+{ 'command': 'block-set-write-threshold',
+  'data': { 'node-name': 'str', 'write-threshold': 'uint64' } }