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, 32 insertions, 0 deletions

diff --git a/qmp-commands.hx b/qmp-commands.hx
index af3fd19935..a85d8479e3 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -2146,6 +2146,8 @@ Each json-object contain the following:
          - "iops_size": I/O size when limiting by iops (json-int)
          - "detect_zeroes": detect and optimize zero writing (json-string)
              - Possible values: "off", "on", "unmap"
+         - "write_threshold": write offset threshold in bytes, a event will be
+                              emitted if crossed. Zero if disabled (json-int)
          - "image": the detail of the image, it is a json-object containing
             the following:
              - "filename": image file name (json-string)
@@ -2223,6 +2225,7 @@ Example:
                "iops_wr_max": 0,
                "iops_size": 0,
                "detect_zeroes": "on",
+               "write_threshold": 0,
                "image":{
                   "filename":"disks/test.qcow2",
                   "format":"qcow2",
@@ -3685,6 +3688,7 @@ Example:
                    "iops_rd_max": 0,
                    "iops_wr_max": 0,
                    "iops_size": 0,
+                   "write_threshold": 0,
                    "image":{
                       "filename":"disks/test.qcow2",
                       "format":"qcow2",
@@ -3921,3 +3925,31 @@ Move mouse pointer to absolute coordinates (20000, 400).
 <- { "return": {} }
 
 EQMP
+
+    {
+        .name       = "block-set-write-threshold",
+        .args_type  = "node-name:s,write-threshold:l",
+        .mhandler.cmd_new = qmp_marshal_input_block_set_write_threshold,
+    },
+
+SQMP
+block-set-write-threshold
+------------
+
+Change the write threshold for a block drive. The threshold is an offset,
+thus must be non-negative. Default is no write threshold.
+Setting the threshold to zero disables it.
+
+Arguments:
+
+- "node-name": the node name in the block driver state graph (json-string)
+- "write-threshold": the write threshold in bytes (json-int)
+
+Example:
+
+-> { "execute": "block-set-write-threshold",
+  "arguments": { "node-name": "mydev",
+                 "write-threshold": 17179869184 } }
+<- { "return": {} }
+
+EQMP