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>

2 files changed, 68 insertions, 0 deletions

diff --git a/include/block/block_int.h b/include/block/block_int.h
index e264be9..7ad1950 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -412,6 +412,10 @@ struct BlockDriverState {
 
     /* The error object in use for blocking operations on backing_hd */
     Error *backing_blocker;
+
+    /* threshold limit for writes, in bytes. "High water mark". */
+    uint64_t write_threshold_offset;
+    NotifierWithReturn write_threshold_notifier;
 };
 
 
diff --git a/include/block/write-threshold.h b/include/block/write-threshold.h
new file mode 100644
index 0000000..f1b899c
--- /dev/null
+++ b/include/block/write-threshold.h
@@ -0,0 +1,64 @@
+/*
+ * QEMU System Emulator block write threshold notification
+ *
+ * Copyright Red Hat, Inc. 2014
+ *
+ * Authors:
+ *  Francesco Romani <fromani@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ */
+#ifndef BLOCK_WRITE_THRESHOLD_H
+#define BLOCK_WRITE_THRESHOLD_H
+
+#include <stdint.h>
+
+#include "qemu/typedefs.h"
+#include "qemu-common.h"
+
+/*
+ * bdrv_write_threshold_set:
+ *
+ * Set the write threshold for block devices, in bytes.
+ * Notify when a write exceeds the threshold, meaning the device
+ * is becoming full, so it can be transparently resized.
+ * To be used with thin-provisioned block devices.
+ *
+ * Use threshold_bytes == 0 to disable.
+ */
+void bdrv_write_threshold_set(BlockDriverState *bs, uint64_t threshold_bytes);
+
+/*
+ * bdrv_write_threshold_get
+ *
+ * Get the configured write threshold, in bytes.
+ * Zero means no threshold configured.
+ */
+uint64_t bdrv_write_threshold_get(const BlockDriverState *bs);
+
+/*
+ * bdrv_write_threshold_is_set
+ *
+ * Tell if a write threshold is set for a given BDS.
+ */
+bool bdrv_write_threshold_is_set(const BlockDriverState *bs);
+
+/*
+ * bdrv_write_threshold_exceeded
+ *
+ * Return the extent of a write request that exceeded the threshold,
+ * or zero if the request is below the threshold.
+ * Return zero also if the threshold was not set.
+ *
+ * NOTE: here we assume the following holds for each request this code
+ * deals with:
+ *
+ * assert((req->offset + req->bytes) <= UINT64_MAX)
+ *
+ * Please not there is *not* an actual C assert().
+ */
+uint64_t bdrv_write_threshold_exceeded(const BlockDriverState *bs,
+                                       const BdrvTrackedRequest *req);
+
+#endif