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authorLuiz Capitulino <lcapitulino@redhat.com>2012-12-04 12:04:39 -0200
committerLuiz Capitulino <lcapitulino@redhat.com>2013-01-25 10:49:46 -0200
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parent7e6ccd9c159450e79f42d08112ebe263b0466644 (diff)
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docs: document virtio-balloon stats
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com>
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+virtio balloon memory statistics
+================================
+
+The virtio balloon driver supports guest memory statistics reporting. These
+statistics are available to QEMU users as QOM (QEMU Object Model) device
+properties via a polling mechanism.
+
+Before querying the available stats, clients first have to enable polling.
+This is done by writing a time interval value (in seconds) to the
+guest-stats-polling-interval property. This value can be:
+
+ > 0 enables polling in the specified interval. If polling is already
+ enabled, the polling time interval is changed to the new value
+
+ 0 disables polling. Previous polled statistics are still valid and
+ can be queried.
+
+Once polling is enabled, the virtio-balloon device in QEMU will start
+polling the guest's balloon driver for new stats in the specified time
+interval.
+
+To retrieve those stats, clients have to query the guest-stats property,
+which will return a dictionary containing:
+
+ o A key named 'stats', containing all available stats. If the guest
+ doesn't support a particular stat, or if it couldn't be retrieved,
+ its value will be -1. Currently, the following stats are supported:
+
+ - stat-swap-in
+ - stat-swap-out
+ - stat-major-faults
+ - stat-minor-faults
+ - stat-free-memory
+ - stat-total-memory
+
+ o A key named last-update, which contains the last stats update
+ timestamp in seconds. Since this timestamp is generated by the host,
+ a buggy guest can't influence its value
+
+It's also important to note the following:
+
+ - Previously polled statistics remain available even if the polling is
+ later disabled
+
+ - As noted above, if a guest doesn't support a particular stat its value
+ will always be -1. However, it's also possible that a guest temporarily
+ couldn't update one or even all stats. If this happens, just wait for
+ the next update
+
+ - Polling can be enabled even if the guest doesn't have stats support
+ or the balloon driver wasn't loaded in the guest. If this is the case
+ and stats are queried, an error will be returned
+
+ - The polling timer is only re-armed when the guest responds to the
+ statistics request. This means that if a (buggy) guest doesn't ever
+ respond to the request the timer will never be re-armed, which has
+ the same effect as disabling polling
+
+Here are a few examples. QEMU is started with '-balloon virtio', which
+generates '/machine/peripheral-anon/device[1]' as the QOM path for the
+balloon device.
+
+Enable polling with 2 seconds interval:
+
+{ "execute": "qom-set",
+ "arguments": { "path": "/machine/peripheral-anon/device[1]",
+ "property": "guest-stats-polling-interval", "value": 2 } }
+
+{ "return": {} }
+
+Change polling to 10 seconds:
+
+{ "execute": "qom-set",
+ "arguments": { "path": "/machine/peripheral-anon/device[1]",
+ "property": "guest-stats-polling-interval", "value": 10 } }
+
+{ "return": {} }
+
+Get stats:
+
+{ "execute": "qom-get",
+ "arguments": { "path": "/machine/peripheral-anon/device[1]",
+ "property": "guest-stats" } }
+{
+ "return": {
+ "stats": {
+ "stat-swap-out": 0,
+ "stat-free-memory": 844943360,
+ "stat-minor-faults": 219028,
+ "stat-major-faults": 235,
+ "stat-total-memory": 1044406272,
+ "stat-swap-in": 0
+ },
+ "last-update": 1358529861
+ }
+}
+
+Disable polling:
+
+{ "execute": "qom-set",
+ "arguments": { "path": "/machine/peripheral-anon/device[1]",
+ "property": "stats-polling-interval", "value": 0 } }
+
+{ "return": {} }