diff --git a/docs/virtio-balloon-stats.txt b/docs/virtio-balloon-stats.txt new file mode 100644 index 0000000000..f74612f468 --- /dev/null +++ b/docs/virtio-balloon-stats.txt @@ -0,0 +1,104 @@ +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": {} }