docs/virtio-balloon-stats.txt

   1 virtio balloon memory statistics
   2 ================================
   3
   4 The virtio balloon driver supports guest memory statistics reporting. These
   5 statistics are available to QEMU users as QOM (QEMU Object Model) device
   6 properties via a polling mechanism.
   7
   8 Before querying the available stats, clients first have to enable polling.
   9 This is done by writing a time interval value (in seconds) to the
  10 guest-stats-polling-interval property. This value can be:
  11
  12   > 0  enables polling in the specified interval. If polling is already
  13        enabled, the polling time interval is changed to the new value
  14
  15   0    disables polling. Previous polled statistics are still valid and
  16        can be queried.
  17
  18 Once polling is enabled, the virtio-balloon device in QEMU will start
  19 polling the guest's balloon driver for new stats in the specified time
  20 interval.
  21
  22 To retrieve those stats, clients have to query the guest-stats property,
  23 which will return a dictionary containing:
  24
  25   o A key named 'stats', containing all available stats. If the guest
  26     doesn't support a particular stat, or if it couldn't be retrieved,
  27     its value will be -1. Currently, the following stats are supported:
  28
  29       - stat-swap-in
  30       - stat-swap-out
  31       - stat-major-faults
  32       - stat-minor-faults
  33       - stat-free-memory
  34       - stat-total-memory
  35       - stat-available-memory
  36       - stat-disk-caches
  37
  38   o A key named last-update, which contains the last stats update
  39     timestamp in seconds. Since this timestamp is generated by the host,
  40     a buggy guest can't influence its value. The value is 0 if the guest
  41     has not updated the stats (yet).
  42
  43 It's also important to note the following:
  44
  45  - Previously polled statistics remain available even if the polling is
  46    later disabled
  47
  48  - As noted above, if a guest doesn't support a particular stat its value
  49    will always be -1. However, it's also possible that a guest temporarily
  50    couldn't update one or even all stats. If this happens, just wait for
  51    the next update
  52
  53  - Polling can be enabled even if the guest doesn't have stats support
  54    or the balloon driver wasn't loaded in the guest. If this is the case
  55    and stats are queried, last-update will be 0.
  56
  57  - The polling timer is only re-armed when the guest responds to the
  58    statistics request. This means that if a (buggy) guest doesn't ever
  59    respond to the request the timer will never be re-armed, which has
  60    the same effect as disabling polling
  61
  62 Here are a few examples. QEMU is started with '-balloon virtio', which
  63 generates '/machine/peripheral-anon/device[1]' as the QOM path for the
  64 balloon device.
  65
  66 Enable polling with 2 seconds interval:
  67
  68 { "execute": "qom-set",
  69              "arguments": { "path": "/machine/peripheral-anon/device[1]",
  70                          "property": "guest-stats-polling-interval", "value": 2 } }
  71
  72 { "return": {} }
  73
  74 Change polling to 10 seconds:
  75
  76 { "execute": "qom-set",
  77              "arguments": { "path": "/machine/peripheral-anon/device[1]",
  78                          "property": "guest-stats-polling-interval", "value": 10 } }
  79
  80 { "return": {} }
  81
  82 Get stats:
  83
  84 { "execute": "qom-get",
  85   "arguments": { "path": "/machine/peripheral-anon/device[1]",
  86   "property": "guest-stats" } }
  87 {
  88     "return": {
  89         "stats": {
  90             "stat-swap-out": 0,
  91             "stat-free-memory": 844943360,
  92             "stat-minor-faults": 219028,
  93             "stat-major-faults": 235,
  94             "stat-total-memory": 1044406272,
  95             "stat-swap-in": 0
  96         },
  97         "last-update": 1358529861
  98     }
  99 }
 100
 101 Disable polling:
 102
 103 { "execute": "qom-set",
 104              "arguments": { "path": "/machine/peripheral-anon/device[1]",
 105                          "property": "stats-polling-interval", "value": 0 } }
 106
 107 { "return": {} }