Bug 1181653

Exposing the new event should be easy; the hard part will be figuring out an interface for the user to request that the event should happen.  I'm suspecting we need a new API (and thus can't rebase this to happen any sooner than RHEL 7.2), that lets a user register a size to use to trigger a threshold event.

(In reply to Eric Blake from comment #1)
> Exposing the new event should be easy; the hard part will be figuring out an
> interface for the user to request that the event should happen.  I'm
> suspecting we need a new API (and thus can't rebase this to happen any
> sooner than RHEL 7.2), that lets a user register a size to use to trigger a
> threshold event.

RHEL 7.2 should be fine for us (= oVirt/RHEV).

(In reply to Eric Blake from comment #1)
> Exposing the new event should be easy; the hard part will be figuring out an
> interface for the user to request that the event should happen.  I'm
> suspecting we need a new API (and thus can't rebase this to happen any
> sooner than RHEL 7.2), that lets a user register a size to use to trigger a
> threshold event.

Regarding the API, I'll briefly describe what oVirt currently does.

The task is done by VDSM, the oVirt node management daemon.
periodically, each disk of each VM is sampled. disk images using format != cow or which are not block devices are immediately discarded.

(pseudo-code python-ish follows)
for each disk
- grab blockInfo:
  capacity, alloc, physical = virDomainGetblockInfo(drive.path, 0)

- check if the drive should be extended or not

  def _shouldExtendVolume(self, drive, capacity, alloc, physical):
     # always use the freshest data
     nextPhysSize = physical + drive.VOLWM_CHUNK_MB * constants.MEGAB

     # NOTE: the intent of this check is to prevent faulty images to
     # trick qemu in requesting extremely large extensions (BZ#998443).
     # Probably the definitive check would be comparing the allocated
     # space with capacity + format_overhead. Anyway given that:
     #
     # - format_overhead is tricky to be computed (it depends on few
     #   assumptions that may change in the future e.g. cluster size)
     # - currently we allow only to extend by one chunk at time
     #
     # the current check compares alloc with the next volume size.
     # It should be noted that alloc cannot be directly compared with
     # the volume physical size as it includes also the clusters not
     # written yet (pending).
     if alloc > nextPhysSize:
         pause_vm_using_the_disk()
         raise Exception

     return physical - alloc < drive.watermarkLimit():

- the drive's watermarkLimit is expressed on percentage of the drive apparent
  size and possibly adjusted to accomodate
  live storage migration.

  constants:


 VOLWM_CHUNK_MB = configfile.getint('volume_utilization_chunk_mb')  # default: 1024
 VOLWM_FREE_PCT = 100 - config.getint('irs', 'volume_utilization_percent')  # default: 50
 VOLWM_CHUNK_REPLICATE_MULT = 2  # Chunk multiplier during replication

  def volExtensionChunk(drive):
    """
    Returns the volume extension chunks (used for the thin provisioning
    on block devices). The value is based on the vdsm configuration but
    can also dynamically change according to the VM needs (e.g. increase
    during a live storage migration).
    """
    if drive.isDiskReplicationInProgress():
        return drive.VOLWM_CHUNK_MB * drive.VOLWM_CHUNK_REPLICATE_MULT
    return drive.VOLWM_CHUNK_MB

  def watermarkLimit(drive):
    """
    Returns the watermark limit, when the LV usage reaches this limit an
    extension is in order (thin provisioning on block devices).
    """
    return (drive.VOLWM_FREE_PCT * volExtensionChunk(drive) *
            constants.MEGAB / 100)

HTH

Current libvirt proposal for exposing this:
https://www.redhat.com/archives/libvir-list/2015-May/msg00580.html

Now upstream in v3.2.0, culminating with this commit:

commit 91c3d430c96ca365ae40bf922df3e4f83295e331
Author: Peter Krempa <pkrempa>
Date:   Thu Mar 16 14:37:56 2017 +0100

    qemu: stats: Display the block threshold size in bulk stats
    
    Management tools may want to check whether the threshold is still set if
    they missed an event. Add the data to the bulk stats API where they can
    also query the current backing size at the same time.