[libvirt] RFC: exposing qemu's block-set-write-threshold

Eric Blake eblake at redhat.com
Mon May 18 20:28:09 UTC 2015

I'm trying to wire up libvirt to do event reporting based on qemu 2.3's
BLOCK_WRITE_THRESHOLD event. Doing this will allow management
applications to do event-based notification on when to enlarge LVM (or
other) storage underlying a qcow2 volume, rather than their current
requirement to frequently poll block statistics.  But I'm stuck on the
best way to expose the new parameter:

One idea is to treat it as part of the domain XML, and have
virDomainSetBlockIoTune add one more typed parameter for a disk's
current write threshold.  Doing this could allow setting a threshold
even for an offline domain (although the threshold is only meaningful
for a running domain), but might get weird because qemu's event is
one-shot (you have to re-arm a new threshold every time an existing
threshold fires - so every time it fires, the domain XML is rewritten,
even though it is not guest-visible ABI that was changing).  At least
with this approach, it is also easy for a client to poll the current
setting of the threshold, via virDomainGetBlockIoTune.  But the
threshold isn't quite a tuning parameter (it isn't throttling how fast
the guest can write to the block device, only how full the host side can
get in order to allow transparent resizing of the host storage prior to
running out of space).

Another idea is to add a completely new API, maybe named
virDomainBlockSetWriteThreshold(virDomainPtr dom, const char *disk, long
long int threshold, unsigned int flags) (with threshold in bytes).
Here, virDomainBlockStatsFlags() could be a way to query the current
threshold.  And if desired, we could add a flag value to treat threshold
as a percentage instead of a byte value (but is 1% too large of
granularity, and how would you scale the percentage to anything finer
while still keeping the parameter as long long int rather than double?)

Of course, I'd want virConnectGetAllDomainStats() to list the current
threshold setting (0 if no threshold or if the event has already fired,
non-zero if the threshold is still set waiting to fire), so that clients
can query thresholds for multiple domains and multiple disks per domain
in one API call.  But I don't know if we have any good way to set
multiple thresholds in one call (at least virDomainSetBlockIoTune must
be called once per disk; it might be possible for my proposed
virDomainBlockStatsFlags() to set a threshold for multiple disks if the
disk name is passed as NULL - but then we're back to the question of
what happens if the guest has multiple disks of different sizes; it's
better to set per-disk thresholds than to assume all disks must be at
the same byte or percentage threshold).

I'm also worried about what happens across libvirtd restarts - if the
qemu event fires while libvirtd is unconnected, should libvirt be
tracking that a threshold was registered in the XML, and upon
reconnection check if qemu still has the threshold?  If qemu no longer
has a threshold, then libvirt can assume it missed the event, and
generate one as part of reconnecting to the domain.

Thoughts?  If I don't hear anything, I'll base my first implementation
on adding a new API, rather than reusing virDomainSetBlockIoTune.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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