[Qemu-devel] [Bug 1338957] Re: RFE: add an event to report block devices watermark

[Francesco Romani]

patch posted on qemu-devel, reviewd, acked and merged into maintainer's branch:
https://github.com/stefanha/qemu/commit/f050ea639522e9dd7e501ef285a2a12709b8726a

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Title:
  RFE: add an event to report block devices watermark

Status in QEMU:
  New

Bug description:
  Add an event to report if a block device usage exceeds a threshold.
  The threshold should be configurable with a monitor command. The event
  should report the affected block device. Additional useful information
  could be the offset of the highest sector , like in the query-
  blockstats output.

  Rationale for the RFE
  Managing applications, like oVirt (http://www.ovirt.org), make extensive use 
of thin-provisioned disk images.
  In order to let the guest run flawlessly and be not unnecessarily paused, 
oVirt sets a watermark and automatically resized the image once the watermark 
is reached or exceeded.

  In order to detect the mark crossing, the managing application has no choice 
than aggressively polling the QEMU monitor
  using the query-blockstats command. This lead to unnecessary system load, and 
is made even worse under scale: scenarios
  with hunderds of VM are becoming not unusual.

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