[Qemu-devel] [Bug 1126369] Re: qemu-img snapshot -c is unreasonably slow

[Stefan Hajnoczi]

Skipping the bdrv_flush() in update_cluster_refcount() gives a huge
speed-up from over 3 minutes down to <1 second.  I think Kevin already
discovered this in the past.

Now we need to figure out how to safely perform the updates without
flushing after each L2 table refcount increment.

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https://bugs.launchpad.net/bugs/1126369

Title:
  qemu-img snapshot -c is unreasonably slow

Status in QEMU:
  New

Bug description:
  Something fishy is going on with qcow2 internal snapshot creation
  times.  I don't know if this is a regression because I haven't used
  internal snapshots in the past.

  QEMU 1.4-rc2:
  $ qemu-img create -f qcow2 test.qcow2 -o size=50G,preallocation=metadata
  $ time qemu-img snapshot -c new test.qcow2
  real  3m39.147s
  user  0m10.748s
  sys   0m26.165s

  (This is on an SSD)

  I expect snapshot creation to take under 1 second.

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