[Qemu-devel] [PATCH][RFC][0/2] REF+/REF- optimization

[Frediano Ziglio]

These patches try to trade-off between leaks and speed for clusters
refcounts.

Refcount increments (REF+ or refp) are handled in a different way from
decrements (REF- or refm). The reason it that posting or not flushing
a REF- cause "just" a leak while posting a REF+ cause a corruption.

To optimize REF- I just used an array to store offsets then when a
flush is requested or array reach a limit (currently 1022) the array
is sorted and written to disk. I use an array with offset instead of
ranges to support compression (an offset could appear multiple times
in the array).
I consider this patch quite ready.

To optimize REF+ I mark a range as allocated and use this range to
get new ones (avoiding writing refcount to disk). When a flush is
requested or in some situations (like snapshot) this cache is disabled
and flushed (written as REF-).
I do not consider this patch ready, it works and pass all io-tests
but for instance I would avoid allocating new clusters for refcount
during preallocation.

End speed up is quite visible allocating clusters (more then 20%).


Frediano Ziglio (2):
  qcow2: optimize refminus updates
  qcow2: ref+ optimization

 block/qcow2-refcount.c |  270 +++++++++++++++++++++++++++++++++++++++++++++---
 block/qcow2.c          |    2 +
 block/qcow2.h          |   16 +++
 3 files changed, 275 insertions(+), 13 deletions(-)