[Qemu-devel] [RFC PATCH 00/16] qcow2: Delayed COW

[Kevin Wolf <kwolf@redhat.com>]

During the last few releases we have got rid of most of the overhead of
metadata writes during cluster allocation. What's left is the COW for
unaligned allocating write requests, and it's quite expensive.

In the general case, this cost cannot be avoided. However, if we're
lucky enough that before the next flush the data copied during COW
would be overwritten, we can do without the COW. Sequential writes
always overwrite the COW area at the end of the cluster immediately,
so delaying the COW a bit and cancelling it if it's overwritten is a
worthwhile optimisation.


The really interesting part of this series should be close to final;
however, you only see the improvements with the last patch applied,
which isn't quite correct yet. Doing it right requires some additional
refactoring, so I thought I'd get this out for a first round of review
before fixing it.


iozone results with and without this series show significant difference
for allocating writes:

                                                         random  random
           KB  reclen   write rewrite    read    reread    read   write

base    65536       8    1727    1945    12546    12539    2449    1836
patch   65536       8    1934    1949    12263    12521    2463    1796

base  1048576     256   22344   38437   105823   106135   37743   32167
patch 1048576     256   35989   38542   105231   105994   38301   33036


Kevin Wolf (16):
  qcow2: Round QCowL2Meta.offset down to cluster boundary
  qcow2: Introduce Qcow2COWRegion
  qcow2: Allocate l2meta dynamically
  qcow2: Drop l2meta.cluster_offset
  qcow2: Allocate l2meta only for cluster allocations
  qcow2: Enable dirty flag in qcow2_alloc_cluster_link_l2
  qcow2: Factor out handle_dependencies()
  qcow2: Reading from areas not in L2 tables yet
  qcow2: Move COW and L2 update into own coroutine
  qcow2: Delay the COW
  qcow2: Add error handling to the l2meta coroutine
  qcow2: Handle dependencies earlier
  qcow2: Change handle_dependency to byte granularity
  qcow2: Execute run_dependent_requests() without lock
  qcow2: Cancel COW when overwritten
  [BROKEN] qcow2: Overwrite COW and allocate new cluster at the same
    time

 block.c               |    5 +
 block/qcow2-cluster.c |  432 ++++++++++++++++++++++++++++++++++++++-----------
 block/qcow2.c         |  239 +++++++++++++++++++++++-----
 block/qcow2.h         |  153 +++++++++++++++++-
 block_int.h           |    3 +
 5 files changed, 692 insertions(+), 140 deletions(-)

-- 
1.7.6.5