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.
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
Changes in v2:
- Many of the v1 patches that make sense independently have already gone
into qemu.git
- Half of the series that basically rewrites qcow2_alloc_cluster_offset()
is new. This was required in order to make the really interesting patch
in this series ("qcow2: Cancel COW when overwritten") correct.
- Addressed a few comments on v1 by Paolo and Blue
Still to do:
- Figure out what to do with failing qemu-iotests cases that rely on a
specific order in which requests complete. This series can very
obviously change that behaviour. Maybe we should redirect the output
of all qemu-io instances to /dev/null if they use AIO and the output
is non-deterministic therefore.
Kevin Wolf (23):
qcow2: Handle dependencies earlier
qcow2: Improve check for overlapping allocations
qcow2: Change handle_dependency to byte granularity
qcow2: Decouple cluster allocation from cluster reuse code
qcow2: Factor out handle_alloc()
qcow2: handle_alloc(): Get rid of nb_clusters parameter
qcow2: handle_alloc(): Get rid of keep_clusters parameter
qcow2: Finalise interface of handle_alloc()
qcow2: Clean up handle_alloc()
qcow2: Factor out handle_copied()
qcow2: handle_copied(): Get rid of nb_clusters parameter
qcow2: handle_copied(): Get rid of keep_clusters parameter
qcow2: handle_copied(): Implement non-zero host_offset
qcow2: Use byte granularity in qcow2_alloc_cluster_offset()
qcow2: Allow requests with multiple l2metas
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: Cancel COW when overwritten
qemu-iotests: Another concurrent multicluster allocation case
qcow2: Move cluster gathering to a non-looping loop
qcow2: Gather clusters in a looping loop
block.c | 5 +
block/qcow2-cluster.c | 707 ++++++++++++++++++++++++++++++++++----------
block/qcow2.c | 209 ++++++++++++-
block/qcow2.h | 110 +++++++
include/block/block_int.h | 3 +
tests/qemu-iotests/038.out | 10 +-
tests/qemu-iotests/046 | 19 ++
tests/qemu-iotests/046.out | 16 +
trace-events | 4 +
9 files changed, 905 insertions(+), 178 deletions(-)
--
1.7.6.5
