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[Qemu-devel] [PATCH v2 0/7] qcow2: implement lazy refcounts optimization

From: Stefan Hajnoczi
Subject: [Qemu-devel] [PATCH v2 0/7] qcow2: implement lazy refcounts optimization
Date: Wed, 25 Jul 2012 13:21:17 +0100

This series aims to improve qcow2 performance with cache=writethrough and
cache=directsync.  In particular it reduces the impact of metadata updates for
allocating writes.

This optimization should help make qcow2 a good choice even for
cache=writethrough and cache=directsync where QED has traditionally had an
advantage due to less metadata - this allows us to converge image format
development in QEMU around the qcow2v3 format.

Allocating writes are expensive because they involve updating L2 tables and
refcount blocks.  In addition they can also cause L2 table allocation and
refcount block allocation but these remain unaffected by this optimization.

The key insight is that refcounts are not required to access data in the image.
This means that we can postpone refcount updates without violating the data
integrity guarantee that cache=writethrough and cache=directsync give.

The trade-off for postponing refcount updates is that the image may not be
completely consistent in case of power failure or crash.  If the image is dirty
then it must be repaired before performing further modifications, in other
words we need an fsck-like scan on startup.

fio benchmark results:
WRITE lazy_refcounts=off io=552650KB
WRITE lazy_refcounts=on  io=708264KB (+28%)

Disk: Intel X25-M 120GB 2nd Gen
Host file system: ext4
Host kernel: Linux 3.2.0-3-amd64 Debian

Note that these random 8k writes are to an empty qcow2 file.  This is the worst
case scenario for cache=writethrough and cache=directsync because each I/O is
likely to involve metadata I/O, including refcount updates.

$ qemu-system-x86_64 -enable-kvm -m 1024 \
                     -drive if=virtio,cache=none,file=../vm/rhel-6.img \
                     -drive if=virtio,cache=writethrough,file=a.qcow2

Job file:


 * Fixed feature bit qcow2 spec formatting [Kevin]
 * Record feature bit names in qcow2 header for better error messages [Kevin]
 * Use separate enums for different feature bit types [Kevin]
 * Check that dirty bit is still set after read-only image access [Kevin]
 * Check that the dirty flag is never set with lazy_refcounts=off [Kevin]

Stefan Hajnoczi (7):
  docs: add dirty bit to qcow2 specification
  qcow2: introduce dirty bit
  docs: add lazy refcounts bit to qcow2 specification
  qemu-iotests: ignore qemu-img create lazy_refcounts output
  qcow2: implement lazy refcounts
  qemu-io: add "abort" command to simulate program crash
  qemu-iotests: add 039 qcow2 lazy refcounts test

 block/qcow2-cluster.c        |    5 +-
 block/qcow2.c                |  121 +++++++++++++++++++++++++++++++++++++++---
 block/qcow2.h                |   21 ++++++++
 block_int.h                  |   26 ++++-----
 docs/specs/qcow2.txt         |   14 ++++-
 qemu-io.c                    |   12 +++++
 tests/qemu-iotests/031.out   |   20 +++----
 tests/qemu-iotests/036.out   |    4 +-
 tests/qemu-iotests/039       |  117 ++++++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/039.out   |   42 +++++++++++++++
 tests/qemu-iotests/common.rc |    3 +-
 tests/qemu-iotests/group     |    1 +
 12 files changed, 351 insertions(+), 35 deletions(-)
 create mode 100755 tests/qemu-iotests/039
 create mode 100644 tests/qemu-iotests/039.out


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