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[Qemu-devel] [RFC PATCH 0/7] Manual writethrough cache and cache mode to
From: |
Paolo Bonzini |
Subject: |
[Qemu-devel] [RFC PATCH 0/7] Manual writethrough cache and cache mode toggle |
Date: |
Fri, 18 May 2012 16:18:25 +0200 |
This is an alternative implementation of writethrough caching. By always
opening protocols in writethrough mode and doing flushes manually after
every write, it achieves two results: 1) it makes flipping the cache mode
extremely easy; 2) it lets formats control flushes during metadata updates
even in writethrough mode, which makes the updates more efficient; 3)
it makes cache=writethrough automatically flush metadata without needing
extra work in the formats.
In practice, the performance result is a wash. I measured "make -j3
vmlinux" on a 2-core guest/4-core host, with 2GB memory in the guest
and 8GB in the host.
Performance was measured starting qemu-kvm with an empty qcow2 image,
a virtio disk and cache=writethrough (F16 installation + exploded kernel
tarball in the backing file), and the results are as follows:
without patches:
real 9m25.057s user 12m11.091s sys 3m48.281s
real 9m23.429s user 11m58.628s sys 3m47.125s
real 9m23.524s user 12m2.458s sys 3m44.722s
with patches:
real 9m25.808s user 12m16.543s sys 3m50.648s
real 9m22.711s user 12m12.172s sys 3m49.426s
real 9m21.516s user 12m18.127s sys 3m50.762s
So 1%-2% more CPU usage was measured in the guest, but that doesn't make
much sense for virtio with ioeventfd, so I assume it is all within noise.
Any opinions? Should I run any more tests, perhaps with cache=directsync?
Does performance of cache=writethrough matter much, especially if we flip
the default?
Thanks,
Paolo
Paolo Bonzini (7):
block: flush in writethrough mode after writes
block: flush in writethrough mode after snapshot operations
savevm: flush after saving vm state
block: do not pass BDRV_O_CACHE_WB to the protocol
block: copy enable_write_cache in bdrv_append
block: add bdrv_set_enable_write_cache
ide: support enable/disable write cache
block: do not handle writethrough in qcow2 caches
block.c | 20 +++++++++++++++++---
block.h | 1 +
block/qcow2-cache.c | 25 ++-----------------------
block/qcow2-refcount.c | 12 ------------
block/qcow2.c | 7 ++-----
block/qcow2.h | 5 +----
hw/ide/core.c | 18 +++++++++++++++---
savevm.c | 2 +-
9 files changed, 39 insertions(+), 51 deletions(-)
--
1.7.10.1
- [Qemu-devel] [RFC PATCH 0/7] Manual writethrough cache and cache mode toggle,
Paolo Bonzini <=
- [Qemu-devel] [RFC PATCH 2/7] savevm: flush after saving vm state, Paolo Bonzini, 2012/05/18
- [Qemu-devel] [RFC PATCH 1/7] block: flush in writethrough mode after writes, Paolo Bonzini, 2012/05/18
- [Qemu-devel] [RFC PATCH 3/7] block: do not pass BDRV_O_CACHE_WB to the protocol, Paolo Bonzini, 2012/05/18
- [Qemu-devel] [RFC PATCH 5/7] block: add bdrv_set_enable_write_cache, Paolo Bonzini, 2012/05/18
- [Qemu-devel] [RFC PATCH 4/7] block: copy enable_write_cache in bdrv_append, Paolo Bonzini, 2012/05/18
- [Qemu-devel] [RFC PATCH 6/7] ide: support enable/disable write cache, Paolo Bonzini, 2012/05/18
- [Qemu-devel] [RFC PATCH 7/7] block: do not handle writethrough in qcow2 caches, Paolo Bonzini, 2012/05/18