[Qemu-devel] [RFC PATCH 0/7] Manual writethrough cache and cache mode toggle

[Paolo Bonzini <pbonzini@redhat.com>]

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(-)

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1.7.10.1