Re: [Qemu-devel] qemu-nbd very slow, patch available

[Paolo Bonzini <pbonzini@redhat.com>]

Il 12/09/2012 10:25, Tristan Wibberley ha scritto:
> The attached diff adds a commandline option "--cache=" with four modes
> and makes no change to the default behaviour of qemu-nbd:
> 
>   --cache=writethrough [default, O_DSYNC, very slow, very resilient]
>   --cache=off [same as --nocache, O_DIRECT, slow, resilient]
>   --cache=none [O_DIRECT, O_DSYNC, very very slow, very very resilient]
>   --cache=writeback [fast, not resilient]

A very similar patch is in QEMU 1.2.0, just released last week. :)

  --cache=writethrough [default, O_DSYNC]
  --cache=none [same as --nocache, O_DIRECT]
  --cache=directsync [O_DIRECT, O_DSYNC]
  --cache=writeback

> I have not included Stephane's extra fsync operation on close because
> that would make --cache=writeback very slow on disconnect. Users can
> sync after disconnect via alternatives available on their platform.

An fsync is already done by bdrv_close; however, this only syncs just
before qemu-nbd exits, not after each and every disconnect.  I think
it's a good compromise.  The best solution would be to add support for
fsync in the NBD kernel driver, so that --cache=writeback would be just
as good.

Paolo