Re: [libvirt] PATCH: Disable QEMU drive caching
On Wed, Oct 08, 2008 at 11:53:14AM -0500, Anthony Liguori wrote:
Daniel P. Berrange wrote:
>On Wed, Oct 08, 2008 at 10:51:16AM -0500, Anthony Liguori wrote:
>
>>A great deal of virtualization users are doing some form of homogeneous
>>consolidation. If they have a good set of management tools or
>>sophisticated storage, then their guests will be sharing base images or
>>something like that. Caching in the host will result in major
>>performance improvements because otherwise, the same data will be
>>fetched multiple times.
>>
>
>NB, this has no impact on caching of backing files - QEMU masks out
>the O_DIRECT flag when opening the backing file
It doesn't mask out O_DIRECT, it just doesn't pass any flags to the
backing file when it opens it. IMHO, this is a bug.
Perhaps I'm interpreting the wrong bit of code, but I was looking at
QEMU's block.c in the bdrv_open2() function. The last thing it does
is this, which masks out all flags except for the open mode:
if (bs->backing_file[0] != '\0') {
if (bdrv_open(bs->backing_hd, backing_filename, flags & (BDRV_O_RDONLY |
BDRV_O_RDWR)) < 0)
goto fail;
}
Daniel
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