Re: [libvirt] PATCH: Disable QEMU drive caching
Daniel P. Berrange wrote:
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;
}
if (bs->backing_file[0] != '\0') {
/* if there is a backing file, use it */
bs->backing_hd = bdrv_new("");
if (!bs->backing_hd) {
fail:
bdrv_close(bs);
return -ENOMEM;
}
path_combine(backing_filename, sizeof(backing_filename),
filename, bs->backing_file);
if (bdrv_open(bs->backing_hd, backing_filename, 0) < 0)
goto fail;
}
Is what's in the latest QEMU tree. Is what you're looking at carrying a
patch, perhaps? If so, there may be a bug in the patch.
Regards,
Anthony Liguori
Daniel