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.