Am 12.10.2011 12:39, schrieb Stefan Hajnoczi: > On Tue, Oct 11, 2011 at 03:46:28PM +0200, Kevin Wolf wrote: >> Am 07.10.2011 17:49, schrieb Stefan Hajnoczi: >>> Image streaming copies data from the backing file into the image file. It >>> is >>> important to represent zero regions from the backing file efficiently during >>> streaming, otherwise the image file grows to the full virtual disk size and >>> loses sparseness. >>> >>> There are two ways to implement zero write detection, they are subtly >>> different: >>> >>> 1. Allow image formats to provide efficient representations for zero >>> regions. >>> QED does this with "zero clusters" and it has been discussed for qcow2v3. >>> >>> 2. During streaming, check for zeroes and skip writing to the image file >>> when >>> zeroes are detected. >>> >>> However, there are some disadvantages to #2 because it leaves unallocated >>> holes >>> in the image file. If image streaming is aborted before it completes then >>> it >>> will be necessary to reread all unallocated clusters from the backing file >>> upon >>> resuming image streaming. Potentionally worse is that a backing file over a >>> slow remote connection will have the zero regions fetched again and again if >>> the guest accesses them. #1 avoids these problems because the image file >>> contains information on which regions are zeroes and do not need to be >>> refetched. >>> >>> This patch series implements #1 with the existing QED zero cluster feature. >>> In >>> the future we can add qcow2v3 zero clusters too. We can also implement #2 >>> directly in the image streaming code as a fallback when the BlockDriver does >>> not support zero detection #1 itself. That way we get the best possible >>> zero >>> write detection, depending on the image format. >>> >>> Here is a qemu-iotest to verify that zero write detection is working: >>> http://repo.or.cz/w/qemu-iotests/stefanha.git/commitdiff/226949695eef51bdcdea3e6ce3d7e5a863427f37 >>> >>> Stefan Hajnoczi (3): >>> block: add zero write detection interface >>> qed: add zero write detection support >>> qemu-io: add zero write detection option >>> >>> block.c | 16 +++++++++++ >>> block.h | 2 + >>> block/qed.c | 81 >>> +++++++++++++++++++++++++++++++++++++++++++++++++++++------ >>> block_int.h | 13 +++++++++ >>> qemu-io.c | 35 ++++++++++++++++++++----- >>> 5 files changed, 132 insertions(+), 15 deletions(-) >> >> It's good to have an option to detect zero writes and turn them into >> zero clusters, but it's something that introduces some overhead and >> probably won't be suitable as a default. > > Yes, this series simply has a bdrv_set_zero_detection() API to toggle it > at runtime. By default it is off to save CPU cycles. > >> I think what we really want to have for image streaming is an API that >> explicitly writes zeros and doesn't have to look at the whole buffer (or >> actually doesn't even get a buffer). > > I didn't take this approach to avoid having block drivers handle the > zero buffers that need to be allocated when the region does not cover > entire clusters. It can be done for sure but I'm not sure how to do it > nicely yet.
If I understand your QED code right, in such cases it ignores that there are some zeros that could be turned into a zero cluster. Considering this and that you always fill a buffer just to be able to check it (which is known to take considerable time from qemu-img convert experience) - how could any solution that works consistently, but requires an allocation in the block driver be less nice? Kevin