On Wed, Oct 12, 2011 at 12:03 PM, Kevin Wolf <kw...@redhat.com> wrote: > 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?
The fallback is easy when you already have a buffer - just do the write :). My point is that this patch is the simplest approach. Other approaches can optimize better and the question is whether they are worth doing. Stefan