Re: [Qemu-block] [Qemu-devel] reduce write bandwidth of qcow2 driver while allocating new cluster

[Liu Qing]

On Mon, Aug 28, 2017 at 10:46:34AM -0500, Eric Blake wrote:
> [adding qemu-block]
> 
> On 08/28/2017 12:56 AM, Liu Qing wrote:
> > Dear list,
> >     Recently I used fio to test qcow2 driver in the guest os, and found out
> > that when a new cluster is allocated the 4K IO will occupy 64K(default 
> > cluster
> > size) bandwith.
> >     From the code qcow2 driver will fill the unused part of new allocated
> > cluster with 0 in perform_cow. These 0s are set in qcow2_co_readv when the 
> > read
> > destination is not allocated and it has no backing file. Could I forbidden 
> > any
> > further write in copy_sectors if the copy source is not allocated and it has
> > no backing file? So only the requested data is written to the cluster. 
> > Function
> > copy_sectors is only used by perform_cow in the master branch.
> 
> There have already been discussions on optimizing COW writes in a manner
> similar to what you are describing; for example,
> 
> https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg00109.html
Thanks Eric, this is what I am looking for.
The only concern I have is in patch '[Qemu-devel] [PATCH v4 12/15] qcow2: skip
writing zero buffers to empty' it says:

It can be detected that
  1. COW alignment of a write request is zeroes
  2. Respective areas on the underlying BDS already read as zeroes
     after being preallocated previously
  If both of these true, COW may be skipped

Will writing zero be skipped if the disk is not preallocated? @Anton

BTW: why the code in the patch is a little different than the latest
master branch? For example I don't have the is_zero function but only
get is_zero_sectors. Is there something wrong with my settings?

My repo:
# git remote -v
origin  git://git.qemu-project.org/qemu.git (fetch)
origin  git://git.qemu-project.org/qemu.git (push)

Thanks.
> 
> -- 
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3266
> Virtualization:  qemu.org | libvirt.org
>