Re: [Qemu-block] [PATCH] block/mirror: enable detect zeroes when driving mirror

[wyang]

On 2016年11月22日 18:09, Kevin Wolf wrote:
> Am 22.11.2016 um 04:45 hat wyang geschrieben:
> > On 2016年11月21日 19:34, Kevin Wolf wrote:
> > > Am 21.11.2016 um 02:24 hat Yang Wei geschrieben:
> > > > In order to preserve sparse disk image, detect_zeroes
> > > > should also be enabled when bdrv_get_block_status_above()
> > > > returns BDRV_BLOCK_DATA
> > > >
> > > > Signed-off-by: Yang Wei <address@hidden>
> > > Just preserving sparseness is exactly why bdrv_get_block_status_above()
> > > is checked, and this happens always.
> > I have a sparse qcow2 image(virtual size: 20G disk size: 12G),
> > # qemu-img info 20G.img
> > image: 20G.img
> > file format: qcow2
> > virtual size: 20G (21474836480 bytes)
> > disk size: 12G
> > cluster_size: 65536
> > Format specific information:
> >      compat: 1.1
> >      lazy refcounts: false
> >      refcount bits: 16
> >      corrupt: false
> >
> > and  I tried to qemu-img map --output json xxxx.img, as you can see,
> > the field of data always is true.
> Did you use metadata preallocation? If so, you explicitly requested all
> clusters to be allocated.

Yes,  I created the image with metadata preallocation. I see. its why 
qcow2_get_cluster_type always return QCOW2_CLUSTER_NORMAL.

> > > detect_zeroes does not preserve sparseness, but detect zeroes in parts
> > > of the source image that are not sparse, and make them sparse in the
> > > target image. This means that it has higher overhead (because all
> > > sectors need to be processed before copying them) and doesn't result in
> > > an exact copy. There may be cases where this is wanted, but in the
> > > common case, it's probably not the right mode of operation.
> > To be frank, customers are more concerned about sparseness than overhead.
> All generalisations are wrong.
>
> We could be talking about the majority of users (the QEMU project
> doesn't have customers),

The majority of users.:-) forgive my word.
>   but even then I doubt you're right, especially
> when using preallocation which is a tool for giving up sparseness in
> favour of improved performance of the first write to a cluster.

This patch is able to handle this case.  In fact I only hope to use 
preallocation to improve performance rather than become a sparseness 
image while live migrating.
it would waste a large mount of disk space, as most of our VM's data 
disk(/dev/vdb) is larger than 1TB. As you said,  We can use 
blockdev-mirror and manually enable detect-zeros
on new qemu verison(>2.6), but for less than 2.6, we also need to do it 
in drive-mirror.

> > > If you really want to detect zeroes, create the target block driver node
> > > manually (with detect-zeroes=on) and use the blockdev-mirror QMP command
> > > to mirror to it.
> > the target block is created by **bdrv_img_create** and opened by
> > **bdrv_open** inside qmp_drive_mirror, it also mean than we only
> > enable detect-zeroes while
> > its opened, i don't find qmp cmd to only enable it, if I understand
> > correctly.
> That's why I said you use blockdev-mirror instead drive-mirror.
Got it,  unfortunately  my qemu version does not contain this command. 
in order to use it, I also need to modify my libvirt.  its not 
temporarily available to do this. I would consider the my next release
to do it.:-)
> Kevin