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

[wyang <w90p710@gmail.com>]

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 <w90p710@gmail.com>
> 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. when I migrated it with --copy-storage-all,
I get non-spareness image. I tried to read qcow2_co_get_blcok_status,  
but I did not find anything issue, if you can give a hint on the case,
i fix it.:-)

# qemu-img map --output json 20G.img
[{ "start": 0, "length": 536870912, "depth": 0, "zero": false, "data": 
true, "offset": 327680},
{ "start": 536870912, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 537264128},
{ "start": 1073741824, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 1074200576},
{ "start": 1610612736, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 1611137024},
{ "start": 2147483648, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 2148139008},
{ "start": 2684354560, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 2685075456},
{ "start": 3221225472, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 3222011904},
{ "start": 3758096384, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 3758948352},
{ "start": 4294967296, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 4295950336},
{ "start": 4831838208, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 4832886784},
{ "start": 5368709120, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 5369823232},
{ "start": 5905580032, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 5906759680},
{ "start": 6442450944, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 6443761664},
{ "start": 6979321856, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 6980698112},
{ "start": 7516192768, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 7517634560},
{ "start": 8053063680, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 8054571008},
{ "start": 8589934592, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 8591572992},
{ "start": 9126805504, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 9128509440},
{ "start": 9663676416, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 9665445888},
{ "start": 10200547328, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 10202382336},
{ "start": 10737418240, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 10739384320},
{ "start": 11274289152, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 11276320768},
{ "start": 11811160064, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 11813257216},
{ "start": 12348030976, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 12350193664},
{ "start": 12884901888, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 12887195648},
{ "start": 13421772800, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 13424132096},
{ "start": 13958643712, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 13961068544},
{ "start": 14495514624, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 14498004992},
{ "start": 15032385536, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 15035006976},
{ "start": 15569256448, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 15571943424},
{ "start": 16106127360, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 16108879872},
{ "start": 16642998272, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 16645816320},
{ "start": 17179869184, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 17182818304},
{ "start": 17716740096, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 17719754752},
{ "start": 18253611008, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 18256691200},
{ "start": 18790481920, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 18793627648},
{ "start": 19327352832, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 19330629632},
{ "start": 19864223744, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 19867566080},
{ "start": 20401094656, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 20404502528},
{ "start": 20937965568, "length": 536870912, "depth": 0, "zero": false, 
"data": true, "offset": 20941438976}]
>
> 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.
>
> 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.

Thanks
Wei
>
> Kevin
>