[Qemu-devel] about qemu-img info on block dev

[Qemu-devel] about qemu-img info on block dev

[wang.guang55]

hello


if we create a qcow2 file on a block dev.


we can,t get the right disk size by qemu-img info。






[root@host-120-79 qemu]# ./qemu-img create -f qcow2 /dev/zs/lvol0 1G

Formatting '/dev/zs/lvol0', fmt=qcow2 size=1073741824 cluster_size=65536 lazy_refcounts=off refcount_bits=16

[root@host-120-79 qemu]# ./qemu-img info /dev/zs/lvol0 

image: /dev/zs/lvol0

file format: qcow2

virtual size: 1.0G (1073741824 bytes)

disk size: 0

cluster_size: 65536

Format specific information:

    compat: 1.1

    lazy refcounts: false

    refcount bits: 16

    corrupt: false






Then we get disk size from L1 table and L2 table in qcow2_get_allocated_file_size.


May be we can submit the patch to qemu master??

Re: [Qemu-devel] about qemu-img info on block dev

[Kevin Wolf]

Am 07.11.2017 um 08:18 hat wang.guang55@zte.com.cn geschrieben:
> hello
> 
> 
> if we create a qcow2 file on a block dev.
> 
> 
> we can,t get the right disk size by qemu-img info。
> 
> 
> 
> 
> 
> 
> [root@host-120-79 qemu]# ./qemu-img create -f qcow2 /dev/zs/lvol0 1G
> 
> Formatting '/dev/zs/lvol0', fmt=qcow2 size=1073741824 cluster_size=65536 lazy_refcounts=off refcount_bits=16
> 
> [root@host-120-79 qemu]# ./qemu-img info /dev/zs/lvol0 
> 
> image: /dev/zs/lvol0
> 
> file format: qcow2
> 
> virtual size: 1.0G (1073741824 bytes)
> 
> disk size: 0
> 
> cluster_size: 65536
> 
> Format specific information:
> 
>     compat: 1.1
> 
>     lazy refcounts: false
> 
>     refcount bits: 16
> 
>     corrupt: false
> 
> 
> 
> 
> 
> 
> Then we get disk size from L1 table and L2 table in qcow2_get_allocated_file_size.
> 
> 
> May be we can submit the patch to qemu master??

No, qcow2 is the wrong layer to do this. If anything,
raw_get_allocated_file_size() could be changed to return the full file
size for block devices. However, even this is not necessarily the right
number, the block device could actually be thin provisioned and the
kernel doesn't tell us how many blocks are actually allocated. So I
think 0 for "we don't know" is in fact the right answer.

Kevin