[Qemu-devel] qemu-img problem when create a file larger than fs's size

[yuxh <yuxinghai@cn.fujitsu.com>]

Hello all,

I have to consult you a qemu-img's problem.

Is this reasonable to create a file which is larger than the available 
size of the fs by qemu-img cmd ?

When I use qemu-img create a file which is larger than the available 
size of the fs, the creation is completed succesfully.

However when I use this file in guest as a guest's disk, and write 
beyond the size the host file can provides, the guest was paused by 
qemu-kvm or libvirt and was in maybe a infinite circle where the guest 
just can't be used except I detach the disk from guest or destroy the guest.

I read the qemu-img's code and found it just create the file as we 
required and doesn't check if the size we specify is reasonable.But this 
may let the guest in a risk of meeting the problem I describe above.

Exp:
[root@build mytest]# df -ah /mytest/
Filesystem            Size  Used Avail Use% Mounted on
/dev/sdb2             2.0G  3.1M  1.9G   1% /mytest
[root@build mytest]# qemu-img create -f raw test.raw 3G
Formatting 'test.raw', fmt=raw size=3221225472
[root@build mytest]# ls -l test.raw
-rw-r--r--. 1 root root 3221225472 May  8 12:23 test.raw
[root@build mytest]#

Thank you.

Best Regards
Xinghai Yu