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

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

Thank you for your reply. I agreed what you said.
And do you think we shall print a prompt to user when the size user 
specified is larger than available space ?

On 05/08/2013 02:36 PM, Fam Zheng wrote:
> I think it's the system admin to be responsible for the risk of over
> provisioning. We have host sparse file[1] (as your example) for
> preallocated image (for example, -f raw), as well as sparse image (as
> supported in qcow2, vmdk, etc.). There are cases that host file system
> is extended or the vm disk is moved to a larger storage when the actual
> data grows closer to full, so it's not very practical to limit the
> creating size, just as this is quietly valid in linux, no matter how
> small your /tmp is.
>
>      # df /tmp/ -h
>      Filesystem      Size  Used Avail Use% Mounted on
>      tmpfs           3.8G   14M  3.8G   1% /tmp
>      # touch /tmp/a
>      # truncate /tmp/a --size 10000T
>      # ls -l /tmp/a
>      -rw-rw-r--. 1 fam fam 10995116277760000 May  8 14:33 /tmp/a
>
> [1]: http://en.wikipedia.org/wiki/Sparse_file
>
> On Wed, 05/08 13:18, yuxh wrote:
>> 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
>>
>

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