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

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

On 05/08/2013 04:20 PM, Fam Zheng wrote:
> On Wed, 05/08 15:34, yuxh wrote:
>>
>> 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 ?
>
> Might be nice, but it's hard to define "available space", and "available
> space" at creating time promises nothing. Even if the user creates a
> small image which seems safe, risk is still there: when other
> applications' data fills up the "available space", the use still
> experiences the same problem.
>

Yes. That means the prompt means nothing. Thank you very much!

yuxh

>>
>> 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
>>>>
>>>
>>
>> --
>> 以上
>>
>> 第一软件事业部 第一开发部 driver组 于星海
>> Best Regards
>> --------------------------------------------------
>> Yu Xinghai
>> Development Dept.I
>> Nanjing Fujitsu Nanda Software Tech. Co., Ltd.(FNST)
>> No.6 Wenzhu Road, Nanjing, 210012, China
>> TEL: +86+25-86630566-8533
>> FUJITSU INTERNAL: 7998-8533
>> FAX: +86+25-83317685
>> MAIL: yuxinghai@cn.fujitsu.com
>> --------------------------------------------------
>

-- 
以上

第一软件事业部 第一开发部 driver组 于星海
Best Regards
--------------------------------------------------
Yu Xinghai
Development Dept.I
Nanjing Fujitsu Nanda Software Tech. Co., Ltd.(FNST)
No.6 Wenzhu Road, Nanjing, 210012, China
TEL: +86+25-86630566-8533
FUJITSU INTERNAL: 7998-8533
FAX: +86+25-83317685
MAIL: yuxinghai@cn.fujitsu.com
--------------------------------------------------