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


From: yuxh
Subject: Re: [Qemu-devel] qemu-img problem when create a file larger than fs's size
Date: Wed, 08 May 2013 15:34:07 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.5) Gecko/20120607 Thunderbird/10.0.5


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

Thank you.

Best Regards
Xinghai Yu



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第一软件事业部 第一开发部 driver组 于星海
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Yu Xinghai
Development Dept.I
Nanjing Fujitsu Nanda Software Tech. Co., Ltd.(FNST)
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