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

[yuxh]

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:
> > > > 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
> >
> > --
> > 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: address@hidden
> > --------------------------------------------------

--
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: address@hidden
--------------------------------------------------