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

[Fam Zheng]

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.

> 
> 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
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-- 
Fam