On Fri, Aug 22, 2014 at 03:13:31PM +0200, Kevin Wolf wrote:
> Am 22.08.2014 um 14:25 hat Richard W.M. Jones geschrieben:
> >
> > On Mon, Jul 28, 2014 at 04:48:46PM +0800, Hu Tao wrote:
> > > ping...
> > >
> > > All the 6 patches have reviewed-by now.
> > >
> > > On Fri, Jul 11, 2014 at 02:09:57PM +0800, Hu Tao wrote:
> > > > This series adds two preallocation mode to qcow2 and raw:
> > > >
> > > > Option preallocation=full preallocates disk space for image by writing
> > > > zeros to disk, this ensures disk space in any cases.
> > > >
> > > > Option preallocation=falloc preallocates disk space by calling
> > > > posix_fallocate(). This is faster than preallocation=full.
> >
> > Sorry if this was discussed before, but why would anyone use
> > preallocation=full if preallocation=falloc was possible?
> >
> > Shouldn't preallocation=full simply use posix_fallocate if it's
> > available, and fall back to writing zeroes if not?
>
> posix_fallocate() is basically metadata preallocation on the file
> system level. If any lower levels involve allocations as well, does
> posix_fallocate() allocate them there?
It's a good question.
>From observation of ext4 using libguestfs, the answer is
(surprisingly to me) no.
Writing zeroes won't necessarily allocate either -- eg. if
detect_zeroes is enabled, or lots of other exotic storage schemes that
look into what you're writing.
I think my point still stands however. There's no point giving
callers an option. QEmu should do the best possible job. It is the
only agent in a position to know how best to fully allocate the
backing file.
Rich.
$ guestfish -N fs:ext4:1G -m /dev/sda1
Welcome to guestfish, the guest filesystem shell for
editing virtual machine filesystems and disk images.
Type: 'help' for help on commands
'man' to read the manual
'quit' to quit the shell
><fs> fallocate64 /test 100M
><fs> ll /
total 102424
drwxr-xr-x 3 root root 4096 Aug 22 13:22 .
drwxr-xr-x 19 root root 4096 Aug 22 13:21 ..
drwx------ 2 root root 16384 Aug 22 13:21 lost+found
-rw-r--r-- 1 root root 104857600 Aug 22 13:22 test
><fs> exit
$ du -sh test1.img
33M test1.img
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines. Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v
