Re: [Qemu-devel] [PATCH v12 0/6] qcow2, raw: add preallocation=full and preallocation=falloc

[Richard W.M. Jones]

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
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