[Libguestfs] [PATCH] Don't use libvirt for volume information when converting with libvirtxml
Daniel P. Berrange
berrange at redhat.com
Wed Jan 26 15:29:10 UTC 2011
On Wed, Jan 26, 2011 at 03:00:32PM +0000, Richard W.M. Jones wrote:
> On Wed, Jan 26, 2011 at 02:34:49PM +0000, Matthew Booth wrote:
> > Unless you're lucky, the target volumes won't be in a storage pool, so libvirt
> > will return an error. For libvirtxml we know that the storage is local, so we
> > can inspect it directly. We use qemu-img to determine format and file size.
> [...]
> > + # Use the output of qemu-img to inspect the path
> > + open(my $qemuimg, '-|', 'env', 'LANG=C', 'qemu-img', 'info', $path)
> > + or die("Unable to execute qemu-img: $!");
>
> You need to make sure qemu-img is an explicit Requires in the
> spec file, ie:
>
> Requires: /usr/bin/qemu-img
>
> and it should be mentioned in the README file too for other distros.
> Also other distros name this differently, eg: 'kvm-qemu-img' and maybe
> other variations.
>
> > + # For $usage, $is_sparse and $is_block, we need to know if it's a block
> > + # device
> > + # N.B. qemu-img's 'disk size' output isn't useful here
> > + my ($usage, $is_sparse, $is_block);
> > + if (-b $path) {
> > + $is_block = 1;
> > + $usage = $size;
> > + $is_sparse = 0;
> > + } else {
> > + $is_block = 0;
> > + my $st = stat($path);
> > + $usage = $st->blocks * 512;
> > + $is_sparse = $usage < $size ? 1 : 0;
> > + }
> > +
> > + die("size ($size) < usage ($usage)") if $size < $usage;
>
> Agreed that qemu-img gets it wrong:
>
> # qemu-img info /dev/vg_pin/CentOS5x64
> image: /dev/vg_pin/CentOS5x64
> file format: raw
> virtual size: 10G (10737418240 bytes)
> disk size: 0
>
> *but* I'm not sure your code is correct. I don't know if $st->blocks
> is always valid, and you can't assume 512-sized blocks.
For files 'st_blocks' always refers to number of 512 byte blocks,
regardless of whether the underlying FS/storage is actually using
512 byte blocks for storing the data. So hardcoding 512 is fine
> I suggest calling 'blockdev --getsize64', with the caveats above about
> dependencies and README file.
No need for calling blockdev actually, the quick way to determine
the size of a block device is to just seek to the end of the device
and then query the file pointer position, eg
open DEV, "/dev/sda";
seek DEV, 0, SEEK_END;
$size = tell DEV;
close DEV;
Regards,
Daniel
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