[Bugs] [Bug 1220173] SEEK_HOLE support (optimization)

bugzilla at redhat.com bugzilla at redhat.com
Sat Jul 18 08:02:49 UTC 2015


--- Comment #44 from Christopher Pereira <kripper at imatronix.com> ---
(In reply to Deepak C Shetty from comment #42)
> (In reply to Christopher Pereira from comment #1)
> > Besides not using SEEK_HOLE/SEEK_DATA to optimize disk IO, I also discovered
> > that GB of zeros are send through the network. That hurts.
> > 
> > To reproduce:
> > 
> > - mount a remote gluster volume
> > - cp a sparse image from the mounted volume to /tmp
> cp doesn't understand qcow2 format, but if we use qemu-nbd to create a 
> local block device (say /dev/nbdX) thats backed by .qcow2 file, then
> dd (with or without conv=sparse) might work ? I m curious to know. Have you
> tried this already ?
> thanx,
> deepak

Hi Deepack,

Sparse files (or "holes") filesystem support [1] and qcow's on-demand space
allocation are two different implementations for solving the same problem.

Yes, qcow files shouldn't have this performance problem.

I wonder where I got this raw image file from.
I guess it was generated by virt-v2v while importing a vmdk image into ovirt.
I wouldn't mind if all images were in qcow2 format.

I also wonder if there is any way to convert images from raw to qcow2 in oVirt.

[1] : https://en.wikipedia.org/wiki/Sparse_file

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