Re: [Qemu-discuss] QCOW2 real hard drive image

[Mike Lovell]

On 08/28/2012 02:02 AM, Jerzy Grzegorek wrote:
> Dnia 27-08-2012 o godz. 18:52 Mike Lovell napisał(a):
> > On 08/18/2012 02:03 AM, Jerzy Grzegorek wrote:
> > > I'd like to create the smallest possible qcow2 (or other format) real
> > > hard drive image. It is possible to create it without "dirty" no used
> > > sectors in the way like this (with additional options)
> > >
> > > qemu-img convert -f raw /dev/sda -O qcow2 disk.qcow2.img
> > unfortunately, i don't think qemu-img can determine whether or not a
> > sector is 'dirty.' i'm guessing by 'dirty' you mean a sector that has
> > had data written to it but the file system on top no longer needs the
> > data because the file was removed or truncated or similar. determining
> > which sector is 'dirty' in this case would depend on knowing what
> > filesystem is used and what parts of the disk the filesystem currently
> > has allocated and what is free space. i don't think qemu-img can do this.
> >
> > you could try just adding -c to the command you listed and qemu-img will
> > create a compressed qcow2. that could reduce the size of the resulting
> > image significantly. other than that, compressing it further would take
> > additional steps.
> >
> > mike
> Thanks Mike for your reply.
> And what do you think about zeroing non used sectors before making and 
> compressing this image?
> Is it a safe and simple enough way to do it? I think it will have influence to 
> compression ratio. Am I right?
>
> Jurek

add'ing the mailing list back to the recipients.

if you can zero the sectors that the filesystem is not currently using 
then it should help the compression significantly. also, i saw some 
commits to the qemu block layer a couple months ago to handle efficient 
writing of zero-ed blocks. so if you are using a recent qemu-img and a 
block format that the driver supports the zero-ed blocks, it should 
efficiently handle those.

unfortunately, i don't have much for you by way of how to zero out those 
blocks. i don't know what file system(s) your using and this isn't 
something i have tried myself yet. i'm also a little skeptical of 
suggesting a method since i don't want to be blamed if it breaks and 
destroys your data. :)

mike