[pve-devel] vma storage format

Andreas Steinel a.steinel at gmail.com
Mon Sep 28 19:17:48 CEST 2015

Hi all,

I had a look at the vma format description file in the qemu git and
like to know if the block data stored in the file is 4K aligned or
not. There is only stated that 'The extend header if followed by the
actual cluster data, where we only store non-zero 4K blocks.'

I assume it is not aligned, yet it would be great if it were so.
Deduplication of uncompressed backup files would be greatly improved
by this.

I tried to find the "real" vma code for this to get a glimpse how this
data is really stored in the vma file, yet I didn't find it in the git
repository. I only found an embedded src.tar.gz and none of the
patches in debian/patches implied the whole vma content, only patches
for vma.

I cannot say for sure, but would it be possible to pad the data in
some way to get 4K block aligned backup data without destroying
backward compatibility? I'd really want to add the requirement of 4K
block aligned storage in backup.txt which would really be another big
advantage of the vma backup mechanism.

Best regards,
Andreas Steinel

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