On Thu, Jul 18, 2024 at 07:33:14PM +0200, Jakob Bohm via wrote: > > > > I accidentally delete the backing file. It was a very old > > > > 200GB file from 2018. The snapshot is 1 TB and I can't use it. [...] > > > > Is there something I can do to recover the files in the snapshot? > > > Well, you deleted all the virtual disk sectors that were > > > unchanged since the backing file. > > > > > > To get back all disk sectors changed since the backing file, > > > just repoint the qcow2 image to an all zero backing file . > > > > Interesting, it's an idea I suggested on IRC (where Leonel came > > first), but someone mentioned it would not work. > > Will snapshot apply the diff "blindly", without checking src block > > (in backing file) was the one expected? > > I believe qcow2 itself has no information about the expected sector > contents, but the file system and applications in the virtual > machine might.
A qcow2 file is a collection of data clusters of the same size (you can see the size with 'qemu-img info'). Each cluster can be allocated (it contains the actual data) or unallocated (the data is read from the backing file, or is zero if there is no backing file). There are no checksums or any kind of information about the expected contents, you can actually replace one backing file with a different one and QEMU won't notice (your virtual machine will, of course). If you deleted the backing file then the clusters that were unallocated in the active file are now zeroes. That's the data that you lost. You can see the affected clusters with 'qemu-img map' but there is no way to recover that information at the QEMU / qcow2 level. Berto
