LUKS volumes, PVE, backups and IO errors

Ever tried to copy a vzdump result containing a LUKS encrypted volume image from your PVE host to a mounted remote share (NFS, SMB, whatever)? Ever succeeded without any “I/O error”? No? Same here.

I am unsure whether the issue is a PVE internal, or more generic. However, it occurs even when trying to copy the disk image directly (bypassing vzdump). Investigation leads to unspecific results such as “probably local or remote storage hardware defect”. However, it can always be narrowed down to crypted volumes being involved, any other images, even far larger in size, do work. Maybe my usecase is not so common, and thus has probably never been part of any testing. Probably some mapper or wrapper just cannot handle the unexpected internal data structure. It actually seems to rather not be a problem of the target system, however, if you look at the surprisingly simple solution:

Workaround

Switch to FTP, done. (I wouldn’t see any benefit in using SFTP or SCP, as both CIFS and NFS are not encrypted either).

I would still like to learn the background, is it some kind of (overdone) parity check? A forgotten escape sequence? Until I know, FTP does the job.