Hello list, I am analyzing unexpected behaviour around incremental tar backups which were so far only seen when
1) running --listed-incremental with copying indices (level N-1 to N as described in tar manpage) of "/home" directory. 2) creating such incremental backups of data from "/home" while a btrfs "subvolid=0" mount was active for a completely unrelated path other than "/home" but affecting the same file system similar to bind mounts (mount -o subvolid=0 /dev/sda1 /mnt) and btrfs snapshots being created/removed there. This btrfs related condition is not proven yet as there was no way to cleanly reproduce the issue yet but I could not think of anything else special going on and affecting the file system around that time. The outcome was that incremental backups started to be identical to full backups in size while usually they are below 3% of full backup size. If the btrfs interaction is really the reason, the issue might be due to btrfs breaking some (inofficial) API assumptions tar is relying on, or maybe even the Linux kernel 5.10 file system API. The reason for asking here is that the only hint to the underlying reason so far is the structure of the tar index files, which is completely different whenever the problem occured. A sequence of "normal" indices would start like that GNU tar-1.34-2 1630841185 9004864 1367129626 208105868 180302 ./[dir-a] GNU tar-1.34-2 1630941984 24586310 1367129626 208105868 180302 ./[dir-a] while when unknown event causes the messup the index starts with GNU tar-1.34-2 1630739742 308717233 1531516583 605322568 178790 ./[dir-b] Both [dir-a] and [dir-b] were present from the first full backup on and where never modified or changed since then, only the access time was updated. Does tar rely on a "constant" ordering of files and directories from "readdir" to match them up against the index files? As btrfs might have copy-on-write interaction for quite a lot of structures (maybe also directory entries) the readdir order may magically change due to snapshot processing due to internal restructuring. Any other ideas that may cause such effects? Kind regards, hd
