Must supply ‘-P’ arg when extracting files from Thuban backup SDLT tape

When restoring files from a Thuban SDLT Backup tape (via Algol’s SDLT tape drive) you must specify the ‘-P’ argument’ to tar, like so:

tar -xvPf /dev/nst1 $RESTOREFILE

If '-P' is not specified, a random set of symlinks in the restored file set will end up being zero length regular files rather than symlinks.

Note that even when you supply the ‘-P’ option, the original date on the symlink is not restored, but is given the current date instead.

This bug doesn’t seem to manifest for trivially small backups, but definitely manifests in our Thuban backups.

This bug is apparently due to a race condition that occurs while tar is restoring the symlink.

See this website for a more in-depth explanation.