Jeffrey writes:

> Looking at the code and the structure of the storage and attrib files,
> it doesn't seem like there is any way for BackupPC to record and
> restore hard links.

Not true.  Hardlinks are stored without using hardlinks.

Hardlinks are stored just like symlinks.  The attribute type is
BPC_FTYPE_HARDLINK and the contents of the file is the path of
the file being linked to.

For example, if there are 4 links to a file, one instance (it
doesn't matter which - depends on the Xfer Method) will be stored
as a regular file.  The remaining three instances will be stored
as type BPC_FTYPE_HARDLINK with pointers to the first file.

Note that this is independent of the hardlinks used to de-duplicate.

There is one subtlety with rsync: it also needs to remember if a
regular file is the target of (other) hardlinks so that the correct
file list can be generated during a restore.  This is done by using
an extra bit in the file's mode stored in the attrib file.

This results in one subtle bug that can't be easily fixed: if you
switch the Xfer method from tar to rsync, old backups that have
hardlinks stored with tar won't be correctly restored with rsync.
The workaround is generate a tar file and extract it, or switch
the Xfer method back to tar before you do the restore.  The
opposite case should work correctly.

Craig

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