-Hard-link handling
-
- At the moment hardlink handling is very expensive, so it's off by
- default. It does not need to be so.
-
- Since most of the solutions are rather intertwined with the file
- list it is probably better to fix that first, although fixing
- hardlinks is possibly simpler.
-
- We can rule out hardlinked directories since they will probably
- screw us up in all kinds of ways. They simply should not be used.
-
- At the moment rsync only cares about hardlinks to regular files. I
- guess you could also use them for sockets, devices and other beasts,
- but I have not seen them.
-
- When trying to reproduce hard links, we only need to worry about
- files that have more than one name (nlinks>1 && !S_ISDIR).
-
- The basic point of this is to discover alternate names that refer to
- the same file. All operations, including creating the file and
- writing modifications to it need only to be done for the first name.
- For all later names, we just create the link and then leave it
- alone.
-
- If hard links are to be preserved:
-
- Before the generator/receiver fork, the list of files is received
- from the sender (recv_file_list), and a table for detecting hard
- links is built.
-
- The generator looks for hard links within the file list and does
- not send checksums for them, though it does send other metadata.
-
- The sender sends the device number and inode with file entries, so
- that files are uniquely identified.
-
- The receiver goes through and creates hard links (do_hard_links)
- after all data has been written, but before directory permissions
- are set.
-
- At the moment device and inum are sent as 4-byte integers, which
- will probably cause problems on large filesystems. On Linux the
- kernel uses 64-bit ino_t's internally, and people will soon have
- filesystems big enough to use them. We ought to follow NFS4 in
- using 64-bit device and inode identification, perhaps with a
- protocol version bump.
-
- Once we've seen all the names for a particular file, we no longer
- need to think about it and we can deallocate the memory.
-
- We can also have the case where there are links to a file that are
- not in the tree being transferred. There's nothing we can do about
- that. Because we rename the destination into place after writing,
- any hardlinks to the old file are always going to be orphaned. In
- fact that is almost necessary because otherwise we'd get really
- confused if we were generating checksums for one name of a file and
- modifying another.
-
- At the moment the code seems to make a whole second copy of the file
- list, which seems unnecessary.
-
- We should have a test case that exercises hard links. Since it
- might be hard to compare ./tls output where the inodes change we
- might need a little program to check whether several names refer to
- the same file.
-
- -- --
-
-
Matt McCutchen's Web Site / rsync / rsync.git / blobdiff