Macos – How to create hard links in OSX

Hardlinking copies of the same message is done by several IMAP servers.

On the other hand, those IMAP servers expect hardlinks. Some other Maildir programs may not. But it should not cause any damage, other than breaking the link.

Could this possibly cause problems for applications using the Maildir

Unlikely.

or could it cause applications to corrupt my mailboxes?

There is nothing that can get corrupted in a Maildir -- except possibly header caches kept by some MUAs and/or IMAP servers, but a cache is something that can be thrown away.

Or would modification of any mail files simply break any hard links associated with that file, without affecting the previous version that was hard-linked elsewhere?

Depends on how the modification is made.

• Truncate and overwrite: Works fine. Other directories have the updated message.
• Write temporary file and rename: Works, but the link is broken and updates will not be visible in other folders (which have the old file).

Although I can't imagine why would offlineimap modify a message...

There is a way to make backups like you say, but I don't believe with Robocopy alone.

There are several programs that will hard link or dedupe backups, and be space efficient.

Here are a few programs I've used or ran across:

• Dupemerge - free - While not a "backup program" it is a bulk hard linker.
• hardlink backup - free for personal use - uses hard links
• Duplicati - free - (dedupes I believe like rsync does)
• rdiff-backup - free - dedupes like rsync

I believe what may be a good fit for your questions is Dupemerge. You could have a batch file execute Robocopy to copy the contents to a directory (ex. d:\backups\YYYY-MM-DD). When that command is done, the batch file could call Dupemerge on d:\backups. That would hard link the files within and across all the subdirectories of d:\backups.

The downside of this is you would need enough space on the external to hold the newly created unhard linked backup. After Dupemerge is run, the newly created backup will be hard linked and the space will be freed.

This would make restoring easy because each dated directory would appear to have a full backup.

As a side note, if you are going to use hard links, I like Link Shell Extension. It helps let you know what isn't hard linked, what is and what it is linked to.

I’ve run across other hard link or deduping programs, but I liked these the best.