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Comment by vvhn | Hacker News Reader

the hard linking of directories is a hack not really a feature. There's a good reason why filesystems ( including HFS+ when not being used for Time Machine) do not do it.

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It's a hack, but it's also the best (only?) way to deduplicate all that metadata without breaking the ability to access the history using pre-existing filesystem operations.

Volume snapshots are a better way. The only filesystems I know of that support them are ZFS, BtrFS, and HAMMER. I'm not sure if snapshots are implemented using hard links or if they are more fundamental.

Another way, which is worse, is block-level deduplication. All of the above filesystems support it, as does NTFS.

I wish Apple would adopt HAMMER for Mac OS. It is BSD-licensed and more suitable for a memory-constrained environment than ZFS.

I'm not aware of any volume snapshot system that presents the history as a plain directory tree. At best, you can use some special mount options to mount a snapshot, but you generally have to use a special tool particular to that filesystem in order to access a snapshot, and creating the snapshot always requires such a tool. There's no standard Unix way to create or access volume snapshots. Time Machine histories can be created, accessed, and analyzed entirely with standard tools except for the modified `ln`.

Btrfs snapshots are implemented using COW-shared trees (multiple trees for data and metadata). They can't be implemented with hardlinks, that would be a very leaky abstraction.

Hm, how about nilfs2?

http://www.nilfs.org

Why is block level deduplication worse?

Why would deduplicating the metadata be important for this use case? Static amount of available inodes on some filesystems is the only reason I can think of.

Because of the aforementioned problem where it takes 100MB and the better part of an hour just to record a new snapshot when literally nothing has changed since the previous snapshot. Deduplicating gets rid of that 100MB overhead, and being able to explicitly do it with hard links instead of having to rely on the filesystem to discover the duplication on its own takes care of most of the running time overhead.

(How) would AuFS fit into this? I've only come across it with Docker.io, but it sounds like it handles incremental backups?

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