Re: HAMMER2 design in progress - 1-2 year time frame
This is mostly just to note it for later, but: BFS, used in BeOS/Haiku
does not support hardlinks either, so this isn't the first time this
problem has happened. It would cause problems in git, for instance,
if I read this correctly:
http://git.661346.n2.nabble.com/PATCH-6-6-Always-provide-a-fallback-when-hardlinks-fail-td729065.html
And here's some vague discussion about it:
http://www.freelists.org/post/haiku-development/BFSs-Lack-Of-Support-For-link-hard-links,11
We have the minor disadvantage that just because a system is DragonFly
doesn't mean that we won't support hardlinks (i.e. a hammer1 or ufs
filesystem), so it may be difficult for 3rd-party software to tell
what the correct behavior is.
It would be interesting to see if the mechanisms for deduplication and
hardlinks could be brought together to solve this; that's a vague
suggestion rather than a feasible plan, though.
On Sun, Jul 3, 2011 at 2:40 AM, Matthew Dillon
<dillon@apollo.backplane.com> wrote:
> ꀠ咗ell, the problem with hardlinks in HAMMER2 is a bit different than
> ꀠ濳agging files and directories with additional meta-data.
>
> ꀠ嗰eta-data is actually very easy to do from a cache coherency standpoint,
> ꀠ毪ut difficult to reliably copy and archive (particularly across
> ꀠ沲ilesystem types).
>
> ꀠ㗎ardlinks create multiple synchronization/coherency paths from each link
> ꀠꀨto the same file) all the way back to the root. 啱n a clustered
> ꀠ沲ilesystem with fine-grained cache coherency on the DIRECTORY TOPOLOGY,
> ꀠ炻erses just the file inode, this can wind up being O(n log n) in overhead,
> ꀠ乸pproximately, for each system call operating on the file (where (n) is
> ꀠ濳he number of hard links). 嘭erformance suffers badly and the cache
> ꀠ氲oherency algorithms also wind up being far more complex... too complex
> ꀠ濳o be desireable, really.
>
> ꀠ啱'll continue to think about how at least a small number of hardlinks
> ꀠ氲ould be implemented without turning already complex cluster algorithms
> ꀠ湶nto a disaster zone. 嚒o far I'm drawing a blank.
>
> ꀠꀠꀠꀠꀠꀠꀠꀠꀠꀠꀠꀠꀠꀠꀠꀠꀠꀠꀠꀠꀠꀠꀠꀭMatt
>
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