On Wed, 06 Jul 2005 23:42:50 -0700, Hans Reiser <[email protected]> said:
> Oh no, don't store the whole path, store just the parent list. This
> will make fsck more robust in the event that the directory gets
> clobbered by hardware error.
> I don't think it affects the cost of detecting cycles though, I think
> it only makes fsck more robust.
It doesn't affect the worst-case cost of detecting cycles; by necessity,
any (static [1]) directed cycle detection algorithm must take O(n) time,
n being the size of the tree (nodes + links). However, under
"reasonable assumptions" (where the reasonableness of those assumptions
may be questioned, but I think they're reasonable), I think that doing a
depth-first-search through the parent links makes the algorithm
not-too-terrible. Namely, the "reasonable assumptions" are that a node
doesn't have "too many" parents, and the filesystem hierarchy is not
"too deep".
[1] BTW, I had also previously looked at online/dynamic algorithms, for
those who are familiar with that area. The best known so far is still
O(n) worst case, but much, much smaller in "most cases".
--
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