Re: If not readdir() then what?

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On 4/10/07, Neil Brown <[email protected]> wrote:
2/ Some data structure using an ordered search key that is based on
  the filename (e.g. a B-tree with a search key that is a hash of the
  filename).

In the first case, you just use a fixed opaque cookie for location in
a directory.
In the second you use the filename.  If the file has been deleted,
that shouldn't stop you finding the place where it would have been in
the overall sort order.
I can think of one (admittedly insane) FS that is between those two:

3/ an unsorted hash table, where each directory entry has an indirect
pointer to its neighbor in case of hash collisions.

  a
  b -> d -> c -> e
  g
  f -> x

Given 'c' as the "last" thing returned, you can hash c to find out
that you are in the bucket with 'b', but if 'c' was deleted, the best
you can do is return b twice or skip the chain entirely.  I maintain
an out-of-tree driver for such an fs (I promise I did not invent it);
the best I could come up with is to encode the hash chain index in the
top byte of f_pos.   Needless to say, readdir is very not performant
with all the seeking this hash scheme entails.

-Bob
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