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SubjectRe: fsync on large files


On Thu, 18 Feb 1999, Raul Miller wrote:

> Alexander Viro <viro@math.psu.edu> wrote:
> > OK, so you are doing lookup on foo from /bar/baz. You see that foo is a
^^^^^^
> > directory and it has another parent. You see that this parent is nowhere
> > near dcache/icache. Worse yet, actually it's the end of *long* chain of
> > directories and none of them (except the root, that is) is in dcache. To
> > make it even nastier, suppose that there are fan-ins in that chain (i.e
> > some of those directories also have multiple parents). Your actions?

> Bring at least one instance of each parent into dcache. I don't think
> this should be significantly worse than having a long path for the first
> reference to a directory.
Erm??? *Their* parents should come into dcache too.

> Note that I'm presuming that, within the filesystem, each directory must
> have an internal unique id (perhaps block # -- if the file system doesn't
> have inodes), and that the test you want to perform is: directory being
> renamed is not in the set of {target directory, ancestors of target
> directory in this same filesystem}.

*All* ancestors, right? How would you recalculate this set on
rename/rmdir/unlink? And besides, it's O(n^2) memory (n - number
of inodes in the game) in the worst case, O(n*depth) in best.

> > Now, assume that another lookup goes through the alternative path in the
> > same time from the other direction. Your actions wrt locking?
>
> Use a per-filesystem directory rename lock when renaming a directory
> and parallel parents are an issue. [I guess, to be safe, we'd have to
> use the lock to determine if parallel parents are an issue.]

Erm... You'll need to do it on any lookup. Right now we lock the
parent on each lookup step and release it before the next one. It is not
rename-specific. Locking issues will become very tricky unless you are
going to throttle lookups in the same way it is done with rename. I.e.
single-threaded fs. Welcome back to Minix. Locking is used *not* only for
rename/rename race prevention. Without it you'll get a race in about any
pair of namespace-related operations.


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