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SubjectRe: fsync on large files
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.

I haven't thought a lot about tricks with mount (union filesystems, etc.)
but I'm assuming that we're not going to allow stunts like mounting a
file system in a subdirectory contained inside itself. So, you don't
have to worry once you cross the mount point, even if the mount point
has multiple identities -- hard links can't span filesystems.

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}.
> 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.]

--
Raul

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