Re: Recursive directory accounting for size, ctime, etc.

J. Bruce Fields
2008-07-15T20:48:24Z

On Tue, Jul 15, 2008 at 01:41:25PM -0700, Sage Weil wrote:
> On Tue, 15 Jul 2008, J. Bruce Fields wrote:
> > > - There is some built-in delay before statistics fully propagate up
> > > toward the root of the hierarchy. Changes are propagated
> > > opportunistically when lock/lease state allows, with an upper bound of (by
> > > default) ~30 seconds for each level of directory nesting.
> >
> > That makes it less useful, e.g., for somebody with cached data trying to
> > validate their cache, or for something like git trying to check a
> > directory tree for changes.
>
> Having fully up to date values would definitely be nice, but unfortunately
> doesn't play nice with the fact that different parts of the directory
> hierarchy may be managed by different metadata servers. A primary goal in
> implementing this was to minimize any impact on performance. The uses I
> had I mind were more in line with quota-based accounting than cache
> validation.

Fair enough.

> I think I can adjust the propagation heuristics/timeouts to make updates
> seem more or less immediate to a user in most cases, but that won't be
> sufficient for a tool like git that needs to reliably identify very recent
> updates. For backup software wanting a consistent file system image, it
> should really be operating on a snapshot as well, in which case a delay
> between taking the snapshot and starting the scan for changes would allow
> those values to propagate.
>
> > > - Ceph internally distinguishes between multiple links to the same file
> > > (there is a single 'primary' link, and then zero or more 'remote' links).
> > > Only the primary link contributes toward the 'rbytes' total.
> >
> > Is that only true for 'rbytes'?
>
> The same goes for rctime. As far as the recursive stats go, the other
> stats (file/directory counts) aren't affected. The primary/remote
> hard link distinction is fundamental to the way metadata is internally
> managed and stored by the MDS, though, if that's what you mean (inode
> content is embedded with the primary link's directory metadata).

I just wonder how one would explain to users (or application writers)
why changes to a file are reflected in the parent's rctime in one case,
and not in another, especially if the primary link is otherwise
indistinguishable from the others. The symptoms could be a bit
mysterious from their point of view.

--b.