[Cluster-devel] [RFC] readdirplus implementations: xgetdents vs dirreadahead syscalls

[Zach Brown]

On Fri, Jul 25, 2014 at 07:08:12PM +0100, Steven Whitehouse wrote:
> Hi,
> 
> On 25/07/14 18:52, Zach Brown wrote:
> >On Fri, Jul 25, 2014 at 01:37:19PM -0400, Abhijith Das wrote:
> >>Hi all,
> >>
> >>The topic of a readdirplus-like syscall had come up for discussion at last year's
> >>LSF/MM collab summit. I wrote a couple of syscalls with their GFS2 implementations
> >>to get at a directory's entries as well as stat() info on the individual inodes.
> >>I'm presenting these patches and some early test results on a single-node GFS2
> >>filesystem.
> >>
> >>1. dirreadahead() - This patchset is very simple compared to the xgetdents() system
> >>call below and scales very well for large directories in GFS2. dirreadahead() is
> >>designed to be called prior to getdents+stat operations.
> >Hmm.  Have you tried plumbing these read-ahead calls in under the normal
> >getdents() syscalls?
> >
> >We don't have a filereadahead() syscall and yet we somehow manage to
> >implement buffered file data read-ahead :).
> >
> >- z
> >
> Well I'm not sure thats entirely true... we have readahead() and we also
> have fadvise(FADV_WILLNEED) for that.

Sure, fair enough.  It would have been more precise to say that buffered
file data readers see read-ahead without *having* to use a syscall.

> doubt, but how would we tell getdents64() when we were going to read the
> inodes, rather than just the file names?

How does transparent file read-ahead know how far to read-ahead, if at
all?

How do the file systems that implement directory read-ahead today deal
with this?

Just playing devil's advocate here:  It's not at all obvious that adding
more interfaces is necessary to get directory read-ahead working, given
our existing read-ahead implementations.

- z