[Gluster-devel] opendir/readdir helper
jdarcy at redhat.com
Fri Feb 1 13:27:22 UTC 2013
As we all know, directory-listing performance (or lack thereof) is a bit
of a sore spot for many GlusterFS users, because it's one of the few
places where FUSE really does make a difference. It will probably
always be a sore spot even with the readdirp changes that are already
under way. The next step would be to add a FUSE enhancement to "inject"
directory entries before they're requested, but that's a lot of work for
an uncertain outcome. The FUSE haters among the kernel leadership would
probably reject such changes without serious consideration, and even if
I'm wrong about that it's likely to be a long time before they make it
into the various distributions (not to mention non-Linux platform issues).
So, let's think outside the box for a bit. What about an LD_PRELOAD
helper? Believe me, I know all about the problems with LD_PRELOAD, but
I still can't think of any reasonable use case that requires readdir to
work across a fork (for example). The basic idea is that the LD_PRELOAD
would catch calls to opendir/readdir and match them against paths
matching GlusterFS volumes. If a match is found, then it would use
libgfapi to serve the results, without any FUSE involvement and with
massive prefetching goodness etc. Without a match, the helper would
naturally fall back to the system functions.
I suspect that this approach would make listings on very large single
directories many times faster than would ever be possible with FUSE.
For deeply nested directories we'd need to add some more complexity so
that we're not going through the whole connection-establishment path
(including authentication etc.) for each directory separately, but
that's all pretty well understood pain for pretty obvious gain.
Any other thoughts?
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