----- Original Message ----- > From: "Dave Chinner" <david@xxxxxxxxxxxxx> > To: "Zach Brown" <zab@xxxxxxxxxx> > Cc: "Abhijith Das" <adas@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, "linux-fsdevel" <linux-fsdevel@xxxxxxxxxxxxxxx>, > "cluster-devel" <cluster-devel@xxxxxxxxxx> > Sent: Friday, July 25, 2014 7:38:59 PM > Subject: Re: [RFC] readdirplus implementations: xgetdents vs dirreadahead syscalls > > On Fri, Jul 25, 2014 at 10:52:57AM -0700, 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? > > The issue is not directory block readahead (which some filesystems > like XFS already have), but issuing inode readahead during the > getdents() syscall. > > It's the semi-random, interleaved inode IO that is being optimised > here (i.e. queued, ordered, issued, cached), not the directory > blocks themselves. As such, why does this need to be done in the > kernel? This can all be done in userspace, and even hidden within > the readdir() or ftw/ntfw() implementations themselves so it's OS, > kernel and filesystem independent...... > I don't see how the sorting of the inode reads in disk block order can be accomplished in userland without knowing the fs-specific topology. From my observations, I've seen that the performance gain is the most when we can order the reads such that seek times are minimized on rotational media. I have not tested my patches against SSDs, but my guess would be that the performance impact would be minimal, if any. Cheers! --Abhi -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
