Re: [RFC] readdirplus implementations: xgetdents vs dirreadahead syscalls

[Andreas Dilger]

On Jul 25, 2014, at 6:38 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> 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.

Sure.

> 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......

That assumes sorting by inode number maps to sorting by disk order.
That isn't always true.

Cheers, Andreas

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