Re: [1/8] readdir-plus system call

[Abhijith Das <adas@redhat.com>]

Hi Boaz/All,

----- Original Message -----
> From: "Boaz Harrosh" <bharrosh@panasas.com>
> To: "Steven Whitehouse" <swhiteho@redhat.com>, "Steve Dickson" <steved@redhat.com>, "Jeff Layton"
> <jlayton@redhat.com>, lsf-pc@lists.linux-foundation.org, "linux-fsdevel" <linux-fsdevel@vger.kernel.org>, "Ganesha
> NFS List" <nfs-ganesha-devel@lists.sourceforge.net>, "Frank S Filz" <ffilz@us.ibm.com>, "J. Bruce Fields"
> <bfields@redhat.com>, "Jim Lieb" <jlieb@panasas.com>, "Venkateswararao Jujjuri" <jvrao@linux.vnet.ibm.com>, "DENIEL
> Philippe" <philippe.deniel@cea.fr>
> Sent: Monday, April 8, 2013 5:22:46 AM
> Subject: [1/8] readdir-plus system call
> 
> By: Steven Whitehouse <swhiteho@redhat.com>)
> 
> I repeat below Steve's original mail. Steve you said you have
> some experimental code, could you post an header and a git URL
> so we can have a look?

The patchset I'm working on is in a local tree, but the latest bits are available in this Red Hat Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=850426#c14

>From a GFS2 perspective, the need for such a system call arose from our talks with Samba folks to better support clustered samba over GFS2. The system call simply collects dirents along with stat and extended attributes and copies the info out to the user buffer. This patchset is a first-attempt at tackling this problem from a GFS2 perspective and is mainly a way to get us talking about possible implementations.

As the patches stand right now, the VFS bits are just hooks and all the real work is done in the GFS2 filesystem. However, there are some bits that could be moved into the VFS so other filesystems can utilize them.

For obtaining stat info, I'm making use of VFS bits of the xstat and fxstat system calls that David Howells proposed here : https://lists.samba.org/archive/samba-technical/2012-April/082906.html

There are 4 parts to my readdirplus (xgetdents()) patches:

Patch 1of4 adds the xgetdents() syscall interface, xreaddir() f_op and the linux_xdirent structure that specifies how the collected data is packaged to the user. From the caller's perspective, it behaves very much like the getdents() syscall except for the -EAGAIN return code. This would require the caller to re-issue the syscall with the same parameters.

Patch 2of4 is a gfs2 patch that adds a data structure that is a resizeable buffer backed by a vector of pages. This is used to collect all the intermediate data before writing it out to the user buffer.

Patch 3of4 is a simple port of the sort() function from lib/sort.c called ctx_sort(). Only difference is that it takes an additional (void *) opaque context pointer and passes it to the compare() and swap() functions. I needed this to be able to sort pointers stored in the vector of pages buffer.

Patch 4of4 has GFS2's implementation of the xreaddir() f_op and all its supporting functions. gfs2_xreaddir() tries to collect the requested data efficiently by ordering disk block accesses based on the filesystem's on-disk layout and also by adjusting the resizeable buffer as needed.

In my quick testing with a 50,000 file directory, xgetdents() is at least twice as fast as getdents()+stat()+getxattr() with a cold cache and nearly thrice as fast when the disk blocks have been cached.

Cheers!
--Abhi