Gluster users - Apr 2015 - PageCache in Gluster Fuse and NFS clients

Hello all,

 I am a graduate student at University of Waterloo and working on
filesystem. I have two questions about GlusterFS that I will appreciate if
anyone can comment about.

First, does GlusterFS FUSE client bypass Linux page cache mechanism(mount
with -o direct_io)? I have a machine with 64 GB of memory and when I write
a about 30GB of data into that machine (less than available memory) and
read it again, I see the reads are server through network, even though
those file could/are in the Linux page cache memory. It seems that's the
case both for NFS and FUSE clients.

My second question is about FS-Cache and if GlusterFS does inode tree cache
on its FUSE clients or not. I know it exist in NFS but is the NFS client of
GlusterFS same as regular NFS?

Thanks in advance,
-- 
Behrooz
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On Mon, Apr 27, 2015 at 02:56:14PM -0400, Behrooz Shafiee
wrote:
> Hello all,
> 
>  I am a graduate student at University of Waterloo and working on
> filesystem. I have two questions about GlusterFS that I will appreciate if
> anyone can comment about.
> 
> First, does GlusterFS FUSE client bypass Linux page cache mechanism(mount
> with -o direct_io)? I have a machine with 64 GB of memory and when I write
> a about 30GB of data into that machine (less than available memory) and
> read it again, I see the reads are server through network, even though
> those file could/are in the Linux page cache memory. It seems that's
the
> case both for NFS and FUSE clients.
Direct IO is intended to not use caches where possible. What you
describe above is the expected behaviour. You can find more details in
'man 2 open' for the O_DIRECT flag.

The Linux kernel has many tunables for the page cache. You can find many
of them under /proc/sys/vm/ , some are documented in 'man 5 proc'.
> My second question is about FS-Cache and if GlusterFS does inode tree cache
> on its FUSE clients or not. I know it exist in NFS but is the NFS client of
> GlusterFS same as regular NFS?
FS-Cache is a client-side option for some filesystems. The Linux kernel
supports FS-Cache for its NFS-client. Some work has been done to add
support for FS-Cache to FUSE, but that never became stable enough to get
included in the upstream Linux kernel. Vimal Kumar did most of the work
for that, and had a proof-of-concept. It will need updating for the
current kernel version and some stability fixes. One of the email
threads is here:

    http://thread.gmane.org/gmane.comp.file-systems.gluster.devel/7877

Vimal was planning to continue working on this, and your email is a
great reminder to get a status update from him.

Cheers,
Niels
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