Gluster users - Nov 2013 - GlusterFS with NFS client hang up some times

I have the following setup with GlusterFS.

Server: 4
  - CPU: Intel(R) Xeon(R) CPU E5-2620 0 @ 2.00GHz
  - RAM: 32G
  - HDD: 1T, 7200 RPM (x 10)
  - Network card: 1G x 4 (bonding)
 
OS: Centos 6.4
  - File system: XFS
    > Disk /dev/sda: 1997.1 GB, 1997149306880 bytes
    > 255 heads, 63 sectors/track, 242806 cylinders
    > Units = cylinders of 16065 * 512 = 8225280 bytes
    > Sector size (logical/physical): 512 bytes / 512 bytes
    > I/O size (minimum/optimal): 512 bytes / 512 bytes
    > Disk identifier: 0x000efb6f
  - GlusterFS:  3.4.0-8.el6
  - Sysctl.conf:
    > vm.swappiness = 0
    > vm.vfs_cache_pressure = 1000
    > net.core.rmem_max = 4096000
    > net.core.wmem_max = 4096000
    > net.ipv4.neigh.default.gc_thresh2 = 2048
    > net.ipv4.neigh.default.gc_thresh3 = 4096
    > vm.dirty_background_ratio = 1
    > vm.dirty_ratio = 16

I use only default config for GlusterFS (follow
http://gluster.org/community/documentation/index.php/Getting_started_overview).
After testing between NFS client and FUSE client, I choose NFS because
the performance is much better. 

NFS mount options:
svr385-1212.localdomain:/gv0 on /glusterfs type nfs
(rw,nfsvers=3,addr=192.168.12.12)

I also use a Nagios plugin to monitor the availability the NFS mount
point by run `stat -f /glusterfs` every minute. 

The problem: Sometimes, the plugin report that 'NFS server is not
responding', because `stat -f` do not return after 2 seconds.

Nothing useful in  client machine log files.
The /var/log/glusterfs/nfs.log (http://pastebin.com/M2sicYBx) contains
some errors that I do not know how to interpreter.

So I want to ask:
  1. Why is this happening to me? How can I debug the problem?
  2. What are recommend settings for my cluster? Can I do something to
  max out the performance?

-- 
PHAM Van Diep