I have a Ovirt cluster with 6 VM hosts and 4 gluster nodes. There are two virtualisation clusters one with two nehelem nodes and one with  four  sandybridge nodes. My master storage domain is a GlusterFS backed by a replica 3 gluster volume from 3 of the gluster nodes.  The engine is a hosted engine 3.5.1 on 3 of the sandybridge nodes, with storage broviede by nfs from a different gluster volume.  All the hosts are CentOS 6.6.

 vdsm-4.16.10-8.gitc937927.el6
glusterfs-3.6.2-1.el6
2.6.32 - 504.8.1.el6.x86_64

Problems happen when I try to add a new brick or replace a brick eventually the self heal will kill the VMs. In the VM's logs I see kernel hung task messages. 

Mar 12 23:05:16 static1 kernel: INFO: task nginx:1736 blocked for more than 120 seconds.
Mar 12 23:05:16 static1 kernel:      Not tainted 2.6.32-504.3.3.el6.x86_64 #1
Mar 12 23:05:16 static1 kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Mar 12 23:05:16 static1 kernel: nginx         D 0000000000000001     0  1736   1735 0x00000080
Mar 12 23:05:16 static1 kernel: ffff8800778b17a8 0000000000000082 0000000000000000 00000000000126c0
Mar 12 23:05:16 static1 kernel: ffff88007e5c6500 ffff880037170080 0006ce5c85bd9185 ffff88007e5c64d0
Mar 12 23:05:16 static1 kernel: ffff88007a614ae0 00000001722b64ba ffff88007a615098 ffff8800778b1fd8
Mar 12 23:05:16 static1 kernel: Call Trace:
Mar 12 23:05:16 static1 kernel: [<ffffffff8152a885>] schedule_timeout+0x215/0x2e0
Mar 12 23:05:16 static1 kernel: [<ffffffff8152a503>] wait_for_common+0x123/0x180
Mar 12 23:05:16 static1 kernel: [<ffffffff81064b90>] ? default_wake_function+0x0/0x20
Mar 12 23:05:16 static1 kernel: [<ffffffffa0210a76>] ? _xfs_buf_read+0x46/0x60 [xfs]
Mar 12 23:05:16 static1 kernel: [<ffffffffa02063c7>] ? xfs_trans_read_buf+0x197/0x410 [xfs]
Mar 12 23:05:16 static1 kernel: [<ffffffff8152a61d>] wait_for_completion+0x1d/0x20
Mar 12 23:05:16 static1 kernel: [<ffffffffa020ff5b>] xfs_buf_iowait+0x9b/0x100 [xfs]
Mar 12 23:05:16 static1 kernel: [<ffffffffa02063c7>] ? xfs_trans_read_buf+0x197/0x410 [xfs]
Mar 12 23:05:16 static1 kernel: [<ffffffffa0210a76>] _xfs_buf_read+0x46/0x60 [xfs]
Mar 12 23:05:16 static1 kernel: [<ffffffffa0210b3b>] xfs_buf_read+0xab/0x100 [xfs]
Mar 12 23:05:16 static1 kernel: [<ffffffffa02063c7>] xfs_trans_read_buf+0x197/0x410 [xfs]
Mar 12 23:05:16 static1 kernel: [<ffffffffa01ee6a4>] xfs_imap_to_bp+0x54/0x130 [xfs]
Mar 12 23:05:16 static1 kernel: [<ffffffffa01f077b>] xfs_iread+0x7b/0x1b0 [xfs]
Mar 12 23:05:16 static1 kernel: [<ffffffff811ab77e>] ? inode_init_always+0x11e/0x1c0
Mar 12 23:05:16 static1 kernel: [<ffffffffa01eb5ee>] xfs_iget+0x27e/0x6e0 [xfs]
Mar 12 23:05:16 static1 kernel: [<ffffffffa01eae1d>] ? xfs_iunlock+0x5d/0xd0 [xfs]
Mar 12 23:05:16 static1 kernel: [<ffffffffa0209366>] xfs_lookup+0xc6/0x110 [xfs]
Mar 12 23:05:16 static1 kernel: [<ffffffffa0216024>] xfs_vn_lookup+0x54/0xa0 [xfs]
Mar 12 23:05:16 static1 kernel: [<ffffffff8119dc65>] do_lookup+0x1a5/0x230
Mar 12 23:05:16 static1 kernel: [<ffffffff8119e8f4>] __link_path_walk+0x7a4/0x1000
Mar 12 23:05:16 static1 kernel: [<ffffffff811738e7>] ? cache_grow+0x217/0x320
Mar 12 23:05:16 static1 kernel: [<ffffffff8119f40a>] path_walk+0x6a/0xe0
Mar 12 23:05:16 static1 kernel: [<ffffffff8119f61b>] filename_lookup+0x6b/0xc0
Mar 12 23:05:16 static1 kernel: [<ffffffff811a0747>] user_path_at+0x57/0xa0
Mar 12 23:05:16 static1 kernel: [<ffffffffa0204e74>] ? _xfs_trans_commit+0x214/0x2a0 [xfs]
Mar 12 23:05:16 static1 kernel: [<ffffffffa01eae3e>] ? xfs_iunlock+0x7e/0xd0 [xfs]
Mar 12 23:05:16 static1 kernel: [<ffffffff81193bc0>] vfs_fstatat+0x50/0xa0
Mar 12 23:05:16 static1 kernel: [<ffffffff811aaf5d>] ? touch_atime+0x14d/0x1a0
Mar 12 23:05:16 static1 kernel: [<ffffffff81193d3b>] vfs_stat+0x1b/0x20
Mar 12 23:05:16 static1 kernel: [<ffffffff81193d64>] sys_newstat+0x24/0x50
Mar 12 23:05:16 static1 kernel: [<ffffffff810e5c87>] ? audit_syscall_entry+0x1d7/0x200
Mar 12 23:05:16 static1 kernel: [<ffffffff810e5a7e>] ? __audit_syscall_exit+0x25e/0x290
Mar 12 23:05:16 static1 kernel: [<ffffffff8100b072>] system_call_fastpath+0x16/0x1b


I am wondering if my volume settings are causing this.  Can anyone with more knowledge take a look and let me know:

network.remote-dio: on
performance.stat-prefetch: off
performance.io-cache: off
performance.read-ahead: off
performance.quick-read: off
nfs.export-volumes: on
network.ping-timeout: 20
cluster.self-heal-readdir-size: 64KB
cluster.quorum-type: auto
cluster.data-self-heal-algorithm: diff
cluster.self-heal-window-size: 8
cluster.heal-timeout: 500
cluster.self-heal-daemon: on
cluster.entry-self-heal: on
cluster.data-self-heal: on
cluster.metadata-self-heal: on
cluster.readdir-optimize: on
cluster.background-self-heal-count: 20
cluster.rebalance-stats: on
cluster.min-free-disk: 5%
cluster.eager-lock: enable
storage.owner-uid: 36
storage.owner-gid: 36
auth.allow:*
user.cifs: disable
cluster.server-quorum-ratio: 51%

Many Thanks,  Alastair