[Bugs] [Bug 1234096] New: rmtab file is a bottleneck when lot of clients are accessing a volume through NFS

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1234096

            Bug ID: 1234096
           Summary: rmtab file is a bottleneck when lot of clients are
                    accessing a volume through NFS
           Product: GlusterFS
           Version: 3.6.3
         Component: nfs
          Keywords: Triaged
          Severity: urgent
          Assignee: ndevos at redhat.com
          Reporter: ndevos at redhat.com
                CC: bugs at gluster.org, gluster-bugs at redhat.com
        Depends On: 1169317



+++ This bug was initially created as a clone of Bug #1166862 +++

Description of problem:

This feature: http://review.gluster.org/#/c/4430/

Create bottleneck when several clients are accessing a nfs volume.

On our setup:

Gluster 3.5.2 under centos7.

Hardware:

    dual Xeon® CPU E5-2640
    64GB RAM
    SSD for rootfs
    10Gb NIC

Context:

    Around 700 nfs clients for small file or vm images.


Version-Release number of selected component (if applicable):

3.5.2

How reproducible:

Always as long as you have enough NFS clients

Steps to Reproduce:
1. Create a volume accessible through gluster nfs
2. Make it accessible for 700 clients
3. See how it hangs

Actual results:

NFS client got intermittent hang (every minutes / for 10s each time). Even an
“rpcinfo -t server nfs 3" will hang.

Gluster nfs process literally eat the CPU of the server

Expected results:

No hanging

Additional info:

The cause:

the rmtab file located in /var/lib/glusterd/nfs/ is flushed from memory to 
/var/lib/glusterd/nfs/rmtab.tmp. During this time, NFS server hang literraly.

Workaroud:

Move the file to memory for faster I/O using this option:

set nfs.mount-rmtab: /dev/shm/glusterfs.rmtab

Result:

We still have some hang but for ~300ms now, the load average of the server is
WAY better.

Personal thought:

This feature is not usable and should be disabled by default.

You can find attached load average and Disk usage before and after using SHM
for rmtab.


Referenced Bugs:

https://bugzilla.redhat.com/show_bug.cgi?id=1169317
[Bug 1169317] rmtab file is a bottleneck when lot of clients are accessing
a volume through NFS
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