Bad NFS performance if share is loopback mounted

This document (7009907) is provided subject to the disclaimer at the end of this document.

Environment

All Linux Products
All Product versions
All Servicepacks

Situation

There is a NFS server which exports some directories. Likely the NFS Server also runs some client applications that need to access the shares that have been exported. For ease of use the exports from this NFS server are mounted by itself via NFS. A so called "loopback" NFS mount.

This looks like to work for normal conditions, but sometimes it is possible to experience bad performance during read or write to the shares. This bad performance is seen on the NFS server itself and/or on other NFS clients. The bad performance can go up to and including timeouts, locks and even deadlocks.

Generally we do NOT recommend or support this "loopback" NFS mounts.

Resolution

In Linux there is no need to loopback mount the NFS shares. Linux has the possibility to use so called "bindmounts". This bindmounts do not need to use NFS. For the NFS server this will produce less CPU usage, less memory consumption, less I/O waits, etc. etc.

To use a bindmount simply use the mount option "bind"

mount -o bind /myexport/myshare /clientmountpoint

instead of

mount -t nfs <myip>:/myexport/myshare /clientmountpoint

mount -t nfs 127.0.0.1:/myexport/myshare /clientmountpoint

The entry for a bindmount in /etc/fstab would look like

/myexport/myshare    /clientmountpoint    none    bind    0 0

Additional Information

The performance issue usually shows up in tough memory conditions. Think about following.

NFS server needs more memory
NFS server tries to flush page cache
page cache tries to free memory by flushing page cache allocated by files over NFS
NFS client tries to flush pages by writing back to NFS server. Go back to 1.

Disclaimer

This Support Knowledgebase provides a valuable tool for SUSE customers and parties interested in our products and solutions to acquire information, ideas and learn from one another. Materials are provided for informational, personal or non-commercial use within your organization and are presented "AS IS" WITHOUT WARRANTY OF ANY KIND.

Document ID:7009907
Creation Date: 19-Dec-2011
Modified Date:03-Mar-2020
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