RHEL 5 NFS client `sync` mount option forces 4KiB writes

Issue

• Red Hat Enterprise Linux V5.x suffers from a performance decrease when NFS is set to mount with the sync option.

• Customers believe they need to do synchronous writes to their remote Netapp filer for application integrity reasons. On other platforms, mounting with the sync option does not come with a major performance penalty, but on their RHEL5 systems, we see a substantial impact.

• Further debug showed that the RHEL5 kernel drops the write size down to 4K when running in FILE_SYNC mode. This is not the case for later kernels (RHEL6/2.6.32, RHEL7/3.10.0) as they still send the maximum negotiated wsize when sending with FILE_SYNC set.

• What is the justification/reason for limiting the writes to such a small value in the RHEL5 kernel and is there a way to get around this limitation?

• NFS client has poor performance when sync option is used