Read and write size adjustments

Some of the most useful NFS tuning options are the rsize and wsize options, which define the maximum sizes of each RPC packet for read and write, respectively.

The following reasons outline why you might want to change the read and write size values:

• The server might not be capable of handling the data volume and speeds inherent in transferring the read/write packets, which are 8 KB for NFS Version 2 and 32 KB for NFS Version 3 and NFS Version 4. This might be the case if a NFS client is using a PC as an NFS server. The PC may have limited memory available for buffering large packets.
• If a read/write size value is decreased, there may be a subsequent reduction in the number of IP fragments generated by the call. If you are dealing with a faulty network, the chances of a call and reply pair completing with a two-packet exchange are greater than if there must be seven packets successfully exchanged. Likewise, if you are sending NFS packets across multiple networks with different performance characteristics, the packet fragments may not all arrive before the timeout value for IP fragments.

Reducing the rsize and wsize values might improve the NFS performance in a congested network by sending shorter packets for each NFS-read reply and write request. But, a side effect of this is that more packets are needed to send data across the network, increasing total network traffic, as well as CPU utilization on both the server and client.

If your NFS file system is mounted across a high-speed network, such as Gigabit Ethernet, larger read and write packet sizes might enhance NFS file system performance. With NFS Version 3 and NFS Version 4, you can set the rsize and wsize values as high as 65536 when the network transport is TCP. The default value is 32768. With NFS Version 2, the maximum values for the rsize and wsize options is 8192, which is also the default.