What are the default and maximum values for rsize and wsize with NFS mounts?

Environment

• Red Hat Enterprise Linux 7
• Red Hat Enterprise Linux 6
• Red Hat Enterprise Linux 5
• Red Hat Enterprise Linux 4

Issue

• How to increase the default read/write (block) size on RHEL with NFS.
• What is the maximum rsize or wsize in Red Hat Enterprise Linux?

Resolution

• There is no 'default' (in the traditional sense of the word) value for rsize and wsize. The 'default' is to use the largest value that both the client and server support. The maximum rsize and wsize values vary across the various versions of Red Hat Enterprise Linux.
• If rsize/wsize is not specified in the mount options, the client will query the server and will use the largest size that both support.
• If rsize/wsize is specified in the mount options and it exceeds the maximum value that either the client or server support, the client will use the largest size that both support.
• The server maximum is calculated based on the amount of memory specified in totalram_pages (see the table below).
• The server maximum can be overridden by writing a value to /proc/fs/nfsd/max_block_size.

RHEL NFS Client and Server Maximum rsize/

wsize

Notes:
[1] NFS v2 has a maximum of 8192.
[2] NFS over UDP has a maximum of 32768.

NFS server max blocksize calculation

Notes:
[1] totalram_pages is the amount of 'usable' memory and does not include memory reserved by the BIOS, reserved by the kernel for global data structures, etc. Therefore a server with exactly 4GB of RAM will use 524288 for the maximum blocksize, not 1048576.

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