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NFSv3 slot tables

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NFSv3 performance on Linux depends on a parameter called tcp_max_slot_table_entries.

TCP slot tables are the NFSv3 equivalent of host bus adapter (HBA) queue depth. These tables control the number of NFS operations that can be outstanding at any one time. The default value is usually 16, which is far too low for optimum performance. The opposite problem occurs on newer Linux kernels, which can automatically increase the TCP slot table limit to a level that saturates the NFS server with requests.

For optimum performance and to prevent performance problems, adjust the kernel parameters that control the TCP slot tables.

Run the sysctl -a | grep tcp.*.slot_table command, and observe the following parameters:

# sysctl -a | grep tcp.*.slot_table
sunrpc.tcp_max_slot_table_entries = 128
sunrpc.tcp_slot_table_entries = 128

All Linux systems should include sunrpc.tcp_slot_table_entries, but only some include sunrpc.tcp_max_slot_table_entries. They should both be set to 128.

Caution

Failure to set these parameters may have significant effects on performance. In some cases, performance is limited because the linux OS is not issuing sufficient I/O. In other cases, I/O latencies increases as the linux OS attempts to issue more I/O than can be serviced.