NFS Performance Tuning for HP-UX 11.0 and 11i Systems

ManualsBrandsHP ManualsSoftwareHP-UX 11i v1 Networking Software

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nfs performance tuning for hp-ux 11.0 and 11i systems page 98

Notes:

Page 98July 22, 2002

nfs/udp

vs.

nfs/tcp

Retransmissions and Timeouts

Overrides Van

Jacobsen Algorithm

(avoid if possible)

Effectively Ignored

(HP behaves the

same as SUN)

“timeo” Mount Option

Min = calculated

Max = 60 seconds

Min = calculated

Max = 20 seconds

Default Timeouts

MTU Size

(typically 1500 Bytes)

RSIZE/WSIZE

(as much as 32KB)

How much DATA is

sent in retransmission

Transport manages 1

NFS manages 2

NFS manages

Managing Timeouts

and Retransmissions

UDP TCPISSUE

Managing Timeouts and Retransmissions

Since the UDP transport is unreliable, it is up to the application (NFS) to manage retransmitting

packets that are not replied to. TCP/IP has built in mechanisms for managing the delivery of

data. Only in extreme situations does NFS get involved in retransmitting data on a TCP mount.

How much DATA is sent in retransmission

When a retransmission occurs on a UDP mount NFS must resend the entire request, which could

be as much as 32KB of data – even if only a single MTU of data was lost. TCP keeps track of

how much data has been sent by the clients and received by the servers. Therefore, if a single

MTU-sized packet of data is lost on the network, only that portion must be retransmitted.

Default Timeouts

Both UDP and TCP use the Van Jacobsen algorithm to calculate timeout values based on the

smooth round trip timers (the current srtt values can be displayed on a per-mount basis by

issuing the “nfsstat –m” command). UDP has a maximum timeout value of 20 seconds, while

NFS allows TCP to retry for 60 seconds before it forces a retransmission of the entire request.

“timeo” Mount Option

The “timeo” mount option is effectively ignored on UDP mounts. The use of this option on TCP-

based filesystems is highly discouraged as it overrides the Van Jacobsen algorithm, potentially

resulting in very poor performance, and causing “NFS server not responding” errors.