Disk Load Balancing, Fault Tolerance, and Configuration Limits for NonStop Systems

number of IOPs which can be launched in a processor. See the explanation in the preceding

“Processors and memory” section.

Ignoring memory limitations, the approximate maximum number of FCSA-attached mirrored disk

volumes which can be configured on a system is:

The number of processors → Divided by 2 (since an IOP has 2 halves and each half uses all 4

paths) → Times the number of FCSA adapters (number of SACs doesn't matter) → Times the 125

limit → Divided by 4 (because each volume has -P, -B, -M, and -MB paths)

For many system configurations, memory limitations will produce a smaller maximum number of

volumes than this calculation based on path count. The effective limit is the smaller of the two limits.

For best load balancing, distribute the IOPs which use an FCSA or CLIM across all available

processors and configure the IOPs which run in the same processor to distribute their paths across all

available FCSAs or CLIMs. This will balance the load on the processors and also spread I/O traffic

across the multiple ServerNet paths leading to each FCSA or CLIM. The I/O paths through an

adapter should be distributed across the SACs on the adapter.

In addition to distributing all configured device paths evenly across all FCSAs, SACs, and CLIMs, all

paths which are active by default (that is, -P and -M) should be distributed evenly across all FCSAs,

SACs, and CLIMs. If the configuration of -P, -B, -M, and -MB paths in the CONFIG file is unbalanced

and it is not convenient to stop the IOPs and change the configuration, it is possible to swap the

current active and inactive paths online with SCF SWITCH. SCF SWITCH only affects the running

IOP. It does not change the configured -P, -B, -M, and -MB paths.

CLIMs connected to any system type have a limit of 1000 paths from each processor through each

CLIM. This limit cannot be reached at the moment because the CLIM itself only allows the

configuration of a total of 512 device paths through it and available memory will limit the number of

IOPs which can be launched in a processor.

A CLIM can be connected to a maximum of 8 ESS ports, with a maximum of 500 ESS LDEVs per ESS

port. Both maxima cannot be reached at the same time due to the limit of 512 total paths per CLIM.

To avoid exceeding these limits, extraneous ESS ports and LDEVs should be hidden from a CLIM by

using SAN fabric zoning and ESS LUN masking.

In an S-series adapter (PMF, IOMF, or SNDA) or FCSA adapter, multiple SACs are packaged on one

adapter. It is possible to configure a mirrored disk volume using only 2 adapters, with different SACs

on one adapter carrying the -P and -MB paths and different SACs on the other adapter carrying the -B

and -M paths. But using 4 different adapters provides better fault tolerance.

For CLIM-connected disk volumes, it is possible to configure 4 paths to a mirrored volume using only 2

CLIMs, with the -P and -MB paths through one CLIM and the -B and -M paths through the other CLIM,

but using 4 separate CLIMs for the 4 paths to a mirrored disk volume is more fault tolerant.