Expand Configuration and Management Manual (G06.24+)

Tuning
Expand Configuration and Management Manual—523347-008
20-5
Variable Packet Size
Multipacket Frame Configuration
The multipacket frame size is determined by the value assigned to the
PATHBLOCKBYTES modifier.
When the variable packet-size feature (PATHPACKETBYTES modifier) is used, the
Expand subsystem should be able to send a full variable-size packet inside a
multipacket frame. For this reason, the PATHBLOCKBYTES modifier must be set to a
value greater than or equal to the PATHPACKETBYTES modifier value. Refer to
Variable Packet Size for an explanation of the variable packet-size feature.
When the multipacket frame feature is used, the value specified for the L2TIMEOUT
modifier should be based on the transmission time required for the configured
PATHBLOCKBYTES modifier value rather than on the configured FRAMESIZE
modifier value. You can calculate the value of the L2TIMEOUT modifier using the
following formula:
where txw is the TXWINDOW modifier value, pathblockbytes is the
PATHBLOCKBYTES modifier value, lspd is the line speed in bits per second (actual,
not configured), and dl is the DELAY modifier value. The result of this formula is a
one-hundredth of a second value.
For more information about configuring the multipacket frame feature, refer to
Multipacket Frame Feature on page 18-64.
Variable Packet Size
The variable packet-size feature is designed to improve bulk transfers across Expand
connections. This feature enables you to configure a maximum packet size for each
path for both single-packet and multipacket frames. This feature effectively overrides
the value configured for the FRAMESIZE modifier between configured nodes. The
variable packet-size feature is supported for all line types.
The variable packet-size feature provides the following benefits:
•
Reduces per-message processor cost for large message sizes
•
Reduces network bandwidth used for Expand overhead for large messages
•
Increases potential throughput in high-bandwidth Expand paths
The variable packet-size feature is especially suited for transferring large messages,
such as in tape backup and restores, and file transfers. While small online transaction
processing (OLTP) requests transfer fastest with smaller frame sizes, larger bulk
(((txw + 1) * pathblockbytes * 8 ) / (lspd / 100) + (2 * dl) + 10
Note. If you use the Expand subsystem SCF ALTER LINE command to set the L2TIMEOUT
modifier, you must convert the result of this formula to a time interval. For example, if the result
is 300 (3 seconds), you will enter the following command:
ALTER LINE $device_name, L2TIMEOUT 3.00