HPDT Packing

Restriction: HPDT Packing can be enabled only for non-XCF point-to-point connections. Therefore XCF, ATM, and OSA devices are excluded.

The HPDT device driver was initially designed for large data transfer and within the architectural limitations of the I/O subsystem. HPDT is efficient at transporting large pieces of data but relatively inefficient when building a data stream with multiple small to medium size packets (packets from 257 to 2KB bytes, including headers). A write data stream built from this type of workload results in a significant number of alignment bytes. These alignment bytes are transmitted across the media but are not used by the receiver. In effect, the alignment bytes significantly reduce the media bandwidth.

HPDT Packing can be enabled to eliminate most of these alignment bytes and effectively increase the media bandwidth. However, HPDT Packing is a compromise between CPU cycles, storage, and channel bandwidth. There are two side effects of enabling HPDT Packing:

CPU use increases because of the cost of moving the data into the transmit packing buffer.
Storage use increases because fixed packing transmit buffers are allocated.

Recommendation: In cases where the HPDT MPC connection is to a router or through a channel extender (and packet size is small to medium), you should consider enabling HPDT Packing. In these cases, the benefits of a densely packed data stream might exceed the costs in CPU and storage resources. If channel bandwidth is not a concern, enabling HPDT Packing is not recommended.