Finding out the maximum frame size

LPAR mode z/VM guest

The inbuf_size attribute returns the maximum frame size (MFS) in KB. To find out the MFS, read the inbuf_size attribute of the devices.

About this task

An OSA-Express CHPID in QDIO mode allows packing of data, and always runs with an MFS of 64 KB.

HiperSockets™ CHPIDs do not pack data and run with a frame size that matches their definition in the hardware configuration (IOCP CHPARM specification). On HiperSockets, the MFS maps to corresponding maximum transmission unit (MTU) sizes, see Table 1.

Table 1. HiperSockets MFS and corresponding MTU sizes
inbuf_size value	MFS	MTU
16k	16 KB	8 KB
24k	24 KB	16 KB
40k	40 KB	32 KB
64k	64 KB	56 KB

Procedure

Issue a command of this form to get a list of all attributes and find the inbuf_size attribute:

# lszdev qeth <device_bus_id> --info --info |grep inbuf_size

Alternatively, use sysfs:

# cat /sys/bus/ccwgroup/drivers/qeth/<device_bus_id>/inbuf_size

Example

To find the inbuf_size of a device 0.0.a100 issue:

# lszdev qeth 0.0.a100 --info --info  | grep inbuf_size 
inbuf_size  "64k"

or:

# cat /sys/bus/ccwgroup/drivers/qeth/0.0.a100/inbuf_size
64k