Finding out the maximum frame size

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7.1 LPAR mode z/VM guest

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

About this task

Issue a command of the form:

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

An OSA-Express CHPID 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

Example

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

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