Setting the maximum buffer size

Red Hat Enterprise Linux 8.6 LPAR mode z/VM guest

Use the buffer device group attribute to set a maximum buffer size for a CTCM group device.

Before you begin

  • Set the maximum buffer size for CTC interfaces only. MPC interfaces automatically use the highest possible maximum buffer size.
  • The device must be online when you set the buffer size.

About this task

You can set the maximum buffer size for a CTC interface. The permissible range of values depends on the MTU settings. It must be in the range <minimum MTU + header size> to <maximum MTU + header size>. The header space is typically 8 byte. The default for the maximum buffer size is 32768 byte (32 KB).

Changing the buffer size is accompanied by an MTU size change to the value <buffer size - header size>.

Procedure

To set the maximum buffer size, issue a command of this form: 
# echo <value> > /sys/bus/ccwgroup/drivers/ctcm/<device_bus_id>/buffer
where <value> is the number of bytes you want to set. If you specify a value outside the valid range, the command is ignored.

Example

In this example, the maximum buffer size of a CTCM group device 0.0.f000 is set to 16384 byte. 
# echo 16384 > /sys/bus/ccwgroup/drivers/ctcm/0.0.f000/buffer