What you should know about SCSI IPL

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SCSI IPL opens the way to a new set of IPL I/O devices with a somewhat different processing compared to CCW-based devices.

At first glance, a traditional IPL (also called CCW IPL) and a SCSI IPL are similar:
  1. A mainframe administrator initiates an IPL at the SE, HMC, or at a z/VM® console.
  2. The machine checks the IPL parameters and tries to access the corresponding IPL devices.
  3. Some code will be loaded from the IPL device into main storage and executed. Usually this initial code will load some more code into storage until the entire operating system is in memory.
The difference between SCSI IPL and CCW IPL is the connection to the IPL device. In the CCW case the IPL device is connected more or less directly to the host. In contrast, in the SCSI IPL case there could be an entire Fibre Channel SAN between the host and the IPL device.

In traditional CCW IPL, a channel command word (CCW) contains a command to perform a read, write, or control operation. A chain of CCWs is called a channel program, and this will be executed in a channel by channel engines that run independently of the usual CPUs.

All I/O is controlled by channel programs. I/O devices are identified by a two-byte device number. The I/O devices are configured within the I/O definition file (IODF). A CCW IPL is also called 24-bytes-IPL because only one PSW and two CCWs are read from the disk initially. These 24 bytes are the first stage boot loader and are enough to allow the reading of more IPL code from the IPL device.

SCSI IPL is more complex than CCW IPL and can:
  • Log in to an Fibre Channel fabric.
  • Maintain a connection through the Fibre Channel SAN.
  • Send SCSI commands and associated data.
To accomplish this, an enhanced set of IPL parameters is required (see SCSI IPL parameters).