Preparing a logical device as a boot device

6.10 LPAR mode z/VM guest

A logical device is a block device that represents one or more real devices.

If your boot directory is on a logical DASD or SCSI device, zipl cannot detect all required information about the underlying real device or devices and needs extra input.

Logical devices can be two DASDs combined into a logical mirror volume. Another examples are a linear mapping of a partition to a real device or a more complex mapping hierarchy. Logical devices are controlled by a device mapper.

Blocks on the logical device must map to blocks on the underlying real device or devices linearly. If two blocks on the logical device are adjacent, they must also be adjacent on the underlying real devices. This requirement excludes mappings such as striping.

You always boot from a real device. zipl must be able to write to that device, starting at block 0. In a logical device setup, starting at the top of the mapping hierarchy, the first block device that grants access to block 0 (and subsequent blocks) is the base device, see Figure 1.

Figure 1. Definition of base device
Illustration of the base device concept
A base device can have the following mappings:
  • A mapping to a part of a real device that contains block 0
  • A mapping to one complete real device
  • A mapping to multiple real devices.
For a mapping to multiple real devices all the real devices must share the device characteristics and contain the same data (for example, a mirror setup). The mapping can also be to parts of the devices if these parts contain block 0. The mapping must not combine multiple devices into one large device.

The zipl command needs the device node of the base device and information about the physical characteristics of the underlying real devices. For most logical boot devices, a helper script automatically provides all the required information to zipl for you.

If you decide not to use the supplied helper script, or want to write your own helper script, you can use parameters to supply the base device information to zipl.