Installation guide

Introduction to Mirroring Data
4-2 Fault Tolerant System Administration (R1004H) HP-UX version 11.00.03
■ A physical volume group is a set of physical volumes, or disks, within a
volume group.
■ A logical volume is a unit of usable disk space divided into sequential logical
extents. Logical volumes can be used for swap, dump, raw data, or file
systems.
■ A logical extent is a portion of a logical volume mapped to a physical extent.
■ A physical extent is an addressable unit on a physical volume.
■ Contiguous means that the physical extents of each mirror are placed
immediately adjacent to one another on the disk and cannot span several
disks. Root volumes must be contiguous.
■ Noncontiguous means that physical extents of each mirror can be allocated to
one or more physical volumes and can be separated by other data.
■ Strict allocation means that physical extents are allocated to different physical
volumes, or disks. Strict allocation is the default for mirroring.
■ PVG-strict allocation means that physical extents of each mirror are allocated
to different physical volume groups, and not just different physical volumes.
In addition to increasing availability, this allows LVM more flexibility in
reading data, resulting in better performance. If you configure physical
volume groups so that disks using the same interface card or SCSI bus are
grouped together, this allocation policy is also called I/O channel separation.
For more information, see the “Setting Up I/O Channel Separation” section
later in this chapter.
■ Nonstrict allocation means that physical extents can be allocated to any
available disk space in the volume group. With this allocation policy, mirrored
physical extents can be allocated to the same disk. If the disk or SCSI bus fails,
both primary and mirrored data can become unavailable or lost.
■ Dual-initiation is a term used when a logical SCSI bus is driven by two
physical SCSI controllers, usually in different PCI card-cages, working
together to support a single set of disks. If one of the controllers fails, the other
controller can still access the disks.
■ Single-initiation is the term used when the logical SCSI bus is driven by a
single SCSI controller.