HP-UX Reference (11i v1 05/09) - 4 File Formats (vol 8)

a
arraytab(4) arraytab(4)
NAME
arraytab - disk array configuration table
DESCRIPTION
Arraytab is a table of supported configurations for
HP SCSI disk array products. Each table entry includes a
set of parameter values that specify an array configuration. The array configuration table is located in
/etc/hpC2400/arraytab
.
HP SCSI disk array devices are highly configurable. The physical disk mechanisms in an array can be
grouped in special ways to provide various levels of data redundancy, and data read/write performance.
These levels are known as RAID (for Redundant Array of Inexpensive Disks) levels.
Using a process called striping, data from each read or write operation can be distributed across multiple
physical disk mechanisms to provide load balancing and/or to add data redundancy for protection against
the failure of physical disk mechanisms. Striping is done in increments of the physical disk block size for
all
RAID levels except RAID_3 (which uses byte striping). The stripe size, also known as segment size,
establishes the degree of data spread across the set of disk mechanisms.
Logical disks are created by defining address regions that include all or part of the address space of a disk
group. Each logical disk are separately addressable. For example:
Physical Physical
Block Drive
Address 1 2 3
0 XXX|
XXX|LogicalDrive 0
XXX|
. YYY|
. YYY|LogicalDrive 1
. YYY|
ZZZ|
ZZZ|LogicalDrive 2
N ZZZ|
In this example, 3 physical drives have been grouped into a single RAID group (1 vertical partition). Three
logical disks have then been formed by partitioning the composite logical address space (in blocks) into 3
logical regions.
A logical configuration which has more than one logical partition per physical disk group is called a sub-
LUN. If the logical partition includes the entire address space of the disk group, the logical partition is
called a regular LUN.
Each array configuration requires two types of specifications—physical specifications, and logical
specifications. A physical specification determines which disk mechanisms form the groups. A logical
configuration specifies the type and location of each physical disk mechanism (in the array) that is to be
used within the logical partition. The logical configuration also specifies the size and characteristics of the
logical partition.
Raid Levels
The disk array can be configured using one of the following
RAID levels, depending on the I/O requirements
of the system, and the degree of data availability required. Data availability (redundancy) is achieved at
the expense of storage capacity, and possibly performance.
RAID_0: This level provides no data redundancy, however disks may be grouped in a set, and data
striped across the disk set to provide load balancing.
A special case exists when a drive group of size 1 is defined (independent mode). In this case
the physical disk mechanisms appear to the system as they would if there were no array con-
troller. The array controller is transparent, providing only address selection among the disks
connected to it. When configured in this manner the disks operate independently for every I/O
request.
RAID_1: This level provides disk mirroring. Two sets of disks maintain identical copies of the data. By
choosing the number of disks in each set larger than one, data can be striped across the disks
in each set (RAID_0) to provide better load balancing; the redundant disk sets provide availa-
bility.
Section 4−−20 Hewlett-Packard Company − 1 − HP-UX 11i Version 1: September 2005