IBM 88553RX, Installation Manual

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32
IBM
Eserver
xSeries 455 Planning and Installation Guide
of the disks. GPT replaces the older Master Boot Record (MBR) partitioning
scheme that has been common to PCs.
There are several reasons for introducing a new partitioning scheme:
MBR disks support only four partition table entries and a volume size of 2 TB
(terabytes). If more partitions are required then one of these partitions must
be an extended partition. Only one extended partition is allowed per disk
drive. Extended partitions are then subdivided into one or more logical disks.
In theory, GPT disks support an unlimited number of partitions. The number
of partitions’ is limited only by the amount of space reserved for making
partitions entries. GPT disks can grow to a very large size. This can be up to
2
64
logical blocks in length (logical blocks are typically 512 bytes). This
equates to 18 EB (Exabytes). In practice, the maximum is less, as there are
practical limitations with other parts of the system accessing this amount of
disk space efficiently.
GPT disks use primary and backup partition tables for redundancy and
CRC32 fields for improved partition data structure integrity.
For backward compatibility with legacy MBR disk tools, all GPT disks contain a
protective MBR . The protective MBR, beginning in sector 0, precedes the GUID
Partition Table on the disk and contains only one partition entry that appears to
span the entire disk. The legacy tools are not aware of GPT and do not know how
to properly access a GPT disk. The benefit of a protective MBR is that these
tools will view a GPT disk as having a single encompassing (possibly
unrecognized) partition, rather than mistaking the disk for one that is
unpartitioned. That is why a GPT-partitioned disk appears to have an MBR.
GPT disks can be converted to MBR disks and vice versa only if all existing
partitioning is first deleted, with associated loss of data. To a legacy tool, the
structure of any GPT disk appears to look like Figure 1-17. The protective MBR is
followed by a theoretically unlimited number of data (or possibly unrecognized)
partitions.
Figure 1-17 General GPT disk structure
Data partition(s)
P
rot
ect
ive M
B
R