3PAR InForm® OS 2.3.1 Concepts Guide (320-200112 Rev B, February 2010)

7.2
Physical Disk Chunklets
3PAR InForm OS Concepts Guide InForm OS Version 2.3.1
7.2 Physical Disk Chunklets
Physical disks are divided into chunklets, where each chunklet occupies 256 MB of contiguous
space.
Space on a physical disk is allocated as follows:
■ 256MB of data is reserved for the table of contents (TOC), which contains the internal
description of the system. The TOCs on all physical disks in the system contain the same
information.
■ 4 MB are reserved for diagnostic use, 2 MB beginning after the TOC and 2 MB from the end
of the disk logical block address.
■ One or more chunklets are allocated as spares. These spare chunklets are used to hold
relocated data during disk failures and during maintenance procedures. Spares are created
during installation and setup. Any chunklet can be reserved as a spare, but the system setup
script selects those chunklets as close to the end of the physical disk’s logical block space as
possible.
■ The remainder of the disk can be used for logical disks.
7.3 Spare Chunklets
Some chunklets are identified as spares when the storage server is first set up at installation.
Data from other chunklets is moved or reconstructed onto these spare chunklets in response to
a chunklet or disk failure or when a drive magazine needs to be serviced. This initial spare
storage totals the amount of storage in a single drive magazine, using the largest size physical
disks.
How spare chunklets work:
■ When a connection is lost to a physical disk or a physical disk fails, all future writes to the
disk are automatically written to a logging logical disk until the physical disk comes back
online or until the time limit for logging is reached. Logging disk space is allocated when
the storage server is set up. This does not apply to RAID 0 chunklets which have no fault
tolerance.
■ If the time limit for logging is reached, or if the logging logical disk becomes full, the
relocation of chunklets on the physical disk to free chunklets designated as spares starts