HP 3PAR StoreServ Concepts Guide: HP 3PAR OS 3.1.3

6 Chunklets
Overview
Physical disks are divided into chunklets. When a physical disk is admitted to the system, it is
divided into chunklets that become available to the system. Some chunklets are used by LDs, and
other chunklets are designated as spares to hold relocated data during a disk failure or during
maintenance procedures.
Creating, moving, and removing chunklets and spares can only be performed with the HP 3PAR
CLI. See the HP 3PAR Command Line Interface Administrator’s Manual for instructions on performing
these tasks.
To view chunklets and spares, use either the HP 3PAR CLI and the HP 3PAR Management Console.
See the HP 3PAR Command Line Interface Administrator’s Manual and the HP 3PAR Management
Console Online Help for instructions on performing this task.
Physical Disk Chunklets
Each chunklet occupies contiguous space on a physical disk.
Space on a physical disk is allocated as follows:
• On F-Class and T-Class systems, all chunklets are 256 MB. On StoreServ 10000 and StoreServ
7000 systems, all chunklets are 1 GB.
• 256 MB of space 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 of space is 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. 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 LDs.
Spare Chunklets
Some chunklets are identified as spares when the system is first set up at installation. Data from
other chunklets is moved or reconstructed to these spare chunklets in response to a chunklet or disk
failure or when a drive magazine must be serviced. This initial spare storage is equal to 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 LD until the physical disk comes back online, or
until the time limit for logging is reached. Logging disk space is allocated when the system 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 LD becomes full, the relocation of
chunklets on the physical disk to free chunklets designated as spares starts automatically. Free
chunklets are any chunklets that are not already allocated for use by LDs.
• For automatic relocations, the system uses up to a maximum of one disk worth of chunklets
per system node.
• When selecting a target chunklet for relocation, the system attempts to identify a local spare
chunklet, a local free chunklet, a remote spare chunklet, and finally, a remote free chunklet.
34 Chunklets