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

7 LDs
Overview
An LD is a collection of physical disk chunklets arranged as rows of RAID sets. Each RAID set is
made up of chunklets from different physical disks. LDs are pooled together in CPGs, which allocate
space to virtual volumes. Creating CPGs maps out the data layout parameters for creating LDs.
LDs are created automatically by the system when virtual volumes are created from CPGs. The
RAID type, space allocation, growth increments, and other LD parameters can be set when you
create a CPG, or can be modified after you create a CPG. For information about CPGs, see
“Common Provisioning Groups” (page 41).
LDs and Common Provisioning Groups
A CPG establishes a virtual pool of LDs that can grow on demand. When you create virtual volumes,
the system creates all underlying LDs for you automatically. Volumes associated with a CPG draw
LD space from the virtual pool, allocating space on demand. As the volumes that draw from a CPG
require additional storage, the system automatically creates additional LDs and adds them to the
pool. After you create a CPG, you can add and remove LDs. You can also specify advanced LD
parameters when you create CPGs. This allows you to exercise a greater degree of control over
how the system creates LDs in the CPG.
NOTE: Creating virtual copies, or snapshots, requires the HP 3PAR Virtual Copy Software license.
For more information, see “HP 3PAR Software” (page 10)
LD Types
The following LD types provide storage space to virtual volumes:
• User LDs provide user storage space to virtual volumes. The user space contains the user data
and is exported as a LUN to the host.
• Snapshot data LDs provide the storage space for snapshots or virtual copies. The snapshot
space contains copies of user data that changed since the previous snapshot of the volume
was created.
• Snapshot administration LDs provide the storage space for snapshot administration. The
administration space is used to track changes to the volume since the previous snapshot was
created.
The system sets aside LDs for logging, for preserved data, and for system administration. These
LDs are multi-level LDs with three-way mirrors for enhanced redundancy and performance. The
following LD types are created by the system:
• Logging LDs are RAID 10 LDs that are used to temporarily hold data during disk failures and
disk replacement procedures. Logging LDs are created by the system during the initial
installation and setup of the system. Depending on the system model you have, each controller
node in the system has a 20 GB or 60 GB logging LD.
• Preserved data LDs are RAID 10 LDs used to hold preserved data. Preserved data LDs are
created by the system during the initial installation and setup of the storage system. The size
of the preserved data LD is based on the amount of data cache in the system.
When multiple disk failures during write operations leave data suspended in cache memory, the
system temporarily preserves this data by writing it to a preserved data LD. By doing so, the system
clears the data cache, and prevents the data cache from locking up and leading to wider system
36 LDs