HP 3PAR StoreServ 7400 2-node HP 3PAR StoreServ Storage Concepts Guide (OS 3.1 - Page 34
Logical Disks, Overview, Logical Disks and Common Provisioning Groups, Logical Disk Types
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7 Logical Disks Overview A Logical Disk (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. Logical disks are pooled together in Common Provisioning Groups (CPGs) which allocate space to virtual volumes. Creating CPGs maps out the data layout parameters for the creating logical disks. Logical disks are created automatically by the system when virtual volumes are created from CPGs. The RAID type, space allocation, growth increments and other logical disk parameters can be set when you create a CPG or modified after creating a CPG. For information about CPGs, see "Common Provisioning Groups" (page 39). Logical Disks and Common Provisioning Groups Creating a Common Provisioning Group (CPG) establishes a virtual pool of logical disks that can grow on demand. When you create virtual volumes, the system creates all underlying logical disks for you automatically. Volumes associated with a CPG draw logical disk space from the virtual pool as needed, allocating space on demand. As the volumes that draw from a CPG require additional storage, the system automatically creates additional logical disks and adds them to the pool. Once you create a CPG, you can add and remove logical disks. You can also specify advanced logical disk parameters when creating CPGs. This allows you to exercise a greater degree of control over how the system creates logical disks within the CPG. NOTE: Creating virtual copies or snapshots requires the HP 3PAR Virtual Copy Software license. For more information, see "HP 3PAR Software" (page 9) Logical Disk Types The following logical disk types provide storage space to virtual volumes: • User logical disks 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 logical disks 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 logical disks 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 logical disks for logging, for preserved data, and for system administration. These logical disks are multi-level logical disks with three way mirrors for enhanced redundancy and performance. The following logical disk types are created by the system: • logging logical disks are RAID 10 logical disks that are used to temporarily hold data during disk failures and disk replacement procedures. Logging logical disks are created by the system during the initial installation and setup of the system. Each controller node in the system has a 60 GB logging LD. • preserved data logical disks are RAID 10 logical disks used to hold preserved data. Preserved data logical disks 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 logical disk. By doing so, the system clears the data cache and prevents it from locking up and leading to wider system 34 Logical Disks