HP Surestore Disk Array FC60 HP SureStore E Disk Array 12H User's and Service - Page 94

Concepts and Management Disk Array Hardware NVRAM Each controller contains NVRAM (non-volatile RAM) used to store vital operating parameters and mapping information used in managing the array. Because data is spread across the disks, the array controller must have some means of determining where each block of data is located. It accomplishes this using a logical-to-physical data map stored in controller NVRAM. This map is vital to managing data-without it, all data on the array is unrecoverable. The contents of the NVRAM is maintained by dual battery backup if power to the disk array is off. Each controller module has its own dual set of batteries. If the batteries fail or lose their charge while power is off, the NVRAM contents will be lost. To protect against this, the Shutdown operation copies the contents of the NVRAM to each disk when power to the array is turned off by the power switch. This allows reconstruction of the data in NVRAM memory if the original map in NVRAM is lost or corrupted. The process of copying the contents of NVRAM to the disks makes the disk set independent of its controller. Because all the necessary mapping information is on the disks, it is possible to install a new controller or move the entire disk set to another controller. The new controller will determine that it has a new disk set, and the controller will attach itself to those disks. Data Maps and Array Recovery Two of the disks in the disk array are used to store data recovery map information. If the data maps in NVRAM memory are lost, you may be able to recover them with the arrayrecover utility. The arrayrecover utility and the data recovery map settings and parameters are described in more detail in the System Administrator's Guide, which is shipped with each disk array. The data resiliency feature for HP AutoRAID provides a means of recovering data after loss of NVRAM when the disk array is not shut down properly. Such a loss can only occur due to: dead battery, battery mishandling, a single failure in single controller configuration, or a multiple failure in dual controller configuration. The data maps in NVRAM memory are reconstructed using the latest copy of mapping information stored on the disk drives. This reconstruction includes a combination of the maps which were written during Shutdown, and the data recovery maps which are written at regular intervals. Part of the recovery operation includes performing a parity scan on the contents of the entire disk array to validate the accuracy of the maps and to correct any parity inconsistencies. This process can take up to several hours depending on the amount of data on the disk array. Controller firmware places a limit on the amount of time either data or map information can reside in battery-backed RAM before the process of writing that information to disk is begun. The firmware can optionally eliminate this limited exposure, which it automatically does during single controller operation. If the data maps in NVRAM memory are lost, all data except that retained only in NVRAM can be recovered using the Recover command via the front panel or the arrayrecover utility. In single-controller operation, the write cache is disabled, resulting in no data exposure to NVRAM loss. Disabling of the write cache is accompanied by a very serious degradation in performance under some workloads. This feature is supported on controller firmware versions later than HP40. 94 Concepts