HP Surestore Disk Array 12h - And FC60 Service Manual 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.
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