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EMRAM (Magneto-resistive Random Access Memory) is nonvolatile, has the speed of SRAM (40ns-70ns), and has unlimited write endurance. This document outlines how EMRAM can be a higher performance, lower cost replacement to existing NVRAM technologies in RAID controllers.

MCU

RAID

RFID

Smart Card

SRAM Replacement

EMRAM for RAID Controllers

RAID Controllers
RAID (Redundant Arrays of Independent Disk Drives) systems are built for EDAP (Extended Data Availability and Protection). In today’s demand for high data availability and reliability, RAID system must ensure data integrity and provide immediate access to stored data after the rectification of any system failure. Most modern RAID systems thus store key data such as restart vectors in a nonvolatile memory on the controller board. All this is managed by the Array Management Software, which runs either in the host or on the controller.

Embedded controllers that offer high data reliability and increased I/O performance in one package have become the standard on RAID. High performance embedded processors, latest software and control algorithms can be found on the RAID controller.

RAID Controller Memory Requirements
The array management software is constantly moving data among the array members, and there are three primary applications that benefit from EMRAM.

RAID Journal
Traditionally, RAID systems are designed with a distributed block of disk to maintain system configuration data and store system recovery address vectors. However, when power failure occurs, the controller’s volatile system memory is lost, and on subsequent power-up the entire disk array must be scanned to recover configuration data and address vectors. This architecture leads to significant recovery time, especially for large arrays, in case of power and system failure.

Immediate data recovery upon power restoration is required, and the restart vectors can be stored in an appropriate nonvolatile memory on the controller board itself. The RAID controller is constantly moving data among the individual array members to optimize I/O balance, maximize I/O rates, and assure redundancy; hence the address vector tables are constantly being updated. Unlimited write endurance is required, and EMRAM, with its unlimited write endurance, is ideal for RAID journal.

RAID Configuration Data
Same as the RAID journal, RAID system configuration data and parity information can also be stored in EMRAM to allow immediate recovery in case of disk failure. It is desirable to move this information at processor bus speed so as not to hold up the system in wait-states and the RAID controller does not waste time searching the disk array for its configuration data. EMRAM with its fast write cycle time (40- 70ns) and unlimited write endurance is a good fit in this regard.

RAID Caches
Write caching allows the array management software to "complete" a write command while the magnetic head on the target disk is still seeking the right sectors for write operation. It removes the need to notify when the desired sectors are found and write operation completed. This cache also affords the controller the option of accumulating data blocks before writing them to the target disk as a consolidated block. All this has the desired result of reducing the seek and write time on the target disks. Although caching EMRAM for RAID Controllers provides system performance improvements in write intensive or RAID 5 applications, it presents the potential problem of data loss should system failure occurs before a disk write is completed.

Adding battery-backed SRAM is an expensive and cumbersome way of solving this problem. Periodic board maintenance is required to ensure proper functioning of batteries. EMRAM is ideal for those critical caches where the maintenance, record keeping and expenses of battery-backup solutions are to be avoided.

EMRAM as the Nonvolatile Memory of Choice
Parallel EMRAM is ideal for RAID controller application:

EMRAM is nonvolatile.

It has unlimited write endurance (SRAM/EEPROM, for example supports 106 nonvolatile write cycles,
and FRAM 1012 write cycles).

It runs at processor bus speeds (40-70ns).

It is pin-compatible to SRAM.

It looks like standard SRAM to the processor and is easily implemented.

It does not require back up battery or capacitors, removing a potential failure mode.

It is by far the lowest cost NVRAM in the market for this application because of its smallest die size and
absence of back-up battery.

Information furnished is believed to be accurate and reliable. However, NLSC assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of NLSC. Specifications mentioned in this publication are subject to change without notice. This publication supercedes and replaces all information previously supplied. NLSC products are not authorized for use as critical components in life support devices or systems without express written approval of NLSC.