Electrical computers and digital processing systems: memory – Storage accessing and control – Specific memory composition
Reexamination Certificate
2000-06-23
2002-08-20
Robertson, David L. (Department: 2187)
Electrical computers and digital processing systems: memory
Storage accessing and control
Specific memory composition
C713S340000
Reexamination Certificate
active
06438647
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to systems and methods for controlling an array of disk drives in a computer system, and more particularly to a method and apparatus for providing battery-backed immediate write back cache for an array of disk drives in a computer system.
2. Description of Related Art
Modern mass storage subsystems are continuing to provide increasing storage capacities to fulfill user demands from host computer system applications. Due to this critical reliance on large capacity mass storage, demands for enhanced reliability are also high. Various storage device configurations and geometries are commonly applied to meet the demands for higher storage capacity while maintaining or enhancing reliability of the mass storage subsystems.
A popular solution to these mass storage demands for increased capacity and reliability is the use of multiple smaller storage modules configured in geometries that permit redundancy of stored data to assure data integrity in case of various failures. In many such redundant subsystems, recovery from many common failures can be automated within the storage subsystem itself due to the use of data redundancy, error codes, and so-called “hot spares” (extra storage modules which may be activated to replace a failed, previously active storage module). These subsystems are typically referred to as redundant arrays of inexpensive (or independent) disks (or more commonly by the acronym RAID). A number of reference articles that describe the design and characteristics of disk array subsystems have been published, including the articles: “Introduction to Redundant Arrays of Inexpensive Disks (RAID)” by D. Patterson, P. Chen, G. Gibson and R. Katz, IEEE, 1989; “Coding Techniques for Handling Failures in Large Disk Arrays” by G. Gibson, L. Hellerstein, R. Karp, R. Katz and D. Patterson, Report No. UCB/CSD 88/477, December 1988, Computer Science Division, University of California Berkeley; and “A Case Study for Redundant Arrays of Inexpensive Disks (RAID)” by D. Patterson, G. Gibson, and R. Katz, presented at the June 1988 ACM SIGMOD Conference in Chicago, Ill.
Generally speaking, a disk array subsystem includes an array of standard disk drives, referred to collectively as a “composite” drive, coupled in parallel. The disk array subsystem further includes a drive array controller for interfacing the composite drive to a computer system. The drive array controller, which is generally installable on an expansion bus of the computer system, converts input-output (“I/O”) read and write requests into a sequence of seeks, delays and other disk commands to read data from or write data to the composite drive.
A drive array controller differs from a conventional disk drive controller (i.e., a single disk controller) in that, with respect to the drive array controller, the set of disk drives coupled thereto emulate a single disk drive having a greater capacity and a higher performance than any individual disk drive included as a portion thereof. To perform an access to a virtual composite drive location within the composite drive, the drive array controller must be cognizant of both the position of the particular disk drive to be accessed as well as the physical sector location within that disk drive which corresponds to the virtual composite drive location for which access is sought. Various hardware and software implementations are well-known for performing these functions.
A significant concern relating to the mass storage of data within disk array subsystems is the possibility of data loss or corruption due to drive failure. A variety of data redundancy and recovery techniques have therefore been proposed to allow restoration of data in the event of a drive failure.
There are several “levels” of standard geometries defined in the Patterson publication. For example, a RAID level 1 system, comprises one or more disks for storing data and an equal number of additional “mirror” disks for storing copies of the information written to the data disks. Additional RAID levels, such as RAID level 2,3,4 and 5 systems, segment the data into portions for storage across several data disks. One of more additional disks are utilized to store error check or parity information.
RAID storage subsystems typically utilize a control module that shields the user or host system from the details of managing the redundant array. The controller makes the subsystem appear to the host computer as a single, highly reliable, high capacity disk drive. In fact, the RAID controller may distribute the host computer system supplied data across a plurality of the small independent drives with redundancy and error checking information so as to improve subsystem reliability. Frequently RAID subsystems provide large cache memory structures to further improve the performance of the RAID subsystem. The cache memory is associated with the control module such that the storage blocks on the disk array are mapped to blocks in the cache. This mapping is also transparent to the host system. The host system simply requests blocks of data to be read or written and the RAID controller manipulates the disk array and cache memory as required.
To further improve reliability, it is known in the art to provide redundant control modules to reduce the failure rate of the subsystem due to control electronics failures. In some redundant architectures, pairs of control modules are configured such that they control the same physical array of disk drives. A cache memory module is associated with each of the redundant pair of control modules.
The redundant control modules communicate with one another to assure that the cache modules are synchronized. Typically, the redundant pair of control modules communicate at their power-on initialization (or after a reset operation). While the redundant control modules complete their communications to assure synchronization of the cache modules, the RAID storage subsystem are unavailable with respect to completing host computer requests. If the cache modules are “out of sync”, the time required to restore synchronization could be significant. In addition, a failure of one of the redundant pair of control modules would further extend the time during which the RAID storage subsystem would be unavailable. Manual (operator) intervention could be required to replace a defective redundant control module in order for the RAID subsystem to begin processing of host computer requests.
During normal operation, dual controllers operate in a write back mode. Write back mode refers to the process of writing data in a receiving controller's cache and then writing data in the other controller's cache before returning a completion status to the host. For dual controller systems with battery-backed data cache, when a controller fails, a replacement controller is installed. The replacement controller needs to recondition its battery before the write back cache mode is reinitiated. This is because the cache is not yet protected from a power failure. During the reconditioning period, the operation of the controller is in a write through cache mode so the data is committed to the storage media before a completion status is returned to the host. Once the battery is reconditioned the controller can change back to the write back cache mode. However, traditional replacement controllers need to condition the batteries attached to them for many hours before write back cache operations can take place. Further, the write back cache mode returns a completion status to the host much faster than the write through cache mode. Thus, the write through cache mode causes a decrease in performance for the many hours it takes to recondition the battery.
Reconditioning is required because a newly installed controller does not know the state of the battery. Reconditioning involves draining the battery down to zero charge, charging it up fully so that the controller knows how long it takes to charge the battery, bring the battery down to zero charge so t
Nielson Michael E.
Richardson Thomas E.
Altera Law Group LLC
Robertson David L.
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