Electrical computers and digital processing systems: memory – Storage accessing and control – Specific memory composition
Reexamination Certificate
2001-01-25
2003-10-28
Sparks, Donald (Department: 2186)
Electrical computers and digital processing systems: memory
Storage accessing and control
Specific memory composition
C711S157000, C710S302000, C365S052000, C365S063000, C365S226000
Reexamination Certificate
active
06640282
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to memory systems and, more particularly, to redundant memory systems.
2. Background of the Related Art
This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
As most people know, the use of computers has increased dramatically over the past few decades. In the early days, computers were relatively few in number and primarily used as scientific tools. However, with the advent of standardized architectures and operating systems, computers soon became virtually indispensable tools for a wide variety of businesses. Perhaps even more significantly, in the past ten to fifteen years with the advent of relatively simple user interfaces and ever increasing processing capabilities, computers have now found their way into many homes. Today, at least one full generation has been raised in a world which, from their perspective, has always known computers, thus making computers a familiar and integral part of many lives.
The types of computing systems have similarly evolved over this period of time. For example, early scientific computers were typically stand-alone systems designed to carry out relatively specific tasks and requiring relatively knowledgeable users. As computer systems evolved into the business arena, mainframe computers emerged. In mainframe systems, users utilized “dumb” terminals to provide input to and to receive output from the mainframe computer, while all processing was done centrally by the mainframe computer. As users desired more autonomy in their choice of computing services, personal computers evolved to provide processing capability on each user's desktop. More recently, personal computers have given rise to relatively powerful computers called servers. Servers are typically multiprocessor computers that couple numerous personal computers together in a network. In addition, these powerful servers are also finding applications in various other capacities, such as in the communications and Internet industries.
Computers today, such as the personal computers and servers mentioned above, rely on microprocessors, associated chip sets, and memory chips to perform most of their processing functions. Because these devices are integrated circuits formed on semiconducting substrates, the technological improvements of these devices have essentially kept pace with one another over the years. In contrast to the dramatic improvements of the processing portions of a computer system, the mass storage portion of a computer system has experienced only modest growth in speed and reliability. As a result, computer systems failed to capitalize fully on the increased speed of the improving processing systems due to the dramatically inferior capabilities of the mass data storage devices coupled to the systems.
While the speed of these mass storage devices, such as magnetic disk drives, has not improved much in recent years, the size of such disk drives has become smaller while maintaining the same or greater storage capacity. Furthermore, such disk drives have become less expensive. To capitalize on these benefits, it was recognized that a high capacity data storage system could be realized by organizing multiple small disk drives into an array of drives. However, it was further recognized that large numbers of smaller disk drives dramatically increased the chance of a disk drive failure which, in turn, increases the risk of data loss. Accordingly, this problem has been addressed by including redundancy in the disk drive arrays so that data lost on any failed disk drive can be reconstructed through the redundant information stored on the other disk drives. This technology has been commonly referred to as “redundant arrays of inexpensive disks”(RAID).
To date, at least five different levels of RAID have been introduced. The first RAID level utilized mirrored devices. In other words, data was written identically to at least two disks. Thus, if one disk failed, the data could be retrieved from one of the other disks. Of course, a level 1 RAID system requires the cost of an additional disk without increasing overall memory capacity in exchange for decreased likelihood of data loss. The second level of RAID introduced an error code correction (ECC) scheme where additional check disks were provided to detect single errors, identify the failed disk, and correct the disk with the error. The third level RAID system utilizes disk drives that can detect their own errors, thus eliminating the many check disks of level 2 RAID. The fourth level of RAID provides for independent reads and writes to each disk which allows parallel input-output operations. Finally, a level 5 RAID system provides memory striping where data and parity information are distributed in some form throughout the disk drives in the array.
The implementation of data redundancy, such as in the RAID schemes discussed above, creates fault tolerant computer systems where the system may still operate without data loss even if one drive fails. This is contrasted to a disk drive array in a non-fault tolerant system where the entire system is considered to have failed if any one of the drives fail. Of course, it should be appreciated that each RAID scheme necessarily trades some overall storage capacity and additional expense in favor of fault tolerant capability. Thus, RAID systems are primarily found in computers performing relatively critical functions where failures are not easily tolerated. Such functions may include, for example, a network server, a web server, a communication server, etc.
One of the primary advantages of a fault tolerant mass data storage system is that it permits the system to operate even in the presence of errors that would otherwise cause the system to malfunction. As discussed previously, this is particularly important in critical systems where downtime may cause relatively major economic repercussions. However, it should be understood that a RAID system merely permits the computer system to function even though one of the drives is malfunctioning. It does not necessarily permit the computer system to be repaired or upgraded without powering down the system. To address this problem, various schemes have been developed, some related to RAID and some not, which facilitate the removal and/or installation of computer components, such as a faulty disk drive, without powering down the computer system. Such schemes are typically referred to as “hot plug” schemes since the devices may be unplugged from and/or plugged into the system while it is “hot” or operating.
Although hot plug schemes have been developed for many computer components, including microprocessors, memory chips, and disk drives, most such schemes do not permit the removal and replacement of a faulty device without downgrading system performance to some extent. Furthermore, because memory chips have been traditionally more reliable than disk drives, error detection and correction schemes for memory chips have generally lagged behind the schemes used for disk drives.
However, certain factors may suggest that the reliability of semiconductor memory systems may also require improvement. For instance, in the near future, it is believed that it will be desirable for approximately 50% of business applications to run continuously 24 hours a day, 365 days a years. Furthermore, in 1998, it was reported that the average cost of a minute of downtime for a mission-critical application was $10,000.00. In addition to the increasing criticality of such computer systems and the high cost of downtime of such systems, the amount of semiconductor memory
Galloway Jeffery
Johnson Jerome J.
Lai Ho M.
Larson John E.
Lester Robert A.
Baker Paul
Hewlett--Packard Development Company, L.P.
Sparks Donald
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