Error detection/correction and fault detection/recovery – Data processing system error or fault handling – Reliability and availability
Reexamination Certificate
1999-01-19
2001-10-16
Ray, Gopal C. (Department: 2181)
Error detection/correction and fault detection/recovery
Data processing system error or fault handling
Reliability and availability
C707S793000, C711S162000
Reexamination Certificate
active
06304980
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to data backup systems. More particularly, the invention concerns a data storage system with primary and redundant backup storage, where the system automatically switches to the mirroring backup storage when an error occurs at the primary storage, and any reservation of the primary storage to a particular host is honored by the secondary storage.
2. Description of the Related Art
Many data processing systems require a large amount of data storage, for use in efficiently accessing, modifying, and re-storing data. Data storage is typically separated into several different levels, each level exhibiting a different data access time or data storage cost. A first, or highest level of data storage involves electronic memory, usually dynamic or static random access memory (DRAM or SRAM). Electronic memories take the form of semiconductor integrated circuits where millions of bytes of data can be stored on each circuit, with access to such bytes of data measured in nanoseconds. The electronic memory provides the fastest access to data since access is entirely electronic.
A second level of data storage usually involves direct access storage devices (DASD). DASD storage, for example, includes magnetic and/or optical disks. Data bits are stored as micrometer-sized magnetically or optically altered spots on a disk surface, representing the “ones” and “zeros” that comprise the binary value of the data bits. Magnetic DASD includes one or more disks that are coated with remnant magnetic material. The disks are rotatably mounted within a protected environment. Each disk is divided into many concentric tracks, or closely spaced circles. The data is stored serially, bit by bit, along each track. An access mechanism, known as a head disk assembly (HDA) typically includes one or more read/write heads, and is provided in each DASD for moving across the tracks to transfer the data to and from the surface of the disks as the disks are rotated past the read/write heads. DASDs can store gigabytes of data, and the access to such data is typically measured in milliseconds (orders of magnitudes slower than electronic memory). Access to data stored on DASD is slower than electronic memory due to the need to physically position the disk and HDA to the desired data storage location.
A third or lower level of data storage includes tapes, tape libraries, and optical disk libraries. Access to library data is much slower than electronic or DASD storage because a robot is necessary to select and load the needed data storage medium. An advantage of these storage systems is the reduced cost for very large data storage capabilities, on the order of terabytes of data. Tape storage is often used for backup purposes. That is, data stored at the higher levels of data storage hierarchy is reproduced for safe keeping on magnetic tape. Access to data stored on tape and/or in a library is presently on the order of seconds.
Having a backup data copy is mandatory for many businesses for which data loss would be catastrophic. The time required to recover lost data is also an important recovery consideration. With tape or library backup, primary data is periodically backed-up by making a copy on tape or library storage. One improvement over this arrangement is “dual copy,” which mirrors contents of a primary device with a nearly identical secondary device. An example of dual copy involves providing additional DASDs so that data is written to the additional DASDs substantially in real time along with the primary DASDs. Then, if the primary DASDs fail, the secondary DASDs can be used to provide otherwise lost data. A drawback to this approach is that the number of required DASDs is doubled.
A different data backup alternative that avoids the need to provide double the storage devices involves writing data to a redundant array of inexpensive devices (RAID). In this configuration, the data is apportioned among many DASDs. If a single DASD fails, then the lost data can be recovered by applying error correction procedures to the remaining data. Several different RAID configurations are available.
The foregoing backup solutions are generally sufficient to recover data in the event that a storage device or medium fails. These backup methods are useful only for device failures since the secondary data is a mirror of the primary data, that is, the secondary data has the same volume serial numbers (VOLSERs) and DASD addresses as the primary data. Data recovery due to system failures or storage controller failures, on the other hand, is not available using mirrored secondary data. Hence still further protection is required for recovering data if the entire system or even the site is destroyed by a disaster such as an earthquake, fire, explosion, hurricane, etc. Disaster recovery requires that the secondary copy of data be stored at a location remote from the primary data. A known method of providing disaster protection is to periodically backup data to tape, such as a daily or weekly basis. The tape is then picked up by a vehicle and taken to a secure storage area usually located kilometers from the primary data location. Nonetheless, this backup plan has its problems. First, it may take days to retrieve the backup data, and additional data is lost waiting for the backup data to be recovered. Furthermore, the same disaster may also destroy the storage location. A slightly improved backup method transmits data to a backup location each night. This allows the data to be stored at a more remote location. Again, some data may be lost between backups since backups do not occur continuously, as in the dual copy solution. Hence, a substantial amount of data may still be lost and this may be unacceptable to some users.
More recently introduced data disaster recovery solutions include “remote dual copy,” where data is backed-up not only remotely, but also continuously (either synchronously or asynchronously). In order to communicate duplexed data from one host processor to another host processor, or from one storage controller to another storage controller, or some combination thereof, a substantial amount of control data is required for realizing the process. A high overhead, however, can interfere with a secondary site's ability to keep up with a primary site's processing, thus threatening the ability of the secondary site to be able to recover the primary in the event a disaster occurs.
Disaster recovery protection for the typical data processing system requires that primary data stored on primary DASDs be backed-up at a secondary or remote location. The physical distance separating the primary and secondary locations can be set depending upon the level of risk acceptable to the user, and can vary from several kilometers to thousands of kilometers. The secondary or remote location, in addition to providing a backup data copy, must also have enough system information to take over processing for the primary system should the primary system become disabled. This is due in part because a single storage controller does not write data to both primary and secondary DASD strings at the primary and secondary sites. Instead, the primary data is stored on a primary DASD string attached to a primary storage controller while the secondary data is stored on a secondary DASD string attached to a secondary storage controller.
The secondary site must not only be sufficiently remote from the primary site, but must also be able to backup primary data in real time. The secondary site needs to backup primary data in real time as the primary data is updated, with some minimal delay. Additionally, the secondary site has to backup the primary data regardless of the application program (e.g., IMS, DB2) running at the primary site and generating the data and/or updates. A difficult task required of the secondary site is that the secondary data must be “order consistent,” that is, secondary data is copied in the same sequential order as the primary data (sequential con
Beardsley Brent Cameron
Iskiyan James Lincoln
Micka William Frank
Yudenfriend Harry Morris
Dan Hubert & Assoc.
International Business Machines - Corporation
Ray Gopal C.
LandOfFree
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