Electrical computers and digital processing systems: multicomput – Remote data accessing – Accessing a remote server
Reexamination Certificate
2002-11-09
2004-10-12
Kim, Kenneth S. (Department: 2111)
Electrical computers and digital processing systems: multicomput
Remote data accessing
Accessing a remote server
C711S141000, C709S218000, C709S203000
Reexamination Certificate
active
06804706
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to the technical field of multi-processor digital computer systems and, more particularly, to multi-processor computer systems in which:
1. the processors are loosely coupled or networked together;
2. data needed by some of the processors is controlled by a different processor that manages the storage of and access to the data;
3. processors needing access to data request such access from the processor that controls the data;
4. the processor controlling data provides requesting processors with access to it.
BACKGROUND ART
Within a digital computer system, processing data stored in a memory; e.g., a Random Access Memory (“RAM”) or on a storage device such as a floppy disk drive, a hard disk drive, a tape drive, etc.; requires copying the data from one location to another prior to processing: Thus, for example, prior to processing data stored in a file in a comparatively slow speed storage device such as hard disk, the data is first copied from the computer system's hard disk to its much higher speed RAM. After data has been copied from the hard disk to the RAM, the data is again copied from the RAM to the computer system's processing unit where it is actually processed. Each of these copies of the data, i.e., the copy of the data stored in the RAM and the copy of the data processed by the processing unit, can be considered to be image of the data stored on the hard disk. Each of these images of the data may be referred to as a projection of the data stored on the hard disk.
In a loosely coupled or networked computer system having several processors that operate autonomously, the data needed by one processor may be accessed only by communications passing through one or more of the other processors in the system. For example, in a Local Area Network (“LAN”) such as Ethernet one of the processors may be dedicated to operating as a file server that receives data from other processors via the network for storage on its hard disk, and supplies data from its hard disk to the other processors via the network. In such networked computer systems, data may pass through several processors in being transmitted from its source at one processor to the processor requesting it.
In some networked computer systems, images of data are transmitted directly from their source to a requesting processor. One operating characteristic of networked computer systems of this type is that, as the number of requests for access to data increase and/or the amount of data being transmitted in processing each request increases, ultimately the processor controlling access to the data or the data transmission network becomes incapable of responding to requests within an acceptable time interval. Thus, in such networked computer systems, an increasing workload on the processor controlling access to data or on the data transmission network ultimately causes unacceptably long delays between a processor's request to access data and completion of the requested access.
In an attempt to reduce delays in providing access to data in networked computer systems, there presently exist systems that project an image of data from its source into an intermediate storage location in which the data is more accessible than at the source of the data. The intermediate storage location in such systems is frequently referred to as a “cache,” and systems that project images of data into a cache are be referred to as “caching” systems.
An important characteristic of caching systems, frequently referred to as “cache consistency” or “cache coherency,” is their ability to simultaneously provide all processors in the networked computer system with identical copies of the data. If several processors concurrently request access to the same data, one processor may be updating the data while another processor is in the process of referring to the data being updated. For example, in commercial transactions occurring on a networked computer system one processor may be accessing data to determine if a customer has exceeded their credit limit while another processor is simultaneously posting a charge against that customer's account. If a caching system lacks cache consistency, it is possible that one processor's access to data to determine if the customer has exceeded their credit limit will use a projected image of the customer's data that has not been updated with the most recent charge. Conversely, in a caching system that possesses complete or absolute cache consistency, the processor that is checking the credit limit is guaranteed that the data it receives incorporates the most recent modifications.
One presently known system that employs data caching is the Berkeley Software Distribution (“BSD”) 4.3 version of the Unix timesharing operating system. The BSD 4.3 system includes a buffer cache located in the host computer's RAM for storing projected images of blocks of data, typically 8 k bytes, from files stored on a hard disk drive. Before a particular item of data may be accessed on a BSD 4.3 system, the requested data must be projected from the hard disk into the buffer cache. However, before the data may be projected from the disk into the buffer cache, space must first be found in the cache to store the projected image. Thus, for data that is not already present in a BSD 4.3 system's buffer cache, the system must perform the following steps in providing access to the data:
Locate the buffer in the RAM that contains the Least Recently Used (“LRU”) block of disk data.
Discard the LRU block of data which may entail writing that block of data back to the hard disk.
Project an image of the requested block of data into the now empty buffer.
Provide the requesting processor with access to the data.
If the data being accessed by a processor is already present in a BSD 4.3 system's data cache, then responding to a processor's request for access to data requires only the last operation listed above. Because accessing data stored in RAM is much faster that accessing data stored on a hard disk, a BSD 4.3 system responds to requests for access to data that is present in its buffer cache in approximately {fraction (1/250)}th the time that it takes to respond to a request for access to data that is not already present in the buffer cache.
The consistency of data images projected into the buffer cache in a BSD 4.3 system is excellent. Since the only path from processors requesting access to data on the hard disk is through the BSD 4.3 system's buffer cache, out of date blocks of data in the buffer cache are always overwritten by their more current counterpart when that block's data returns from the accessing processor. Thus, in the BSD 4.3 system an image of data in the system's buffer cache always reflects the true state of the file. When multiple requests contend for the same image, the BSD 4.3 system queues the requests from the various processors and sequences the requests such that each request is completely serviced before any processing commences on the next request. Employing the preceding strategy, the BSD 4.3 system ensures the integrity of data at the level of individual requests for access to segments of file data stored on a hard disk.
Because the BSD 4.3 system provides access to data from its buffer cache, blocks of data on the hard disk frequently do not reflect the true state of the data. That is, in the BSD 4.3 system, frequently the true state of a file exists in the projected image in the system's buffer cache that has been modified since being projected there from the hard disk, and that has not yet been written back to the hard disk. In the BSD 4.3 system, images of data that are more current than and differ from their source on the hard disk data may persist for very long periods of time, finally being written back to the hard disk just before the image is about to be discarded due to its “death” by the LRU process. Conversely, other caching systems exist that maintain data stored on the hard disk current with
Kim Kenneth S.
Network Caching Technology, L.L.C.
Schreiber Donald E.
LandOfFree
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