Electrical computers and digital processing systems: memory – Storage accessing and control – Access timing
Reexamination Certificate
1996-09-20
2001-02-13
Cabeca, John W. (Department: 2752)
Electrical computers and digital processing systems: memory
Storage accessing and control
Access timing
C711S170000, C711S004000
Reexamination Certificate
active
06189080
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally to disk storage systems having a disk cache, and in particular, to such a system in which there is a requirement for maintaining a minimum read rate in order to satisfy an application requirement.
In a typical high speed disk array system, such as that manufactured by EMC Corporation under the name Symmetrix, the disk array system is a cached disk array system which receives, from connected hosts, input/output (I/O) requests through an interconnection channel. In an open-systems environment, a typical channel is a SCSI channel and the I/O requests are composed of read operations and write operations. In the cached disk array, the write operations can all be handled by cache, thus allowing 100% of the write operations to be immediately written to cache and to be destaged later, that is, written from cache to disk. This allows for a very high rate for the write operation. On the other hand, read operations will be serviced either from cache, resulting in a high read response and designated a read “hit,” or from the disk which imposes a slower rate of response since a mechanical system is interposed into the read process. This latter operation is designated a read “miss.”
When the write rate is high, the method uses more and more of the disk cache to hold the data to be written (the pending write data) before it is destaged to disk. Accordingly, since cache memory for write operations is expended at the expense of the cache area used for read operations, less and less cache memory becomes available for the read operations as the pending write data builds up. The write operations also consume processing cycles of the control unit which also has to handle the read requests.
Typically, this may not be a problem. However in instances where the system is required to maintain a minimum rate of read operations, for example when the stored information on the disk represents digitized audio or digitized video that is being heard or seen in real time, a reduction in the read rate may interrupt the required continuous stream of audio or video information. Accordingly, such processes require at least a predetermined minimum read rate to insure uninterrupted decoding of the stored audio and/or video data.
SUMMARY OF THE INVENTION
The invention relates to a method and apparatus for maintaining a predetermined minimum data read rate from a disk drive system having a read/write cache memory. The method of the invention features the steps of sending, from a host computer to the drive system, a write cache utility factor inquiry; receiving, at the host system, the write cache utility factor information as provided by the disk drive system; determining whether a write rate to the data read/write cache should be changed based upon the utility factor information, and modifying the write rate, if necessary, in response to the determining step.
According to another aspect of the invention, the write cache utility factor information includes at least two of the following: a value representative of the total available cache memory in the disk drive system; a value representative of the maximum write data cache memory permitted to enable the minimum read rate to be maintained; and a value representative of the current write cache memory being used for pending write data.
The method further features determining the write cache utility factor information at a plurality of time instances, determining from this cache utility factor information the rate at which write data destages in the disk drive system and adjusting the write rate in response to the destage rate and the cache utility factor information. In one aspect, the write rate is maintained approximately equal to the destage rate.
In another aspect of the invention, the method features modifying the write rate in response to the unused write cache availability as determined from the cache utility factor information.
In a preferred aspect of the invention, the sending of the write cache utility factor features sending a SCSI Inquiry command to the disk drive system from the host computer and the receiving step features receiving a SCSI response to the Inquiry command that contains the write cache utility factor information.
The apparatus of the invention features a host system having a host system interface for requesting write cache utility factor information from the disk drive system and the disk drive system has a memory system interface for receiving the host factor request, and a controller connected to the memory system interface for processing the request and for generating a response to the request. The response includes the write cache utility factor information. The host further has a write rate control element for controlling the write rate in response to the drive system controller response.
This system provides advantageous operation where a minimum read rate must be maintained, and where at the same time, a mechanism is provided to service write operations consistent with the minimum read rate requirement. This system can automatically adjust to different cache sizes and operating parameters, and additional hardware units need not be added where, for example, the SCSI Inquiry command is used to acquire certain information about the cache usage, and the disk drive system has been configured to respond to that command.
REFERENCES:
patent: 5353430 (1994-10-01), Lautzenheiser
patent: 5499378 (1996-03-01), McNeill, Jr. et al.
patent: 5561823 (1996-10-01), Anderson
patent: 5586291 (1996-12-01), Lasker et al.
patent: 5603062 (1997-02-01), Sato et al.
patent: 5636355 (1997-06-01), Ramakrishnan et al.
Cabeca John W.
EMC Corporation
Fish & Richardson P.C.
Tzeng Fred F.
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