Browse Prior Art Database

Cache Storage of DASD Tracks in a Split Mode

IP.com Disclosure Number: IPCOM000060497D
Original Publication Date: 1986-Apr-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 3 page(s) / 33K

Publishing Venue

IBM

Related People

Benhase, MT: AUTHOR [+2]

Abstract

Elapsed time for promoting or staging data from a DASD (direct-access storage device) to its cache is minimized by using a roll mode. That is, reading of any DASD track occurs at the first opportunity to read a full record rather than waiting for index to begin reading. For reduced overhead, the first read record is stored in the lowest address portion of a cache-tracked slot (allocation of cache for storing one track of DASD data). As a result, the index point is at an intermediate point of the track slot.

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Cache Storage of DASD Tracks in a Split Mode

Elapsed time for promoting or staging data from a DASD (direct-access storage device) to its cache is minimized by using a roll mode. That is, reading of any DASD track occurs at the first opportunity to read a full record rather than waiting for index to begin reading. For reduced overhead, the first read record is stored in the lowest address portion of a cache-tracked slot (allocation of cache for storing one track of DASD data). As a result, the index point is at an intermediate point of the track slot. A set of three track-slot pointers (one set per slot) includes an index pointer for indicating the address within the slot where index occurs; a home-address pointer indicating the location in the track slot of the home address; and a break-track pointer indicating the address of the track slot which represents the end of the last record read during the roll mode, which is the record on DASD immediately preceding the first record read in the roll mode. In most instances, a DASD track is not filled with data; therefore, the break address of a track slot represents the last address containing valid data. The addresses following the break address to the end of the track slot having the highest slot address contain no data. On a read miss, the control independently accesses DASD to promote the entire addressed track of DASD data to cache after the end of the channel program causing the read miss. When the entire DASD track is to be promoted to cache, the transfer of data from the DASD to the cache may begin with any DASD record. The control implements a function known as a "roll mode stage". Before the track of DASD data can be promoted to the cache, the DASD actuator is positioned to the proper DASD track (via a Seek) but no Set Sector is issued. This activates the "roll mode" which allows data transfer to begin as soon as the Seek has completed, saving the overhead of a Set Sector and latency delays. In a roll mode example, record "N" is the first record transferred from DASD to cache. Record N is stored at the lowest address of the cache track slot. Therefore, when the data transfer is complete, the resulting DASD track image that resides in the cache is called a "split track".

The first half of the split track contains record "N" to Index. The second half of the split track contains the home address (HA) to record "N-1". Since the data transfer from the DASD to the cache may begin with any record on the track, a mechanism is required for determining when the entire track is in the cache so that the data transfer is ended. The home-address field and every count field on high performance DASD devices contain a cell number which indicates the location on the disk surface in which the field resides. This cell number can be used to uniquely identify every record...