Decoupling storage controller cache read replacement from write retirement

Abstract

In a data storage controller, accessed tracks are temporarily stored in a cache, with write data being stored in a first cache and a second cache and read data being stored in a second cache . Corresponding least recently used (LRU) lists are maintained to hold entries identifying the tracks stored in the caches. When the list holding entries for the first cache (the A list) is full, the list is scanned to identify unmodified (read) data which can be discarded from the cache to make room for new data. Prior to or during the scan, modified (write) data entries are moved to the most recently used (MRU) end of the list, allowing the scans to proceed in an efficient manner and reducing the number of times the scan has to skip over modified entries. Optionally, a status bit may be associated with each modified data entry. When the modified entry is moved to the MRU end of the A list without being requested to be read, its status bit is changed from an initial state to a second state, indicating that it is a candidate to be discarded. If the status bit is already set to the second state, then it is left unchanged. If a modified track is moved to the MRU end of the A list as a result of being requested to be read, the status bit of the corresponding A list entry is changed back to the first state, preventing the track from being discarded. Thus, write tracks are allowed to remain in the first cache only as long as necessary.

Description

technical field [0001] The present invention relates generally to data storage controllers, and more particularly to establishing cache discard and degradation policies. Background technique [0002] A data storage controller, such as the International Business Machines Corporation Enterprise Storage Server(R), receives input / output (I / O) requests directed to the attached storage system. An attached storage system may include one or more enclosures that include a large number of interconnected disk drives, such as Random Access Storage Devices (DASD), Redundant Array of Independent Disks (RAID Array), Simple Disk Cluster ( JBOD) and so on. If I / O read and write requests are received at a rate faster than they can be processed, the storage controller will queue the I / O requests in the primary cache, which may consist of one or more gigabytes volatile storage devices, such as Random Access Memory (RAM), Dynamic Random Access Memory (DRAM), and the like. A copy of some modif...

AI Technical Summary

Problems solved by technology

It should be appreciated that jumping through so many cache tracks takes a significant amount of time and wastes processor cycles

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