Browse Prior Art Database

Record Mode Extension to Track Mode Caching DASD Subsystem

IP.com Disclosure Number: IPCOM000100350D
Original Publication Date: 1990-Apr-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 8 page(s) / 393K

Publishing Venue

IBM

Related People

Legvold, VJ: AUTHOR [+3]

Abstract

Present caching DASD subsystems are functionally structured for general-purpose usage and exhibit a significant subsystem performance improvement over non-caching subsystems. These caching subsystems are identified as operating in Track Mode. These subsystems cache data with a high probability of being initially referenced or re-referenced in a device track image format, e.g., all of the formatted fields within a record (such as Count, Key and Data) for all records on the device track from the first referenced record up to the logical or physical end-of-track.

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Record Mode Extension to Track Mode Caching DASD Subsystem

       Present caching DASD subsystems are functionally
structured for general-purpose usage and exhibit a significant
subsystem performance improvement over non-caching subsystems.  These
caching subsystems are identified as operating in Track Mode.  These
subsystems cache data with a high probability of being initially
referenced or re-referenced in a device track image format, e.g., all
of the formatted fields within a record (such as Count, Key and Data)
for all records on the device track from the first referenced record
up to the logical or physical end-of-track.  Some data transfer
command chains (read hit, fast write hit, and fast write miss for
full track reformat) can be satisfied with access to the cache only,
but other command chains (read miss, retentive write hit or miss, and
fast write miss for update) require device access.  When device
access is required, host channel reconnection must be aligned with
device rotational positioning before data transfers between host
channel and device can be effected, and data branching from host
channel to both the device and cache storage for write operations and
from the device to both the host channel and cache storage for read
operations is practiced when the caching algorithms prescribe that
data is to be retained in the cache storage at the completion of the
command chain.

      When the use of a caching DASD subsystem includes Transaction
Processing applications, e.g., random access to either a single
device record or a small group of sequentially positioned device
records, the attributes of a Track Mode caching DASD subsystem,
described above, do not produce the desired levels of subsystem
performance or cache storage space utilization:
o    In device track image caching, cache space will be used for data
records that will not be referenced before the track is aged out of
the cache.
o    Cache storage space allocation algorithms initially assign a
number of fixed sized segments sufficient to contain a complete
formatted device track.  Unused segments are returned to the
available list but unused portions of a segment cannot be re-claimed.
o    Excessive device interface busy time staging full or partial
tracks to cache storage when only a single record or a small group of
records from that track will ever be referenced.
o    Some data records accessed, but not cached, are potential re-
referenced records in a Transaction Processing environment.
o    Device access chains require device-to-host channel
synchronization which involves device rotational delays and host
channel re-connection miss delays.

      This article describes a Record Mode facility added to a
conventional Track Mode caching DASD subsystem.  This Record Mode
facility offers the performance enhancement and cache storage space
utilization improvement of individual record image caching in place
of track image caching, and the...