Browse Prior Art Database

Expanded Cylinder Mode for High-End DASD

IP.com Disclosure Number: IPCOM000119259D
Original Publication Date: 1991-Jan-01
Included in the Prior Art Database: 2005-Apr-01
Document File: 3 page(s) / 146K

Publishing Venue

IBM

Related People

Menon, MJ: AUTHOR [+2]

Abstract

This article proposes an extended cylinder mode of storage allocation and hardware management which allows multiple actuator access to the extended cylinder while preserving the "seek affinity" characteristics of a cylinder. The result is that device skew is reduced with array DASD while throughput performance and data access are improved.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 37% of the total text.

Expanded Cylinder Mode for High-End DASD

      This article proposes an extended cylinder mode of
storage allocation and hardware management which allows multiple
actuator access to the extended cylinder while preserving the "seek
affinity" characteristics of a cylinder.  The result is that device
skew is reduced with array DASD while throughput performance and data
access are improved.

      Environment It is a characteristic of high-end computer
operating systems, such as MVS/TSO, IMS, and VM, that storage space
on DASD (disk storage) is allocated in a physically contiguous manner
by operating system function, application, or by user.  For example,
a user may be allocated twenty cylinders of storage on a particular
device ("volume").  (A cylinder is a number of corresponding tracks
on adjacent disks which can be accessed at a given position of the
access mechanism, i.e., without moving the access mechanism.)  The
result is that when the stated user's program is active, many of the
I/O requests can be handled with little or no motion of the DASD
access mechanism.  The result is a substantial improvement of I/O
response time for the user compared to the case where the data is
placed randomly on multiple tracks of the same device or on multiple
devices.  This feature of high-end storage is a substantial asset.

      The price paid for this improved response-time performance is
that at any given time most of the I/O requests are concentrated on
certain of the DASD volumes. As a result, the system throughput is
limited to the throughput of the active devices while a number of
devices stand by idle.  This situation is called "device skew" and is
a principal limitation to the system-level storage throughput.

      What is proposed is a storage allocation scheme which spreads a
given function's or user's data over a number of devices while
preserving a cylinder concept of contiguous data.  Advantageously,
the response-time of cylinder-wise allocation can be preserved by
this method while significantly improving skew-limited system
throughput.

      The physical embodiment of the proposed allocation scheme is
conceived to be in array-DASD, which facilitates the synchronization
of disk rotation and actuator movement. However, there is no
fundamental reason for restricting the present proposal to small
diameter disks configured in physical arrays; it would be physically
possible to achieve the same effect with large diameter DASD. The
Proposed "New" Cylinder and Its Access Procedures

      The usual cylinder comprises the corresponding tracks on the
disks on a common spindle that are accessed by a common actuator.
Thus, no actuator motion is necessary to switch between the tracks of
a cylinder.  One merely switches electronically between the heads on
the various disk surfaces.  A typical cylinder might have 15 tracks.

      What is proposed is the extension of the cylinder concept to
corresponding tracks in...