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Fast Dual Write

IP.com Disclosure Number: IPCOM000104207D
Original Publication Date: 1993-Mar-01
Included in the Prior Art Database: 2005-Mar-18
Document File: 4 page(s) / 138K

Publishing Venue

IBM

Related People

Cord, JH: AUTHOR [+5]

Abstract

The algorithms for Dual Copy with DASD Fast Write do not provide the performance desired.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 31% of the total text.

Fast Dual Write

      The algorithms for Dual Copy with DASD Fast Write do not
provide the performance desired.

      This publication applies to the IBM 3990* Model 3.  It is based
upon current algorithms for Dual Copy and DASD Fast Write.

      In the current environment with Fast Dual Copy, the performance
for a write hit is equivalent to that of a non-DASD Fast Write
Caching device.  With Fast Dual Write, the performance for a write
hit is equivalent to that of DASD Fast Write.  Experience in the
current customer environment shows that write hit ratios are above
ninety percent.  Thus for most applications, Fast Dual Write would be
nearly equivalent to DASD Fast Write in terms of performance.

      Fast Dual Write combines elements of DASD Fast Write and Fast
Dual Copy.  Its purpose is to provide DASD Fast Write performance on
write hits (including predictable writes) while maintaining two
copies of data on Duplex Pairs of DASD.  Therefore, it provides DASD
Fast Write with the added availablility due to the maintenance of two
DASD copies of data.

      Fast Dual Write can be characterized as follows:

      1.   It has DASD Fast Write performance on write hits.

      2.   It has Dual Copy performance on write misses.

      3.   It destages two copies of written data compared to the
single
     copy of DASD Fast Write, and therefore uses more GALLUP
     resources than DASD Fast Write.

      For reasons 2 and 3, Fast Dual Write generally provides less
performance than DASD Fast Write.  However, for reason 1, it provides
greater performance than Fast Dual Copy.

      Fast Dual Write enhances the availability characteristics of
DASD Fast Write by providing two DASD copies of data, but it does not
equal the availability of Dual Copy and DASD Fast Write, since the
latter provides a synchronous copy on the primary volume.  Device
availability is protected in the case of device failure; otherwise
availability is equal only to that of DASD Fast Write.

Fast Dual Write Objectives

o   Enhanced performance while maintaining two DASD copies of data

Fast Dual Write will enable users to achieve write hit performance
comparable to DASD Fast Write while maintaining two DASD copies of
data.

o   Enhanced availability for DASD Fast Write eul.  Fast Dual Write
    will enable DASD Fast Write users to experience increased
    availability through the automatic maintenance of two copies of
    data.  This provides a level of protection against DASD failures
    not available to the standard DASD Fast Write user.

    Fast Dual Write Description:  Fast Dual Write can most easily be
    described by listing its prerequisites and components.

    -   Prerequisites:

        In order for the Fast Dual Write function to operate, Duplex
        Pairs of DASD must be established for the volumes which are
        objects of the Fast Dual Write operation...