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Data Volatility Solution for Direct Access Storage Device Fast Write Commands

IP.com Disclosure Number: IPCOM000120612D
Original Publication Date: 1991-May-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 6 page(s) / 279K

Publishing Venue

IBM

Related People

Berg, WF: AUTHOR [+2]

Abstract

A method for handling the data volatility problem that is encountered during the execution of a direct access storage device (DASD) Fast Write command is disclosed. System CPU resources are used to maintain a temporary backup copy of the data transferred by a DASD Fast Write command until the system is notified by the DASD Subsystem Control Element that the data has been successfully written to the appropriate DASD.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 24% of the total text.

Data Volatility Solution for Direct Access Storage Device Fast Write
Commands

      A method for handling the data volatility problem that is
encountered during the execution of a direct access storage device
(DASD) Fast Write command is disclosed.  System CPU resources are
used to maintain a temporary backup copy of the data transferred by a
DASD Fast Write command until the system is notified by the DASD
Subsystem Control Element that the data has been successfully written
to the appropriate DASD.

      Refer to the figure for a simplified block diagram representing
the elements of a system that are pertinent to a description of this
disclosure.  The execution of a DASD Fast Write command would involve
the following sequence of events:
1.  A process running in the system processor issues a DASD Fast
Write command to the DASD Subsystem Control Element.
2.  The DASD Subsystem Control Element executes the command and
transfers data from main memory to the DASD Subsystem Control Element
RAM.
3.  The DASD Subsystem Control Element then signals command complete
to the process that issued the DASD Fast Write command.
4.  The DASD Subsystem Control Element then transfers data from the
DASD Subsystem Control Element RAM to the DASD.

      Note that command complete for the DASD Fast Write command is
signalled (event 3 above) to the process running in the system
processor before the data transferred by the command is actually
written (event 4 above) from the DASD Subsystem Control Element RAM
to the DASD.  This enhances the performance of any process that
generates synchronous write operations to DASD, since the physical
delays (arm motion, latency, data transfer) that are normally
associated with DASD Write commands are removed form the synchronous
path of the process, and performed by the DASD Subsystem Control
Element asynchronously.

      The time period bounded by events 3 and 4 above will be
referred to subsequently as the Data Volatility Window. During this
time period, data which the process running in the system processor
believes to be on DASD is actually stored in the DASD Subsystem
Control Element RAM.  It is, therefore, important from a data
integrity viewpoint that the DASD Subsystem Control Element RAM have
the same nonvolatility characteristics (with respect to various DASD
Subsystem failure mechanisms) as the DASD itself (which may be
protected using dual copy, redundant arrays, etc.)

      This invention describes a method of providing data integrity
for DASD Fast Write data during the DATA Volatility Window.  Refer to
the figure.  Using this new method, nonvolatility of the DATA in the
DASD Subsystem Control Element RAM (during the DATA Volatility
Window) is accomplished by keeping a copy of the DASD Fast Write data
in system main store until the DASD Subsystem Control Element signals
the system that the DASD Fast Write data has actually been
transferred to DASD.  The sequence of events for this...