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Synchronizing Data On a Mirrored IMPL Disk Unit

IP.com Disclosure Number: IPCOM000102179D
Original Publication Date: 1990-Nov-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 6 page(s) / 288K

Publishing Venue

IBM

Related People

Lawrence, KJ: AUTHOR [+2]

Abstract

Disk mirroring improves the availability/recovery of the system by keeping the same data on a pair of disk units. When the load source unit is mirrored, either load source unit can be used when an initial program load (IPL) is requested. However, the IPL process can cause the data on the two load source units to become different. A method for minimizing the time to synchronize the data on the load source units during the IPL is disclosed. Also, a method for handling I/O errors that occur while accessing the load source unit during the IPL process is disclosed.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 21% of the total text.

Synchronizing Data On a Mirrored IMPL Disk Unit

       Disk mirroring improves the availability/recovery of the
system by keeping the same data on a pair of disk units. When the
load source unit is mirrored, either load source unit can be used
when an initial program load (IPL) is requested.  However, the IPL
process can cause the data on the two load source units to become
different.  A method for minimizing the time to synchronize the data
on the load source units during the IPL is disclosed.  Also, a method
for handling I/O errors that occur while accessing the load source
unit during the IPL process is disclosed.

      SYNCHRONIZATION OF THE LOAD SOURCE UNIT.  The load source unit
that is used to obtain the initial load is referred to in this
article as the IPL device.  At the start of the IPL, I/O can only be
performed to the IPL device. The other load source unit is not
available for I/O operations until later in the IPL process.
Consequently, when a write operation occurs to the IPL device before
access to the other load source unit is available, the data on the
two load source units are no longer the same.

      The data on the two load source units can be synchronized by
copying all of the data from the IPL device to the other load source
unit when the other load source unit becomes available.  However, the
increase in time to IPL the system would be unacceptable to the user.
Therefore, to minimize the amount of data that must be copied, a
table is created for tracing which sectors of the IPL device were
modified during the IPL while access to the other load source unit
was unavailable.  However, there are two problems with this solution:
-  The amount of time before the other load source unit becomes
available is not predictable.  Therefore, the number of writes to the
IPL device before the other load source unit is available is not
predictable.  Consequently, the trace table could overflow no matter
what size is chosen for the trace table.
-  The system could abnormally terminate before all trace table
entries are processed.  For example, a power failure can occur at any
point in time. To solve these problems, the following are done:
-  The load source is divided into data stripes.  With the exception
of the last data stripe, the size of each stripe is the same.  The
last stripe can be either a full data stripe (same size as the
others) or a partial data stripe.
-  Starting at one end of the load source unit, each data stripe is
assigned a stripe number.  The stripe numbers are assigned
sequentially, and can start with any value that is convenient for the
system.  For our purposes, we will assume the first data stripe is
assigned stripe number 1.
-  One or more sectors on the IPL device is reserved for a data
stripe array.  The stripe number is used to access the array entry.
Each array element contains the following flags for the data stripe:
;  MUST COPY - When this flag is set, it indicates t...