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Data Protection for DASD Array after Device Failure using Double Error-Correcting Code

IP.com Disclosure Number: IPCOM000115756D
Original Publication Date: 1995-Jun-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 2 page(s) / 58K

Publishing Venue

IBM

Related People

Ouchi, NK: AUTHOR

Abstract

In a DASD array, the parity domains are defined with a double error correcting code that would use N+2 devices per domain. N+1 devices are used and a small pool of devices are kept in case of failure. If a device in a domain fails, a device is taken from the pool and a double error correcting domain is formed to protect against a second failure.

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This is the abbreviated version, containing approximately 62% of the total text.

Data Protection for DASD Array after Device Failure using Double
Error-Correcting Code

      In a DASD array, the parity domains are defined with a double
error correcting code that would use N+2 devices per domain.  N+1
devices are used and a small pool of devices are kept in case of
failure.  If a device in a domain fails, a device is taken from the
pool and a double error correcting domain is formed to protect
against a second failure.

      Dumphy teaches that a device from a pool of spare devices may
be used to substitute for a failed device.  Data on the failed device
is reconstructed and recorded on the spare device.  This permits the
array to operate for long periods without repair.

      Disclosed is a mechanism that permits the use of spare devices
to protect data in an array until the failed device is repaired or
replaced.  The data on the failed device is not recorded on the spare
device.  In the disclosed mechanism, the array is defined as a set of
one or more parity domains each with double error-correction
capability.  This requires N+2 devices per domain.  However, only N+1
devices are used and the double error-correction capability is kept
latent.  A pool of devices, called Q drives, are assigned.  The Q
drives can be switched to any domain.  This is illustrated in Fig.
1a.

      When a device in a domain fails, the latent double
error-correction capability is activated by switching a Q device from
the pool.  The Q drive is co...