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Browse Prior Art Database

Continuous Data Stream Read Mode for DASD Arrays

IP.com Disclosure Number: IPCOM000115727D
Original Publication Date: 1995-Jun-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 2 page(s) / 61K

Publishing Venue

IBM

Related People

Ouchi, NK: AUTHOR

Abstract

In a RAID 3 Array, the N+1 devices are read and written in parallel. This provides a very high data rate. However, if the data transfer crosses cylinder boundaries, there is an interruption in the data stream as each device seeks to the next track. In many applications of RAID 3, the READ operation is done in real time while the WRITE operation is done in computational time. For example, a graphic image is rendered and stored frame by frame into the array but viewed as a stream of frames when viewed. Disclosed is a mechanism where the block written on the RAID 3 devices is offset so that only one of the N+1 devices is seeking to the next cylinder and the data for that device is reconstructed using the remaining N devices. The seek time is hidden by the RAID 3 reconstruction.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 59% of the total text.

Continuous Data Stream Read Mode for DASD Arrays

      In a RAID 3 Array, the N+1 devices are read and written in
parallel.  This provides a very high data rate.  However, if the data
transfer crosses cylinder boundaries, there is an interruption in the
data stream as each device seeks to the next track.  In many
applications of RAID 3, the READ operation is done in real time while
the WRITE operation is done in computational time.  For example, a
graphic image is rendered and stored frame by frame into the array
but viewed as  a stream of frames when viewed.  Disclosed is a
mechanism where the block written on the RAID 3 devices is offset so
that only one of the N+1 devices is seeking to the next cylinder and
the data for that device is reconstructed using the remaining N
devices.  The seek time is hidden by the RAID 3 reconstruction.  The
data that are recreated during the seek are written at the end of the
stream of blocks for use in case of a real loss of a drive.

      This is illustrated in the Figure where four drives are used in
a RAID 3 configuration.  Three drives are used for data and the
fourth is the parity drive.  The A blocks are all read in parallel.
The fourth drive is seeking in the B blocks.  Since it is the parity
drive, the block is not reconstructed.  The third drive is seeking in
the C blocks.  Block C3 is reconstructed from the other three drives.
Blocks D2 and E1 are reconstructed in a similar manner hiding the
seek time for de...