Browse Prior Art Database

Redundant Array of Independent Disks 5 Parity Cache Enhanced by Data Write Cache

IP.com Disclosure Number: IPCOM000114467D
Original Publication Date: 1994-Dec-01
Included in the Prior Art Database: 2005-Mar-28
Document File: 2 page(s) / 53K

Publishing Venue

IBM

Related People

Larson, JA: AUTHOR

Abstract

A method of guaranteeing Redundant Array of Independent Disks (RAID) 5 parity cache hits is disclosed. The method assumes a host write-back data cache exists on the RAID 5 controller. Parity is not placed in the parity cache unless the host cache guarantees a parity cache hit.

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Redundant Array of Independent Disks 5 Parity Cache Enhanced by Data
Write Cache

      A method of guaranteeing Redundant Array of Independent Disks
(RAID) 5 parity cache hits is disclosed.  The method assumes a host
write-back data cache exists on the RAID 5 controller.  Parity is not
placed in the parity cache unless the host cache guarantees a parity
cache hit.

      Fig. 1 defines two RAID 5 parity groups.  This particular
configuration has 4 disks, indexed 0-3.  The parity groups are
indexed 0 and 1.  As in conventional RAID 5, P0 checksums D01, D02
and D03, and P1 checksums D10, D12 and D13.

      Assume this RAID 5 controller has two caches: a host write-back
data cache and a parity cache (either write-through or write-back).

      Fig. 2 illustrates a scenario.  An X indicates that the data
band has data staged in the host write cache (the disk has old data,
the cache has new data).  Assume that neither P0 or P1 are in the
parity cache initially.

      The idea is that parity is not cached unless it is guaranteed
to be hit.  Consider two cases of parity group 0:
  o  Case 1: In this case, assume that the data in D01 is ripe to be
      destaged from the host write cache, and that the data is
      contiguous in location.  A search of the host write cache
reveals
      that neither D02 or D03 exist in the cache, and that the cached
      data for band D01 can be destaged in a single destage
operation.
     ...