Browse Prior Art Database

Algorithm to Limit Sequential Access Domination of Non-Volatile Store

IP.com Disclosure Number: IPCOM000118356D
Original Publication Date: 1996-Dec-01
Included in the Prior Art Database: 2005-Apr-01
Document File: 2 page(s) / 73K

Publishing Venue

IBM

Related People

Blount, LC: AUTHOR [+6]

Abstract

A new threshold for Non-Volatile Store (NVS) management is proposed for sequential write channel programs. This threshold is set at 1 Megabyte (MB) of remaining freespace in NVS. When a sequential write channel program requests NVS space and the free space is below the 1MB sequential threshold, the chain is placed in Channel Command Retry (CCR) status. Four destages must complete prior to restarting the chain. Restarted chains do not check the sequential threshold.

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Algorithm to Limit Sequential Access Domination of Non-Volatile Store

      A new threshold for Non-Volatile Store (NVS) management is
proposed for sequential write channel programs.  This threshold is
set at 1 Megabyte (MB) of remaining freespace in NVS.  When a
sequential write channel program requests NVS space and the free
space is below the 1MB sequential threshold, the chain is placed in
Channel Command Retry (CCR) status.  Four destages must complete
prior to restarting the chain.  Restarted chains do not check the
sequential threshold.

      When Dasd Fast Write is active, the 3990-6 will branch all
writes to the control unit cache and NVS.  The data will be
asynchronously destaged from the control unit to the Direct Access
Storage Device (DASD) to maintain freespace in the NVS.  NVS manager
is the microcode process used to manage NVS space, including the
scheduling of destages to maintain adequate NVS space for channel
requests.  NVS management with a sequential allocation threshold
would work as follows:
  1.  If the channel program requesting NVS space is sequential
       (sequential access mode or sequential prestage mode set in
       the global attributes byte or the channel command is a
       format write (op codes 1D, 9D or 11)) NVS management is
       handled as follows:
      o  NVS manager will keep a flag bit available for easy
          query by the channel process to indicate whether there
          is enough NVS space available to satisfy sequential
          requests.  This threshold is set to 1MB of freespace
          but could be changed for tuning purposes.
      o  A sequential channel program requiring NVS checks the
          sequential threshold bit to see whether NVS allocation
          can proceed.  If the bit is off, NVS allocation proceeds
          normally.
      o  When the sequential threshold bit is on, the chain is
          placed in CCR status.
      o  As each sequential request is CCR'ed, a FIFO queue is
          built to allow the CCR'ed requests to be re-driven in
          the original order.
      o  After four destages have been processed, device ends
          are posted for the first four devices in t...