US 2003/0158996 A1 · System and method for efficiently sorting DASD queued commands with unknown rotational latency

Abstract

A method and apparatus are provided for efficiently sorting queued commands with unknown rotational latency in a direct access storage device (DASD). Simultaneously with executing a current command, for each command in a DASD command queue an initial expected access time (EAT) including a probability of success calculation is calculated and stored together with a plurality of predefined variables in a respective row in a miss table. Utilizing the stored miss table, a best candidate command list for a plurality of different starting locations for a next command to be executed is calculated. An actual starting location for the next command to be executed is identified. Utilizing the best candidate command list for the plurality of different starting locations, a command in a DASD command queue is selected based upon the actual starting location. The initial expected access time is calculated for each command in the DASD command queue using a target starting location defined as an expected ending location of the currently executing command. The best candidate command list for the plurality of different starting locations is calculated by first sorting the stored miss table rows by increasing expected access time values and sequentially incrementing a miss time value by the predefined unit of time and updating all variables in the first row and adjusting a potential value in the second row of the sorted miss table. The updated EAT in the first row is compared to the adjusted potential variable in the second row. When the updated EAT in the first row is greater than the potential variable in the second row in the sorted miss table, then the EAT and the stored predefined variables in the second row are updated. The miss table rows then are sorted by increasing expected access time values and a new entry is stored in the best candidate command list. The sequential steps continue until the currently executing command is stored in the first row of the sorted miss table.

Claims20

Dependent claims

A method for efficiently sorting DASD queued commands with unknown rotational latency comprising the steps of:

executing a current command;
for each command in a DASD command queue, simultaneously with executing said current command calculating an initial expected access time including a probability of success calculation and storing said calculated expected access time together with a plurality of predefined variables in a respective row in a miss table;
utilizing said stored miss table, simultaneously with executing said current command calculating a best candidate command list for a plurality of different starting locations for a next command to be executed;
identifying an actual starting location for said next command to be executed;
and
utilizing said best candidate command list for said plurality of different starting locations, selecting a command in a DASD command queue to execute for said actual starting location.

3 more elements

Apparatus in a direct access storage device (DASD) for efficiently sorting DASD queued commands with unknown rotational latency comprising:

a processor;
said processor for executing a current command;
said processor for each command in a DASD command queue simultaneously with executing said current command, for calculating an initial expected access time (EAT) including a probability of success calculation and storing said calculated initial EAT together with a plurality of predefined variables in a respective row in a miss table;
said processor utilizing said stored miss table simultaneously with executing a current command, for calculating a best candidate command list for a plurality of different starting locations for a next command to be executed;
said processor for identifying an actual starting location for said next command to be executed;
and
said processor utilizing said best candidate command list for said plurality of different starting locations, for selecting a command in a DASD command queue to execute for said actual starting location.

4 more elements

Dependencies are inferred from the claim text.

System and method for efficiently sorting DASD queued commands with unknown rotational latency

Life of this patent

System and method for efficiently sorting DASD queued commands with unknown rotational latency

Life of this patent

Abstract

Claims20