Browse Prior Art Database

Method for Skip Write Emulation

IP.com Disclosure Number: IPCOM000013069D
Original Publication Date: 2001-Apr-26
Included in the Prior Art Database: 2003-Jun-12
Document File: 4 page(s) / 63K

Publishing Venue

IBM

Abstract

Disclosed is a method for an I/O adapter (IOA) to emulate the Skip Write function. This emulation is performed transparent to the operating system software while minimizing any performance degradation vs Disk Drive implemented Skip Write function. The Skip Write function is a method for writing closely located, non-contiguous blocks of data to a random access storage medium such as a Disk Drive (or Direct Access Storage Device DASD). The requestor provides: a starting block address a length count of the number of blocks to write

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Method for Skip Write Emulation

   Disclosed is a method for an I/O adapter (IOA) to emulate the Skip Write function. This emulation is performed transparent to the operating system software while minimizing any performance degradation vs Disk Drive implemented Skip Write function.

The Skip Write function is a method for writing closely located, non-contiguous blocks of data to a random access storage medium such as a Disk Drive (or Direct Access Storage Device - DASD). The requestor provides:
a starting block address

a length count of the number of blocks to write

a Skip mask, where each bit in the mask corresponds to a DASD sector offset from

     the Starting DASD Logical Block Address. A '1' in a bit signifies that the DASD sector corresponding to that bit position will be written. A '0' in a bit signifies that the DASD sector corresponding to that bit position will not be written (i.e., Skipped). This function is especially useful in virtual memory paging operating systems and write caching systems since data with locality of reference can be combined into a single command. System performance improvements are achieved since: only the data that needs to be updated is transferred and

all of the data can be written by the Disk Drive without requiring additional

revolutions of the disk between non-contiguous blocks of data.

Some, but not all, Disk Drive vendors provide the Skip Write function. Described is a method for an I/O adapter to allow Disks that do not support Skip Write to attach to a system that requires all Disks that attach to it to support Skip Write.

A simplified representation of a system consists of a host processor, an I/O adapter, and a Disk drive.

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A scatter/gather list identifies the physical locations and lengths of the Host data buffers that are to be written to the DASD. The scatter/gather list could reside in either storage in the I/O adapter or in host system memory as shown in the figure.

Skip Write Command Emulation Sequencing

Host Operating System I/O Adapter (IOA) DASD

(200) Build Skip Write command and send it to the IOA

(210) Translate host operating system Skip Write command into multiple DASD Write commands and send all of the commands to the DASD. A separate DASD Wri...