Browse Prior Art Database

Reducing Delay to Starting Disk File Seek

IP.com Disclosure Number: IPCOM000104709D
Original Publication Date: 1993-May-01
Included in the Prior Art Database: 2005-Mar-19
Document File: 2 page(s) / 106K

Publishing Venue

IBM

Related People

Jewell, CR: AUTHOR

Abstract

Disclosed is a method to reduce delay between a disk file receiving a media-accessing command and starting the physical seek to the data, where fixed block architecture is employed.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 50% of the total text.

Reducing Delay to Starting Disk File Seek

      Disclosed is a method to reduce delay between a disk file
receiving a media-accessing command and starting the physical seek to
the data, where fixed block architecture is employed.

      Disk files employing Fixed Block Architecture have a large
number of sectors each comprising an ID field and a data field.  The
ID field contains a Logical Block Address (LBA) which uniquely
identifies the block and is used by the file's control system to
quickly access the data in the sector.  The mapping of LBA's to
physical locations on the file is arbitrary, although it is usually
sequential with allowances for defects on the magnetic media.  A
command to the file involving a data access (eg:  a Read or a Write)
will specify the LBA.  The file control system must convert this LBA
into a physical location (ie: cylinder and head).  This conversion
usually involves a divide operation.  When the physical location has
been calculated the servo system must be instructed to start to move
the actuator to the appropriate cylinder.

      The physical seek is a major component of the time the file
takes to respond to a command as the most commonly used commands
usually involve a seek.  Once the seek has been started its progress
is handled by the servo system leaving the file control system free
to perform other tasks such as preparing the data channel ready for
data transfer to or from the head.  When the head arrives on the
specified cylinder it has to settle before reading can occur and must
settle more accurately for writing.

The current technique for dealing with a command is as follows:

1.  Receive the command and parameters.
2.  Decode the command completely.
3.  If the command requires a disk access, examine the LBA.
4.  Convert the LBA to a physical location either by means of lookup
    tables or integer dividing.
5.  Instruct the servo system to move the actuator to the correct
    cylinder and settle with the appropriate accuracy.
6.  Perform tasks necessary to prepare for data transfer (eg: by
    setting up the data channel).
7.    Await completion of the seek.

To reduce the time between receiving a command which may require a
seek and starting the seek the following assumptions have been made
in the method.

1.  A command can be partially decoded to determine whether a seek
    may be required or not.
2.  Once the seek has started there will be ample time before its
    completion in which processing in the file control system can
    take place.
3.  Only a part of the LBA needs to be converted to initiate a seek
    to a 'band' of cylinders.  The servo system can be reinstructed
    duirng the initial seek without disrupting the optimum trajectory
    to the exact cylinder provided that the servo system is
    reinstructed before the actuator needs to decelerate for the
    nearest possible cylinder in the band.

      The last as...