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Background Operations for Removable Media Libraries

IP.com Disclosure Number: IPCOM000109333D
Original Publication Date: 1992-Aug-01
Included in the Prior Art Database: 2005-Mar-23
Document File: 3 page(s) / 141K

Publishing Venue

IBM

Related People

Burke, WT: AUTHOR [+4]

Abstract

The performance of libraries of removable media can be improved by segregating the overall workload into high priority foreground activity and lower priority background activity. Foreground activity may consist of reading information from or writing information to the media in the library. Such media may consist of magnetic tape, magneto-optical or write-once disks, or any other type of removable media. Activity of lower priority than the reading and writing of data may be relegated to the background so that it does not compete against foreground activity for control unit, channel, device, etc., resources.

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Background Operations for Removable Media Libraries

       The performance of libraries of removable media can be
improved by segregating the overall workload into high priority
foreground activity and lower priority background activity.
Foreground activity may consist of reading information from or
writing information to the media in the library.  Such media may
consist of magnetic tape, magneto-optical or write-once disks, or any
other type of removable media.  Activity of lower priority than the
reading and writing of data may be relegated to the background so
that it does not compete against foreground activity for control
unit, channel, device, etc., resources.

      This article enumerates several types of background activities
which can improve the performance of foreground activity.  Examples
of preferred background activity include:
*    Pre-erasing magneto-optical (MO) disks so that subsequent write
operations may proceed without an erase,
*    Defragmenting rewritable disks,
*    Determining the Least Recently Used (LRU) inactive disk in the
devices, and
*    Determining the devices which have been inactive for a long
period of time for spin-down. Other background activities could
include:
*    Periodically checking the status of each device, and
*    Checking the location of cartridges in the library.

      The current technology of MO recording requires that old
information be erased in one pass under the laser before a second
recording pass can be made.  During periods of library or device
quiescence, files previously marked for deletion could be erased from
MO disks.  This will allow faster write activity during foreground
operations.

      Rewritable disk media needs periodic defragmentation.  Items
could be erased from the disks, leaving zones of erased and written
tracks.  Unless the written tracks are aggregated, subsequently
written files will have to utilize "extents" as they span a plurality
of formerly erased zones.  Extents cost write and read performance
because a seek and latency period is involved with each extent.  By
defragmenting the disks during periods of library quiescence, future
read and write activity on that disk will be improved.  This may be
done during periods of device quiescence simply by aggregating
sections of files to remove extents.  Should periods of library
quiescence occur, then whole disks may be defragmented en masse.

      Many customers use file folder applications.  Rather than have
an individual's file scattered across several cartridges, we suggest
that this information could be aggregated during periods of library
quiescence.  In a generalized form of defragmentation, file folder
segments from different cartridges could be aggregated to one
cartridge for faster foreground operations.  This way, for example,
when a hospital wants to see a patient's records, they can be
retrieved in one cartridge mount rather than through a series...