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Identification of Timeout Parameter for Switching Cartridge Disposition

IP.com Disclosure Number: IPCOM000108938D
Original Publication Date: 1992-Jul-01
Included in the Prior Art Database: 2005-Mar-23
Document File: 7 page(s) / 324K

Publishing Venue

IBM

Related People

Willson, LD: AUTHOR [+3]

Abstract

There are two performance-oriented cartridge dispositions which may be selected in an optical or tape library, KEEP and Unload_LRU. This article gives an automated means for selecting the timeout parameter X used for switching from KEEP to Unload_LRU, (see the above figure based on known mechanical parameters of the library and a running average of parameters which quantify the workload. This article will assume that the library holds optical drives.

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Identification of Timeout Parameter for Switching Cartridge Disposition

       There are two performance-oriented cartridge dispositions
which may be selected in an optical or tape library, KEEP and
Unload_LRU.  This article gives an automated means for selecting the
timeout parameter X used for switching from KEEP to Unload_LRU, (see
the above figure based on known mechanical parameters of the library
and a running average of parameters which quantify the workload.
This article will assume that the library holds optical drives.

      In the cartridge disposition of KEEP, inactive cartridges are
kept mounted in their respective drives until the next mount request
arrives.  This disposition of KEEP maximizes the on-line capacity of
the library box, which is valuable when the probability of additional
accesses to the mounted cartridge is high.  However, the time to
empty a drive must be added to the time to mount a new cartridge in
that drive, as shown in Table 1.  So, the disposition of KEEP can
lengthen the response time when the workload is primarily one of
random retrievals.

      In Table 1, the invidual times T1, T2, etc., are incremental
times.  Cumulative times are shown with an underscore, i.e., T1_8
represents T1 + T2 + T3 + T4 + T5 + T6 + T7 +
T8.

      An alternative cartridge disposition is that of preemptively
Unloading the Least Recently Used (Unload_LRU) inactive cartridge in
anticipation of the next mount request.  This cartridge disposition
is valuable when the robotic picker in the library has only one
gripper.  Such a single gripper picker must demount the cartridge
currently in the drive and put it away prior to fetching the new
mount request and loading it in that drive.  Because it is inherently
faster to load a new mount request into an empty drive (Table 1), the
Unload_LRU disposition offers substantial time savings when the
pattern of cartridge retrievals is random.

      In the figure, the value of X of zero results in the cartridge
disposition of Unload_LRU and the value of X of a large number, say,
999 seconds, results in the cartridge disposition of KEEP.  A value
of X in between 0 and 999 allows the user to keep inactive cartridges
mounted pending a possible additional access for X seconds before
that inactive cartridge is eligible for preemptive dismount under the
Unload_LRU algorithm.

      A library user could manually set the timeout parameter X via a
console, if that is part of the library.  However, if the workload
varies during the workday, a static value of this timeout parameter
could be detrimental to performance.

      This article proposes an automated means of determining the
timeout parameter X in the figure based on the following parameters.
It should be noted that cartridge flipping occurs in optical
libraries when the optical disks are dual-sided and the optical
drives only have one head.  In optical libraries with single-headed
drives, each side of t...