Browse Prior Art Database

Improved Disconne-t/Reconnect Criteria in Data Storage Devices

IP.com Disclosure Number: IPCOM000106201D
Original Publication Date: 1993-Oct-01
Included in the Prior Art Database: 2005-Mar-20
Document File: 4 page(s) / 159K

Publishing Venue

IBM

Related People

Bish, JE: AUTHOR [+4]

Abstract

Automated storage libraries employ data storage devices, such as tape and optical disk drives, which have a device-level buffer that can be used to optimize the throughput of data between the control unit of the library and the media. The device-level buffer can be viewed as a speed matching device, as the I/O operations between device and the control unit are much faster than the I/O operations between the head and the media. Because the communications between the device and the control unit are so much faster, the lower interface of the control unit can effect concurrent operations over a plurality of devices by disconnecting and reconnecting with individual devices as required.

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Improved Disconne-t/Reconnect Criteria in Data Storage Devices

      Automated storage libraries employ data storage devices, such
as tape and optical disk drives, which have a device-level buffer
that can be used to optimize the throughput of data between the
control unit of the library and the media.  The device-level buffer
can be viewed as a speed matching device, as the I/O operations
between device and the control unit are much faster than the I/O
operations between the head and the media.  Because the
communications between the device and the control unit are so much
faster, the lower interface of the control unit can effect concurrent
operations over a plurality of devices by disconnecting and
reconnecting with individual devices as required.  It is the goal of
this article to optimize data throughput in the library by maximizing
the effective data rate of individual devices and maximizing the
concurrency of the devices within the library.  Although applicable
to any interface between the devices and the control unit, it is
assumed that the SCSI (Small Computer Systems Interface) interface is
used between the control unit and the devices it services.

      The optimum time to reconnect on a read operation is such that
the drive and the interface finish transferring data at the same
time, the Figure.  Essentially, this means that the reconnect occurs
before the read operation is concluded.

      If the device connects at the beginning of the read operation
and stays connected during the entire read, the single drive read
effective data rate (EDR) is optimized.  However, this results in the
lower interface of the control unit being totally dedicated to a
single drive during any I/O operation, preventing concurrent
operations with the other drives in the library.

      If the device connects when its buffer is 100% full, the device
will remain idle until the interface transfer between drive and
control unit is complete.  This will maximize the availability of the
lower interface of the the control unit for concurrent operations.
However, this serialization of the data flow reduces the read EDR of
each device.

      The optimum case is where the device reconnects so that the
data transfers between the device & buffer and buffer & control unit
complete at the same time.  At this exact point, the overall read
process will be optimized since (a) another I/O request may be
immediately received by that device and (b) the control unit
interface can service other devices in the same library, maximizing
concurrent operations in the library.

      The optimum case listed above assumes that the requested read
data will all fit in the buffer.  This is usually the case since
buffer sizes range between 64K and 256K with most devices tending
toward the high end.

      One means that the user may employ to specify when the device
should reconnect to the control unit to send the data on a read
command is the bu...