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Media Information Region
The T9840 tape drives use information recorded on each tape cartridge to access and
manage that tape cartridge while it is loaded in the drive. This information is
recorded at the beginning of the tape in an area known as the Media Information
Region (MIR). The information contained in the MIR falls into two major categories:
• Statistical Counters
Statistical counters include read/write activity, error activity, cumulative mounts,
and other information that reflects tape cartridge usage.
• Data Pointers
The data pointer information is basically a directory (map) used to locate the data
on the physical tape media. Since user data is compressed and written in drive
controlled blocks on the tape, a map is needed to efficiently locate the data after it
is written. This map provides an index between user block ID's and the physical
block on the tape media. Once the data is written, the drive accesses this map to
optimize access to the user data.
A read to a user block ID is translated to the physical location on the tape media,
and the drive determines the quickest method to read the block. If the block is
some physical distance from the current location, a calculation results in a high-
speed locate to the block location and is followed by a normal speed read.
The existence of the MIR is usually transparent to the user unless the MIR has a
problem. This could occur if the MIR update fails during a dismount. The impact of
an invalid MIR occurs in several areas. Since the MIR enables high speed positioning,
an invalid MIR forces all operations to a slow speed mode. This has no impact on a
sequential read from the beginning of the tape. However, an operation that could use
high-speed locate defaults to a sequential slow speed read to the requested block,
which can result in a longer processing time.
An invalid MIR might be suspected if you observe poor performance on a specific
tape cartridge. The T9x40 drive also posts a 36B2 informational FSC whenever a tape
cartridge with an invalid MIR is loaded.
The following sections describe MIR processing and some potential implications of
MIR problems.
Normal MIR Processing
Every time a tape cartridge is loaded, the MIR is read from the tape media and saved
in the drive memory. When the MIR is loaded in drive memory, an invalid flag is
written in the tape-resident MIR. The tape-resident MIR is marked invalid because it
does not reflect results of activity in the current mount session. All subsequent MIR
accesses during the current mount session are saved in the memory-resident MIR.
December 2012
Note – For additional information about mixed-media
management, see
"Cross-Density MIR
Mixed Media Management
Processing".
Overview 25
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