Sun Microsystems StorageTek T9840, Руководство пользователя

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Mixed Media Management
32 T9840 User’s Reference Manual • September 2008 Revision YA • 95739
The T9840C or T9840D tape drive can read data from a tape cartridge written by a
T9840A/B tape drive in the low-density format, but does not append data to that
cartridge. An attempt to append a low-density data cartridge on a T9840C or T9840D
drive will fail, and sense byte data indicates an error (similar to that of a file-protected
data cartridge).
Note – For additional information about mixed-media management see “Cross-Density
MIR Processing” .
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.