Exabyte EXB-8205 Instrucciones de operaciones Pagina 71

  • Descarga
  • Añadir a mis manuales
  • Imprimir
  • Pagina
    / 446
  • Tabla de contenidos
  • MARCADORES
  • Valorado. / 5. Basado en revisión del cliente
Vista de pagina 70
3.3 Using Data Compression
When writing in either 8200c or 8500c format, the CTS uses the
Improved Data Recording Capability (IDRC) compression algorithm
licensed from IBM. During compression, the CTS uses the Exabyte
Compression Integrity Check feature to ensure that data is
accurately compressed and decompressed into the original form sent
by the initiator.
When the CTS writes data to tape in compressed format, it
compresses the data at an average ratio of 2:1. However, the actual
compression ratio may be higher or lower depending on the type of
data.
Logical Block CRC
To ensure that user data is compressed and decompressed accurately,
the CTS adds two bytes of cyclic redundancy check (CRC) data to
every logical block written in compressed format.
Note: These logical block CRC bytes are in addition to the two bytes
of physical block CRC data that the CTS adds to every physical
block on tape.
Adding CRC bytes to each logical block reduces the data capacity of
the tape by two bytes for every logical block. For example, if you are
writing 1,024-byte logical blocks, the data capacity of the tape will be
reduced by 0.2% (that is, 2 ÷ 1,024 × 100%).
Data Compression Monitoring
As it compresses data, the CTS constantly monitors the compression
ratio to determine whether the size of each data block will actually
decrease when compressed. When a logical block expands (as it
might, for example, when it has already been compressed by the
initiator), the CTS automatically turns off compression until it
encounters a compressible logical block. Then, it switches back to the
compressed format.
Note: The benefit of data compression monitoring is seen only when
data is transferred from the initiator in multiple logical blocks.
If each command transfers only a single block of data at a
time, the CTS’s ability to turn compression off for subsequent
blocks has no effect on performance (since there are no
subsequent blocks with the current command).
May 1994 EXB-8205 and EXB-8505
(Standard and XL)
3-9
3 Implementing CTS Operations
Vista de pagina 70
1 2 ... 66 67 68 69 70 71 72 73 74 75 76 ... 445 446

Comentarios a estos manuales

Sin comentarios