User's Manual

Chapter 7 Theory of operations
Page 56 STT20000A Product Manual
Read-While-Write
The STT20000N-C (NS-20) drive is able to read back the data it has just written,
hence the term read-while-write check. A different head is utilized in the NS drive to
accomplish this task along with some additional playback circuitry. The digital
control ASIC is the same for NS and standard drives, but the NS drive enables the
CRC checking circuitry necessary to perform the read-while-write check. While the
data is being written the control ASIC passes the data through the CRC generating
hardware. After a block of data has been written the CRC generator appends the 4
byte CRC to the end of the block. As the data is written, the trailing read head
element is reading the data. This data is passed through the control ASIC’s CRC
checking hardware . Once the block has been read the checking hardware verifies
the data against what was written. If the data verifies, the block is deemed good
and the checking process begins again on the next block of data. If the data fails,
the data block is deemed bad and is re-written. The drive will only allow the data to
be re-written 16 times before aborting the write and posting an error to the host.
ALDC data compression
The STT20000N-C version of the drive also provides hardware data compression.
This feature is enabled at power on and can be enabled or disabled through specific
SCSI commands. The data compression ASIC implements the ALDC compression
algorithm. A larger DRAM buffer is added to facilitate data compression.. During
writes, uncompressed data is transfer to the drive from the host and is stored in the
DRAM buffer. The data is then transferred to the compression ASIC where it is
compressed and returned to the DRAM buffer. ECC is then applied to the
compressed data and written to tape in the method discussed in Section 6 (Tape
Format). During reads the process is reversed, the data is read into the DRAM
buffer. The compressed data is then sent to the compression ASIC where it is
decompressed and returned to the DRAM buffer. The drive then returns the
uncompressed data back to the host.