Cyclic Redundancy Check (Crc); Track Sparing; Look Ahead Reads - HP C2246 Technical Reference Manual

The ECC function is enabled or disabled via the DCR (Disable Correction) bit in parameter
page OlH of the MODE SELECT command. When enabled, the ECC algorithm divides a
sector's data field into three interleaves, or rows, with a selectable correction factor of one,
two, or three bytes per interleave. Ma.thematically, this converts to a maximum burst size of
72 bits per sector. However, the maximum number of bits that is guaranteed .to fit into nine
contiguous bytes is 65. Therefore, if an error burst longer than 65 bits falls exactly within the
boundaries of nine contiguous bytes, it will be corrected. If it spreads across more than nine
contiguous bytes, it will be flagged as unrecoverable after the error correction algorithm has·
been executed (i.e. read retry count, recalibrations, read with offset, reseek, etc.).
The number of bytes that will be corrected in an interleave is selectable via the Correction
Span field in parameter page Olh of the MODE SELECT command. The Correction
Span field value is stated in bits-per-sector. The ECC algorithm converts this value to
bytes-per-interleave by dividing it by 24 (8-bits per byte times three interleaves) and rounding
it up to the nearest byte value.
Cyclic Redundancy Check (CRC)
ECC is aided by a 2-byte cyclic redundancy check (CRC) to decrease the probability of error
miscorrection. With the correction span set to 72 bits, if a random error distribution is
assumed, the calculated probabilities of error misdetection and miscorrection are as follows:
• Probability of misdetection (an error exists, but ECC does not recognize it) is less than
1x10-1s.
• Probability of miscorrection (an error is detected, but is improperly corrected) with CRC is
less than
1X10-
47 •
Track Sparing
Refer to Table 3-1 for a complete track allocation list. The spare cylinders are divided into
eight "pools", one located at the inside diameter of each zone. If all of the spare cylinders in a
given zone have been used, the drive will use
th~
spare cylinders in the adjacent zones nearer
to the outer diameter.
Track sparing is implemented for any defect within the track. Defects causing a spare
operation may exist in the header, data field, or any other area within the physical sector.
During a Format operation, "Slip Track Sparing" is used: defective tracks are passed over,
and the logical volume is slipped one track into the spare pool. During subsequent reads, the
defective track is passed over and the read continues at the next logical track.
For a Reassign Block operation, "Skip Track Sparing" is used: the data in the defective track
is reassigned to an alternate track located in the spare pool. The "new" location is maintained
in a RAM look-up table and supplied to the servo system prior to seeking. When the drive
encounters the defective track, it
will
seek to the alternate location, read the data, and return
to the original track to continue the read.
Look Ahead Reads
The Look Ahead Read capability can improve the performance of a drive doing sequential
READs by preloading the track buffer with the data most likely to be requested with the next
READ command. After a READ command is received by the controller, the drive seeks to the
proper track and loads the requested data into the buffer. While that data is being transferred
to the host, the Look Ahead Read function continues to read the remainder of the current
3-2 Product Features