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digital address data. The digital portions of the spoke data are read and used to locate the desired track, spoke, and head number. The quadrature analog signal portion is detected and used by a servo feedback control loop to precisely position the head on the track center.
6.8DATA INTEGRITY AND SECURITY
The disk drives use a combination of parity checking, error detection coding (EDC), error correction coding (ECC), and checkpointing to protect stored data from media errors, transfer or addressing errors, or errors introduced during block reallocation.
6.8.1Media Error Protection
To ensure that data read is the same as data written, the drive computes and appends an Error Correction Code (ECC) to each block of data stored. The drive uses a 352- bit Reed Solomon code with a 4:1 interleave, which can correct up to 20 bytes in each block.
The drive can also correct up to 2 bytes per interleave (up to 8 per block) in hardware
6.8.2Transfer Error Protection
An
An EDC is calculated and added to each data block as the data arrives from the SCSI bus (after SCSI bus parity is checked). The EDC is stored with the data and protected by the block ECC for added security. On reading or writing, the EDC is checked as the data is transferred between buffer RAM and the media or the SCSI bus.
6.8.3Addressing Error Protection
Each data block on the media is identified and located by a servo spoke address. The spoke address consists of a
To further protect against addressing errors, the logical address (LBA) of the data is added to the EDC of each block. If data is written to the wrong block and subsequently read, or read from the wrong block, the error will be flagged.
The hardware does not allow a blind read of a data block; the firmware must request specific data blocks. Even if the head selection hardware malfunctions, it is not possible for the drive to return data from the wrong head.
6.8.4Data Sector Reallocation Error Protection
In any SCSI disk drive, bad blocks may be reallocated. However, a power failure or unrecoverable data could threaten data integrity during a block reallocation.
The reallocation and defect list storage algorithms prevent a reallocation from being lost due to a power failure. Once a reallocation starts, the information about the data block to be moved is stored on the media. As the reallocation progresses, checkpoint