scsidisk(HW)

Description

Sdsk is the peripheral device driver for SCSI hard disks.

SCSI hard disks, although very reliable, suffer from physical defects like any other magnetic media. These defects cause soft errors where data loss is recoverable, and hard errors where data loss is irrecoverable. SCSI disk controllers that support an error correction code (ECC) typically can correct at least 15 to 25 incorrect bits per sector; this is known as their correction span. A hard error is reported if the number of incorrect bits is greater than the correction span.

Most SCSI disk controllers allow bad blocks to be remapped to good blocks on the disk; note that these are SCSI logical blocks not filesystem logical blocks. (SCSI logical blocks can range from 48 to 4096 bytes in size depending on the manufacturer. The logical block size is usually set to 512 bytes when the disk is formatted.) The logical addresses of bad blocks are added to the disk's grown defect list (G-List). Whenever a bad block is accessed, the disk controller invisibly remaps the request to the good block. The manufacturer's defect list or primary defect list (P-List) is written to the disk during its manufacture and is unchangeable.

Depending on the disk controller, it may be possible to configure it to add defects to the G-List automatically using Automatic Write Relocation (AWR) and Automatic Read Relocation (ARR). These are SCSI-2 features that reallocate bad blocks on detecting any soft error. It is usually undesirable to reallocate soft errors since these may be as small as a single incorrect bit; if provided, the ECC should be capable of correcting such errors.

The driver cannot recover from hard errors that occur while reading from disk. Such errors mean that data has almost certainly been lost. The default action of the driver is to report these errors to the system administrator as a WARNING message (see messages(M) for more details).

Tuning error reporting

Sdsk_verbosity values

Setting	Errors reported
SDSK_V_NONE	none
SDSK_V_HARD	unrecoverable hard errors only
SDSK_V_WARN	recoverable soft errors, and unrecoverable hard errors
SDSK_V_ALL	all

 +------------+-------------------------------------------------+
 |Setting     | Errors reported                                 |
 +------------+-------------------------------------------------+
 |SDSK_V_NONE | none                                            |
 +------------+-------------------------------------------------+
 |SDSK_V_HARD | unrecoverable hard errors only                  |
 +------------+-------------------------------------------------+
 |SDSK_V_WARN | recoverable soft errors, and unrecoverable hard |
 |            | errors                                          |
 +------------+-------------------------------------------------+
 |SDSK_V_ALL  | all                                             |
 +------------+-------------------------------------------------+

Diagnostics

NOTICE: Sdsk, Spurious interrupt

WARNING: Sdsk: Bad block size
         SDsk: Block size (n) must be between NBPSCTR and SBUFSIZE

Limitations

See also