The SCSI Format Unit command is used to destructively format a SCSI

SCSI

Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...

 target device—usually a disk—and prepare the medium to store data. Defect management options may be specified in the CDB

SCSI CDB

In SCSI computer storage, commands are sent in a Command Descriptor Block .Each CDB can be a total of 6, 10, 12, or 16 bytes, but later versions of the SCSI standard also allow for variable-length CDBs. The CDB consists of a one byte operation code followed by some command-specific parameters.A...

. Depending on the device characteristics and capacity, formatting may be a lengthy process. In the Seagate SCSI manual, it is stated that a disk drive will disconnect during formatting, thus implying a time-consuming operation.

The CDB structure is:

SCSI Format Unit Command

Bit	0	1	2	3	4	5	6	7
+0	Operation Code = 0x04
+8	LUN			FmtData	CmpLst	Defect List Format
+16	Vendor-specific
+24	Interleave
+32
+40	Control

The special parameter fields in the CDB have the following meaning:

• FmtData - Format Data - if this is one then the command is followed by two or three extra parameters:
  • defect list header -
  • initialization pattern descriptor -
  • defect descriptors (optional) -

• CmpLst - Complete List:
  • 0 - the initiator will send a partial list of defects, to be added to the existing list
  • 1 - the initiator will send a complete list of defects, overwriting the existing list

• Defect list format - if the FmtData bit is one then this describes the defect options:
  • 000 - block format - 32-bit LBAs
  • 100 - bytes from index format - cylinder + head + byte-offset
  • 101 - physical sector format - cylinder + head + sector
  • 110 - vendor-specific

• Interleave -
  • 0000h - the target can use its default interleave
  • 0001h - consecutive LBAs must be contiguous
  • all other values are vendor-specific

In some cases, issuing this command to a tape drive will result in the tape being erased.