12. Write-Once Devices
12.1. Model for Write-Once Devices
The model for the write-once device is a variation on the optical memory
model. Most of the aspects of a write-once device are similar to optical
memory devices. The differences are summarized below.
12.1.1. Logical Blocks
Data may be written to a logical block only once. A subsequent write to a
logical block already written may or may not be corrupted, depending on the
implementation. Write-once physical media is non-volatile.
The "updating" of blocks (see 15.1) is discouraged. SCSI write-once devices
are intended to be "archival" in nature. Data at a logical block address is
not expected to change once written. The "update" commands are not defined
for this device type. Devices are not precluded from implementing the
"update" function by this standard, though devices that require the update
function should use the optical memory device type.
Devices may be able to determine the state of a logical block prior to
access. These devices can determine whether a block is blank or written. This
is useful in detecting previously written blocks, preventing a destructive
overwrite. This is also useful in finding blank areas for later writing. The
MEDIUM SCAN command can be used to find blank and written areas prior to WRITE
and READ access.
12.1.2. Initialization
The FORMAT UNIT command is not defined for write-once devices. Write-once
media is shipped pre-formatted by the manufacturer and is ready for use when
mounted.
12.1.3. Physical Medium Defects
The raw defect rate is typically higher for optical medium than for magnetic
medium. Data is usually recovered through the use of sophisticated error
correction algorithms. The level of error correction used for data recovery
is often can be selected. However, many write-once devices have a minimum
level that is always used and cannot be de-selected through the error recovery
mode parameter. Control of the error correction algorithms and level of
correction is vendor specific.
Defect management on write-once devices is usually vendor specific. However
there are standards for some types of write-once media that specify defect
management techniques. These standards, where they exist, may supersede the
implementation requirements pertaining to error and defect reporting in this
standard.
12.1.4. Error Reporting
If any of the following conditions occur during the execution of a command
the target shall return CHECK CONDITION status. The appropriate sense key and
additional sense code should be set. The following table illustrates some
error conditions and the applicable sense keys. The table does not provide an
exhaustive enumeration of all conditions that may cause the CHECK CONDITION
status.
Condition Sense Key
---------------------------------------------- ----------------------------
Invalid logical block address ILLEGAL REQUEST
Unsupported option requested ILLEGAL REQUEST
Target reset or medium change since
last command from this initiator UNIT ATTENTION
Self diagnostic failed HARDWARE ERROR
Unrecovered read error MEDIUM ERROR
HARDWARE ERROR
Recovered read error RECOVERED ERROR
Overrun or other error that might
be resolved by repeating the command ABORTED COMMAND
Attempt to write on write protected medium DATA PROTECT
Attempt to read a blank or previously
unwritten block BLANK CHECK
Attempt to write a previously written
block and blank checking is enabled BLANK CHECK
In the case of an invalid logical block address, the sense data information
field shall be set to the logical block address of the first invalid address.
In the case of an attempt to read a blank or previously unwritten block, the
sense data information field shall be set to the logical block address of the
first blank block encountered. The data read up to that block shall be
transferred.
In the case of an attempt to write a previously written block and blank
checking is enabled, the sense information field shall be set to the logical
block address of the first non-blank block encountered.
12.2. Commands for Write-Once Devices
The commands for write-once devices shall be as shown in Table 12-1.
Table 12-1: Commands for Write-Once Devices
==============================================================================
Command Name Code Type Section
------------------------------------------------------------------------------
CHANGE DEFINITION 40h O 7.2.1
COMPARE 39h O 7.2.2
COPY 18h O 7.2.3
COPY AND VERIFY 3Ah O 7.2.4
INQUIRY 12h M 7.2.5
LOCK UNLOCK CACHE 36h O 8.2.2
LOG SELECT 4Ch O 7.2.6
LOG SENSE 4Dh O 7.2.7
MEDIUM SCAN 38h O 15.2.3
MODE SELECT(6) 15h O 7.2.8
MODE SELECT(10) 55h O 7.2.9
MODE SENSE(6) 1Ah O 7.2.10
MODE SENSE(10) 5Ah O 7.2.11
PRE-FETCH 34h O 8.2.3
PREVENT ALLOW MEDIUM REMOVAL 1Eh O 8.2.4
READ(6) 08h O 8.2.5
READ(10) 28h M 8.2.6
READ(12) A8h O 15.2.4
READ BUFFER 3Ch O 7.2.12
READ CAPACITY 25h M 8.2.7
READ LONG 3Eh O 8.2.9
REASSIGN BLOCKS 07h O 8.2.10
RECEIVE DIAGNOSTIC RESULTS 1Ch O 7.2.13
RELEASE 17h M 8.2.11
REQUEST SENSE 03h M 7.2.14
RESERVE 16h M 8.2.12
REZERO UNIT 01h O 8.2.13
==============================================================================
Table 12-1 continued
==============================================================================
Command Name Code Type Section
------------------------------------------------------------------------------
SEARCH DATA EQUAL(10) 31h O 8.2.14.1
SEARCH DATA EQUAL(12) B1h O 15.2.8
SEARCH DATA HIGH(10) 30h O 8.2.14.2
SEARCH DATA HIGH(12) B0h O 15.2.8
SEARCH DATA LOW(10) 32h O 8.2.14.3
SEARCH DATA LOW(12) B2h O 15.2.8
SEEK(6) 0Bh O 8.2.15
SEEK(10) 2Bh O 8.2.15
SEND DIAGNOSTIC 1Dh M 7.2.15
SET LIMITS(10) 33h O 8.2.16
SET LIMITS(12) B3h O 15.2.9
START STOP UNIT 1Bh O 8.2.17
SYNCHRONIZE CACHE 35h O 8.2.18
TEST UNIT READY 00h M 7.2.16
VERIFY(10) 2Fh O 15.2.11
VERIFY(12) AFh O 15.2.12
WRITE(6) 0Ah O 8.2.20
WRITE(10) 2Ah M 8.2.21
WRITE(12) AAh O 15.2.14
WRITE AND VERIFY(10) 2Eh O 8.2.22
WRITE AND VERIFY(12) AEh O 15.2.16
WRITE BUFFER 3Bh O 7.2.17
WRITE LONG 3Fh O 8.2.23
==============================================================================
Key: M = Command implementation is mandatory.
O = Command implementation is optional.
The following command codes are vendor specific: 02h, 05h, 06h, 09h, 0Ch,
0Dh, 0Eh, 0Fh, 10h, 11h, 13h, 14h, 19h, 20h, 21h, 22h, 23h, 24h, 26h, 27h,
29h, and C0h through FFh. All remaining command codes for write-once devices
are reserved for future standardization.
12.3. Parameters for Write-Once Devices
Refer to the parameters for optical memory devices (see 15.3).
12.4. Glossary for Write-Once Devices
Refer to the glossary for optical memory devices (see 15.4).