SCSI-3流命令标准：序列设备与打印机接口详解

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SCSI-3 Stream Commands (SSC) 是一个由美国国家标准委员会NCITS制定的草案标准，旨在规范在SCSI-3 Architectural Model (X3.270-1996)框架下运行的设备服务器的行为。该标准主要针对两类设备：Sequential-access devices（顺序访问设备）和Printer devices（打印机设备），这些统称为Stream Devices。这些设备在INQUIRY命令响应数据中的device type字段中被识别。 SCSI-3 Stream Commands的工作原理遵循T10 Technical Committee（SCSI技术委员会）的标准开发流程，通过公共审查过程接受反馈并进行修改，以确保标准的准确性和适用性。然而，由于这仍是一个草案，用户在使用相关信息时需自行承担风险，因为其可能尚未完成或正式发布。该标准允许NCITS、ISO及其相关的技术委员会和任务组成员复制文档，用于标准化活动，但前提是保留所有权利，并且商业或营利性使用是被严格禁止的。David A. Peterson，来自StorageTek Network Business Group的SSC技术编辑，负责此文档的维护，提供了他的联系方式以便于获取更多信息或反馈。文档的引用编号包括ISO/IEC的国际标准号码以及ANSI的编号，但具体数字在此处未给出。最后，文档的印刷日期为2000年1月17日，表明它可能是在当时的技术环境下提出的，可能需要与后续的修订版进行对比以获取最新信息。在使用SCSI-3 Stream Commands时，关键要点包括理解设备的序列访问特性，如何有效地发送和接收数据流，以及如何确保兼容性和互操作性。此外，开发者和管理员需要关注公共审查期间的任何更新，以便及时调整系统设计以符合最新的标准要求。

BSR NCITS 335

future extension to this standard. Recipients are not required to check reserved bits, bytes,

words or fields for zero values. Receipt of reserved code values in defined fields shall be

reported as an error.

3.3.11 vendor-specific

: Items (e.g., a bit, field, code value, etc.) that are not defined by this standard

and may be vendor defined.

3.4 Conventions

Lower case is used for words having the normal English meaning. Certain words and terms used in this

standard have a specific meaning beyond the normal English meaning. These words and terms are

defined either in Clause 3.3 or in the text where they first appear.

Listed items in this standard do not represent any priority. Any priority is explicitly indicated. Formal lists

(e.g., a) red; b) blue; c) green) which use letters are in an arbitrary order. Formal lists (e.g., 1) red; 2) blue;

3) green) which use numbers are in a required sequential order.

If a conflict arises between text, tables, or figures, the order of precedence to resolve conflicts is text, then

tables, and finally figures. Not all tables or figures are fully described in text. Tables are used to show data

format and values.

The ISO convention of numbering is used (i.e., the thousands and higher multiples are separated by a

space and a comma is used as the decimal point as in 65 536 or 0,5).

The following additional conventions are used:

a) The names of abbreviations, commands, and acronyms used as signal names are in all uppercase

(e.g., IDENTIFY DEVICE);

b) field names are in

SMALL CAPS

to distinguish them from normal English;

c) fields containing only one bit are referred to as the

NAME

bit instead of the

NAME

field;

d) numbers, except clause numbers, that are not immediately followed by lower-case b or h are decimal

values;

e) numbers immediately followed by lower-case b (xxb) are binary values;

f) numbers and the upper case letters A, B, C, D, E, and F immediately followed by lower-case h (xxh)

are hexadecimal values;

g) the most significant bit of a binary quantity is shown on the left side and represents the highest

algebraic value position in the quantity; and

h) if a field is specified as not meaningful or the field is to be ignored, the entity that receives the field

shall not take any action based on the value of that field.

BSR NCITS 335

4 General

4.1 Overview

The SCSI stream device class specifies the behavior of a logical unit that is primarily a streaming data

device. Two device types are members of this class: sequential-access and printer devices. Both have

the common characteristic of primarily handling data in a sequential manner (i.e., a stream). This does

not limit the device's ability to position randomly within the data. However, the physical model defined for

each of these device types indicates whether random-access operations are impossible or just inefficient.

A sequential-access device is not truly random-access (see SBC for a description of a random-access

device). Commands to read and write on a sequential-access device do not contain any positioning

information fields. Instead, the device position is normally determined by previous commands.

Commands are available for absolute and relative positioning. Writing to a sequential-access device may

cause all data starting at the point at which the data is written to be invalidated. There may be restrictions

on where write operations may be initiated. Reading or writing data as a long string of data, as in a

stream, tends to be the most efficient.

A printer device is a sequential-access device since it also transfers data based on a current position,

rather than at an absolute location. However, a printer is different in that it is a data sink and it is not

expected to return the same data back during normal operation.

4.2 Physical models

For a sequential-access device, a recording media exists between two reels, the supply reel and take-up

reel. The read/write mechanism may only access the media between the reels. As media is taken out of

one reel, it passes by the read/write mechanism and into the other reel. Transferring data as a stream is

most efficient, since the media may traverse the read/write mechanism producing a flow of data. To

position to a given point requires moving the media until the appropriate position is found.

In the printer model, the read/write mechanism may only write and the media may only move from the

supply. Unlike a sequential-access device, data may be formatted on a printer device by font selection

and positioning options.

BSR NCITS 335

5 Sequential-access devices

5.1 Definitions specific to sequential access devices

5.1.1 beginning-of-medium

: The extreme position along the medium in the direction away from the

supply reel that is accessible by the device. This position may not coincide with a beginning-of-

partition position.

5.1.2 beginning-of-partition:

The position at the beginning of the permissible recording region of a

partition. If only one partition is defined for a volume, this position is typically equivalent to the

beginning-of-medium. This position may not coincide with a beginning-of-medium position.

5.1.3 buffered mode:

A mode of data transfer in write operations which facilitates tape streaming (see

5.2.5), as reported in the mode parameter header device-specific parameter (see 5.4.3). Buffered

mode is

indicated by a non-zero value (1h or 2h) in the

BUFFER MODE

field in the mode parameter

header (see 5.4.3). Buffered mode is the opposite of unbuffered mode (see 5.1.21).

5.1.4 early-warning:

A physical mark or device computed position near but logically before the end-of-

partition, independent of physical direction (see 5.2.2).

5.1.5 end-of-data:

A recorded indication that no valid logical elements are recorded between this

position and end-of-partition. End of data is denoted in a format-specific manner (see 5.2.4).

5.1.6 end-of-medium:

The extreme position along the medium in the direction away from the take-up

reel that is accessible by the device. This position may not coincide with a end-of-partition

position. This position may be accessed by logical units that support the LOAD UNLOAD

command with the

EOT

bit set to one (see 5.3.3).

5.1.7 end-of-partition:

The position at the end of the permissible recording region of a partition.

5.1.8 filemark:

A special recorded element within a partition, not containing user data, which provides

a segmentation scheme for the contents of a partition.

5.1.9 gap

: A non-data element recorded on the medium. Gaps may be recorded between logical

elements. The format and method of recording a gap may vary.

5.1.10 logical block

: A logical element that is a unit of data supplied or requested by an initiator.

5.1.11 logical element

: A unit of data, either a block or a mark. Each logical element has an unique

logical block identifier (see 5.2.7), if supported, within the partition.

5.1.12 mark

: A logical element that does not contain any initiator defined data. A mark is either a

setmark or filemark.

5.1.13 overlength:

The incorrect length condition that exists after executing a read command when the

length of the actual block read exceeds the requested transfer length in the command descriptor

block or the mode header block size field, whichever is appropriate.

5.1.14 partition:

The entire usable region for recording and reading in a volume or in a portion of a

volume, defined in a vendor-specific manner.

5.1.15 principal density code:

The principal density code is a density code selected by the device

server. The logical unit indicates the principal density code by reporting a

DEFLT

bit of one in the

density support data block descriptor for supported densities in response to the REPORT

DENSITY SUPPORT command (see 5.3.10). The selection of the principal density code is

vendor-specific.

5.1.16 setmark:

A special recorded element within a partition, not containing user data, that provides a

segmentation scheme similar to filemarks. Setmarks may be ignored based on the

RSMK

mode

parameter (see 5.4.3.2).

5.1.17 spacing:

The act of positioning the medium on a sequential access device.

5.1.18 tape:

Tape is the medium on which data is recorded. The medium is normally a long thin

medium which is spooled onto one or two reels, possibly within a cassette or cartridge.

5.1.19 track:

A contiguous line on the medium consisting of a pattern of recorded signals written by one

write component.

5.1.20 track group

: A set of tracks that are recorded at the same time.

BSR NCITS 335

5.1.21 unbuffered mode

: The mode of operation where write data is written directly to the medium

without being buffered. Unbuffered mode is indicated by a zero value (0h) in the

BUFFER MODE

field in the mode parameter header (see 5.4.3). Unbuffered mode is the opposite of buffered

mode (see 5.1.3).

5.1.22 underlength:

The incorrect length condition that exists after executing a read command when

the requested transfer length in the command descriptor block or the mode header block size

field, whichever is appropriate, exceeds the length of the actual block read.

5.1.23 volume:

A recording medium together with its physical carrier.

5.2 Sequential-access device model

5.2.1 Physical elements

Sequential-access devices optimize their use in storing or retrieving user data in a sequential manner.

Since access is sequential, position changes typically take a long time, when compared to random-access

devices.

Sequential-access devices are described herein from the point of view of a tape device. However, other

implementations are not precluded.

The recording medium for tape devices consists of various widths and lengths of a flexible substrate

coated with a semi-permanent magnetic material. The recording medium may be wound onto single reels

or encapsulated into cartridges containing both a supply reel and a take-up reel. Several American

National Standards exist covering the construction of reels and cartridges for interchange as well as

recording techniques for many of the format or density combinations.

A volume is composed of the recording medium and its physical carrier (e.g. reel, cartridge, cassette).

Volumes have an attribute of being mounted or de-mounted on a suitable transport mechanism.

Mounted is the state of a volume when the device is physically capable of executing commands that

cause the medium to be moved. A volume is de-mounted when it is being loaded, threaded, unloaded,

unthreaded, or when not attached to the device.

Ready is the state of the logical unit when medium access and non-medium access commands may be

executed. The logical unit is not ready when no volume is mounted or, from the initiator's perspective,

whenever any medium access command reports CHECK CONDITION status and a NOT READY sense

key. The logical unit is not ready during the transition from mounted to not mounted, or not mounted to

mounted. Devices may have a physical control that places the device in a not ready state even when a

volume is mounted.

The RESERVE and RELEASE commands (see SPC-2) are mandatory while the PERSISTENT

RESERVE OUT and PERSISTENT RESERVE IN commands (see SPC-2) are optional for sequential-

access devices. Element reservations are not supported by this model.

The write enabled or write protected state determines when an initiator may write information on a volume.

This attribute is usually controlled by the user of the volume through manual intervention (e.g. mechanical

lock). Other mechanisms for write protect, including software controlled methods, may be available (see

5.2.9). When a logical unit is in write protected state, writing to media is prohibited. Any of the following

commands could end with CHECK CONDITION status because write protection is enabled: COPY, COPY

AND VERIFY, ERASE, FORMAT MEDIUM, MODE SELECT, WRITE, and WRITE FILEMARKS. In this

situation, the sense key is DATA PROTECT.

The recording medium has two physical attributes called beginning-of-medium (BOM) and end-of-medium

(EOM). Beginning-of-medium is at the end of the medium that is attached to the take-up reel. End-of-

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