The SCSI subsystem has a 3 level architecture with the "upper" level
being closest to the user/kernel interface while the "lower" level
is closest to the hardware. The upper level drivers are commonly known
by a terse two letter abbreviation (e.g. "sd" for SCSI disk driver).
The names of the corresponding module drivers which, for historical
reasons, sometimes differ from the built in driver names are shown in
braces in the following diagram.
The upper level supports the user-kernel interface. In the case of sd and sr
this is a block device interface while for st and sg this is a character
device interface. Any operation using the SCSI subsystem (e.g. reading a
sector from a disk) involves one driver at each of the 3 levels (e.g. sd,
SCSI mid level and aic7xxx drivers).
As can be seen from the diagram, the SCSI mid level is common to all
operations. The SCSI mid level defines internal interfaces and provides
common services to the upper and lower level drivers. Ioctls provided by
the mid level are available to the file descriptors belonging to any of
the 4 upper level drivers.
The most common operation on a block device is to "mount" a
file system. For a sd device typically a partition is mounted
(e.g. mount -t ext2 /dev/sda6 /home). For a
sr device usually the whole device is mounted (e.g. mount -t iso9660 /dev/sr0 /mnt/cdrom). The dd command can be used to read or write from block devices.
In this case the block size argument ("bs") needs to be set to
the block size of the device (e.g. 512 bytes for most disks)
or an integral multiple of that device block size (e.g. 8192 bytes).
A recent addition to the block subsystem allows a device (or partition)
to be mounted more than once, at different mount points.
Sd is a member of the generic disk family, as is the hd device from the
IDE subsystem. Apart from mounting sd devices, the fdisk command is available to view or modify a disk's
partition table. Although the hdparm command is
primarily intended for ATA disks (also known as IDE or EIDE disks), some
options work on SCSI disks.
Sr is a member of the CD-ROM subsystem. Apart from mounting file
systems (e.g. iso9660), audio CDs can also be read. The latter
action does not involve mounting a file
system but typically by invoking some ioctls. General
purpose Linux commands such as dd cannot be
used on audio CDs.
St is a char device for reading and writing tapes. Typically the
mt command is used to perform data transfers and
other control functions.
Sg is a SCSI command pass through device that uses a char device
interface. General purpose Linux commands should not be used on sg devices. Applications such
as SANE (for scanners), cdrecord and
cdrdao (for cd writers) and
cdparanoia (for reading audio CDs digitally) use sg.
