SD(4)

NAME
sd - scsi disk driver

SYNOPSIS
disk sd
disk sd1 target 4 lun 0

DESCRIPTION
The sd driver provides support for a scsi disk. It allows the disk to be divided up into a set of pseudo devices called partitions. A Partition can have both a raw interface and a Block mode interface. In general the interfaces are similar to those described by wd(4) or dk(4).

Where the wd(4) device has a fairly low level interface to the system, SCSI devices have a much higher level interface and talk to the system via a SCSI Adapter and a Scsi Adapter driver e.g. AHA1542. A scsi adapter must also be separatly configured into the system before a scsi disk can be configured.

As the scsi adapter is probed during boot, the SCSI bus is scanned for devices. Any devices found which answer as 'Direct' type devices will be 'attached' to the sd driver. In FreeBSD releases prior to 2.1, the first found will be attached as sd0 and the next, sd1 etc. Beginning in 2.1 it is possible to specify what sd unit a device should come on line as; refer to scsi(4) for details on kernel configuration.

PARTITIONING
The sd driver allows the disk to have two levels of partitioning. One which allows it to have partitions for different Operating systems, (one of which is BSD unix), (see also for the 386 port, fdisk(1) ), and within a BSD partition, further partitions which are individually addressable as separate entries in the /dev directory. The second level of partitioning is controlled by the program disklabel(1) and is common in format across most BSD operating systems. In most of the original BSD ports, what is the BSD part here, is the entire disk, and the outer layer of partitionning does not exist. sd will also run in this manner if disklabel(1) is run with a blank disk, without first partitioning it with fdisk(1) (or similar).

Apologies for the two conflicting usages of the word Partition, but it's a historical artifact, and the meaning must be judged from context in each case. The next paragraph will discuss partitions exclusively in the context of WITHIN a BSD partition on the disk.

The first few blocks of the BSD section (maybe all) of the disk contain some boot code, and a structure, known as the disklabel(5) which describes the disk's characteristics and partitioning for BSD. It is set up by the disklabel(1) program, and read in by the kernel when the device is first initialised during boot. It describes how the drive is further divided. The disklabel(5) structure contains room for 8 (usually) partitions. Usually these partitions are calculated so as to fall evenly on cylinder boundaries, however on a SCSI disk this is sometimes not possible. The reason for doing this is historically to get better performance, however modern SCSI disks often have a variable format, so that it is hard to know at any point in the disk, where the cylinder or track boundaries are. Added to this, the fact that SCSI disk blocks are addressed soley by their 'block number' and not by any geometry, leads to the common occurance on SCSI disks, of laying out partitions on arbitrary boundaries. Because modern disks often have large track caches, this often leads to only small degadations of performance, and is in fact sometimes unavoidable. The boot messages will suggest a geometry similar in heads and cylinders to the real geometry, but the disklable need not agree with this for the system to be able to successfully work with the disk.

During booting with an uninitialised disk, the sd driver will initialise the 'in-core' copy of the disklabel to the suggested values, however they are not written to the disk.

The fourth partition is special. No matter what the disklabel says, the fourth partition (partition d) reflectls the entire disk, including those areas OUTSIDE the BSD partitions. At some times it is suggested that the c partition might be used to represent the entire BSD partition, so these two partitions should be avoided when laying out filesystems. The fourth partition must be used for general scsi(4) ioctls.

While partitions are only theoretically valid within the BSD partition, they are specified in terms of absolute block numbers, so it is possible to specify a partition that lies outside of the BSD partition. This is useful if one wants to have a /dev entry that points to a partition belonging to another OS (e.g. DOS).

KERNEL CONFIGURATION
In configuring, if an optional count is given in the specification, that number of scsi disks are configured; Most storage for them is allocated only when found so a large number of configured devices is cheap. (once the first has included the driver).

IOCTLS
The following ioctl(2) calls apply to scsi disks as well as to other disks. They are defined in the header file disklabel.h.

DIOCSBAD
    Usually used to set up a bad-block mapping system on the disk. Scsi drive incorporate their own bad-block mapping so this is not implimented, however it MAY be implimented in the future as a 'kludged' interface to the scsi bad-block mapping.

DIOCGDINFO
    Read, from the kernel, the in-core copy of the disklabel for the drive. This may be a ficticious disklabel if the drive has never been initialised, in which case it will contain information read from the scsi inquiry commands, and should be the same as the information printed at boot.

DIOCSDINFO
    Give the driver a new disklabel to use. The driver will NOT try write the new disklabel to the disk.

DIOCWLABEL
    Enable or Disable the driver's software write protect of the disklabel on the disk.

DIOCWDINFO
    Give the driver a new disklabel to use. The driver WILL try write the new disklabel to the disk.

In addition, the scsi(4) general ioctls may be used with the sd driver, but only against the fourth (whole disk) partition.

NOTES
If a removable device is attached to the sd driver, then the act of changing the media will invalidate the disklabel and information held within the kernel. To stop corruption, All accesses to the device will be discarded until there are no more open file descriptors referencing the device. During this period, all new open attempts will be rejected. When No more open file descriptors reference the device, the first next open will load a new set of figures (including disklabel) for the drive. An ioctl to map out a bad block is planned. (the code is already present in the driver).

FILES
/dev/sd[0-9][a-h]
    block mode scsi disks
/dev/rsd[0-9][a-h]
    raw scsi disks

DIAGNOSTICS
None.

SEE ALSO
disklabel(1) disklabel(5) fdisk(1) wd(4) dk(4) (on other systems)

HISTORY
The sd driver appeared in MACH 2.5 .