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+.Dd August 27, 1993
+.Dt SD 4
+.Os FreeBSD
+.Sh NAME
+.Nm sd
+.Nd scsi disk driver
+.Sh SYNOPSIS
+.Nm device-driver sd
+.Op Ar count
+.Sh DESCRIPTION
+The
+.Xr sd
+driver provides support for a 
+.Em scsi
+disk. It allows the disk
+to be divided up into a set of pseudo devices called
+.Em partitions.
+A Partition can have both a 
+.Em raw
+interface
+and a
+.Em Block mode
+interface.
+In general the interfaces are similar to those described by 
+.Xr wd 4 
+or
+.Xr dk 4 .
+
+.Pp
+Where the 
+.Xr wd 4
+device has a fairly low level interface to the system, 
+.Em SCSI
+devices have a much higher level interface and talk to the system via
+a 
+.Em SCSI Adapter
+and a
+.Em Scsi Adapter driver
+e.g. 
+.Xr AHA1542 .
+A scsi adapter must also be separatly configured into the system
+before a scsi disk can be configured.
+.Pp
+As the scsi adapter is probed during boot, the 
+.Em SCSI
+bus is scanned for devices. Any devices found which answer as 'Direct'
+type devices will be 'attached' to the 
+.Nm
+driver. The first found will be attached as
+.Em sd0
+and the next, 
+.Em sd1
+etc.
+.Pp
+.Sh PARTITIONING
+The 
+.Nm
+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, 
+.Xr fdisk 1
+), and within a BSD partition, further partitions which are individually 
+addressable as separate entries in the 
+.Em /dev
+directory. The second level of partitioning  is controlled by the program
+.Xr 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. 
+.Nm
+will also run in this manner if
+.Xr disklabel 1
+is run with a blank disk, without first partitioning it
+with
+.Xr fdisk 1
+(or similar).
+
+.Pp
+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.
+.Pp
+The first few blocks of the BSD section (maybe all) of the disk contain 
+some boot code, and a structure, known as the 
+.Xr disklabel 5
+which describes the disk's characteristics and partitioning for BSD.
+It is set up by the 
+.Xr 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
+.Xr 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 
+.Em SCSI
+disk this is sometimes not possible. The reason for doing this is historically
+to get better performance, however modern 
+.Em 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 
+.Em SCSI
+disk blocks are addressed soley by their 'block number' and not by
+any geometry, leads to the common occurance on 
+.Em 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. 
+.Pp
+During booting
+with an uninitialised disk, the
+.Nm 
+driver will initialise the 'in-core' copy of the disklabel to the suggested
+values, however they are not written to the disk.
+.Pp
+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
+.Xr scsi 4
+ioctls.
+.Pp
+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).
+.Pp
+.Sh KERNEL CONFIGURATION
+In configuring, if an optional
+.Ar 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).
+
+.Pp
+.Sh IOCTLS
+The following 
+.Xr ioctl 2
+calls apply to scsi disks as well as to other disks. They are defined
+in the header file
+.Em disklabel.h.
+
+.Bl -tag -width DIOCSDINFO
+
+.It Dv 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.
+.It Dv 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.
+.It Dv DIOCSDINFO
+Give the driver a new disklabel to use. The driver will NOT try write the new
+disklabel to the disk.
+.It Dv DIOCWLABEL
+Enable or Disable the driver's software
+write protect of the disklabel on the disk.
+.It Dv DIOCWDINFO
+Give the driver a new disklabel to use. The driver WILL try write the new
+disklabel to the disk.
+.El
+.Pp
+In addition, the 
+.Xr scsi 4
+general ioctls may be used with the 
+.Nm
+driver, but only against the fourth (whole disk) partition.
+.Sh NOTES
+If a removable device is attached to the 
+.Nm
+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).
+
+.Sh FILES
+.Bl -tag -width /dev/rsd[0-9][a-h] -compact
+.It Pa /dev/sd[0-9][a-h]
+block mode scsi disks
+.It Pa /dev/rsd[0-9][a-h]
+raw scsi disks
+.El
+.Sh DIAGNOSTICS
+None.
+.Sh SEE ALSO
+.Xr disklabel 1
+.Xr disklabel 5
+.Xr fdisk 1
+.Xr wd 4
+.Xr dk 4
+(on other systems)
+.Sh HISTORY
+The
+.Nm
+driver appeared in MACH 2.5 .
+