1.Dd August 27, 1993 2.Dt SD 4 3.Os FreeBSD 4.Sh NAME 5.Nm sd 6.Nd scsi disk driver 7.Sh SYNOPSIS 8.Nm disk sd 9.Nm disk sd1 target 4 lun 0 10.Sh DESCRIPTION 11The 12.Xr sd 13driver provides support for a 14.Em scsi 15disk. It allows the disk 16to be divided up into a set of pseudo devices called 17.Em partitions. 18A Partition can have both a 19.Em raw 20interface 21and a 22.Em Block mode 23interface. 24In general the interfaces are similar to those described by 25.Xr wd 4 26or 27.Xr dk 4 . 28 29.Pp 30Where the 31.Xr wd 4 32device has a fairly low level interface to the system, 33.Em SCSI 34devices have a much higher level interface and talk to the system via 35a 36.Em SCSI Adapter 37and a 38.Em Scsi Adapter driver 39e.g. 40.Xr AHA1542 . 41A scsi adapter must also be separatly configured into the system 42before a scsi disk can be configured. 43.Pp 44As the scsi adapter is probed during boot, the 45.Em SCSI 46bus is scanned for devices. Any devices found which answer as 'Direct' 47type devices will be 'attached' to the 48.Nm 49driver. 50In FreeBSD releases prior to 2.1, the first found will be attached as 51.Em sd0 52and the next, 53.Em sd1 54etc. 55Beginning in 2.1 it is possible to specify what sd unit a device should 56come on line as; refer to 57.Xr scsi 4 58for details on kernel configuration. 59.Pp 60.Sh PARTITIONING 61The 62.Nm 63driver allows the disk to have two levels of partitioning. 64One which allows it to have 65partitions for different Operating systems, (one of which is BSD unix), 66(see also for the 386 port, 67.Xr fdisk 1 68), and within a BSD partition, further partitions which are individually 69addressable as separate entries in the 70.Em /dev 71directory. The second level of partitioning is controlled by the program 72.Xr disklabel 1 73and is common in format across most BSD operating systems. In most of 74the original BSD ports, what is the 75BSD part here, is the entire disk, and the outer layer of partitionning 76does not exist. 77.Nm 78will also run in this manner if 79.Xr disklabel 1 80is run with a blank disk, without first partitioning it 81with 82.Xr fdisk 1 83(or similar). 84 85.Pp 86Apologies for the two conflicting usages of the word Partition, but 87it's a historical artifact, and the meaning must be judged from context 88in each case. The next paragraph will discuss partitions exclusively 89in the context of WITHIN a BSD partition on the disk. 90.Pp 91The first few blocks of the BSD section (maybe all) of the disk contain 92some boot code, and a structure, known as the 93.Xr disklabel 5 94which describes the disk's characteristics and partitioning for BSD. 95It is set up by the 96.Xr disklabel 1 97program, and read in by the kernel when the device is first initialised 98during boot. It describes how the drive is further divided. The 99.Xr disklabel 5 100structure contains room for 8 (usually) partitions. Usually these 101partitions are calculated so as to fall evenly on cylinder boundaries, 102however on a 103.Em SCSI 104disk this is sometimes not possible. The reason for doing this is historically 105to get better performance, however modern 106.Em SCSI 107disks often have a variable format, so that it is hard to know at any point 108in the disk, where the cylinder or track boundaries are. Added to this, the 109fact that 110.Em SCSI 111disk blocks are addressed soley by their 'block number' and not by 112any geometry, leads to the common occurance on 113.Em SCSI 114disks, of laying out partitions on arbitrary boundaries. Because 115modern disks often have large track caches, this often leads to only small 116degadations of performance, and is in fact sometimes unavoidable. The 117boot messages will suggest a geometry similar in heads and cylinders 118to the real geometry, but the disklable need not agree with this for the 119system to be able to successfully work with the disk. 120.Pp 121During booting 122with an uninitialised disk, the 123.Nm 124driver will initialise the 'in-core' copy of the disklabel to the suggested 125values, however they are not written to the disk. 126.Pp 127The fourth partition is special. No matter what the disklabel 128says, the fourth partition (partition d) reflectls the entire disk, including 129those areas OUTSIDE the BSD partitions. At some times it is suggested that 130the c partition might be used to represent the entire BSD partition, so these 131two partitions should be avoided when laying out filesystems. The fourth 132partition must be used for general 133.Xr scsi 4 134ioctls. 135.Pp 136While partitions are only theoretically valid within the BSD partition, they 137are specified in terms of absolute block numbers, so it is possible to 138specify a partition that lies outside of the BSD partition. This is useful 139if one wants to have a /dev entry that points to a partition belonging 140to another OS (e.g. DOS). 141.Pp 142.Sh KERNEL CONFIGURATION 143In configuring, if an optional 144.Ar count 145is given in 146the specification, that number of scsi disks are configured; 147Most storage for them is allocated only when found so a large number 148of configured devices is cheap. (once the first has included the driver). 149 150.Pp 151.Sh IOCTLS 152The following 153.Xr ioctl 2 154calls apply to scsi disks as well as to other disks. They are defined 155in the header file 156.Em disklabel.h. 157 158.Bl -tag -width DIOCSDINFO 159 160.It Dv DIOCSBAD 161Usually used to set up a bad-block mapping system on the disk. Scsi 162drive incorporate their own bad-block mapping so this is not implimented, 163however it MAY be implimented in the future as a 'kludged' interface to the 164scsi bad-block mapping. 165.It Dv DIOCGDINFO 166Read, from the kernel, the in-core copy of the disklabel for the 167drive. This may be a ficticious disklabel if the drive has never 168been initialised, in which case it will contain information read 169from the scsi inquiry commands, and should be the same as 170the information printed at boot. 171.It Dv DIOCSDINFO 172Give the driver a new disklabel to use. The driver will NOT try write the new 173disklabel to the disk. 174.It Dv DIOCWLABEL 175Enable or Disable the driver's software 176write protect of the disklabel on the disk. 177.It Dv DIOCWDINFO 178Give the driver a new disklabel to use. The driver WILL try write the new 179disklabel to the disk. 180.El 181.Pp 182In addition, the 183.Xr scsi 4 184general ioctls may be used with the 185.Nm 186driver, but only against the fourth (whole disk) partition. 187.Sh NOTES 188If a removable device is attached to the 189.Nm 190driver, then the act of changing the media will invalidate the 191disklabel and information held within the kernel. To stop corruption, 192All accesses to the device will be discarded until there are no more 193open file descriptors referencing the device. During this period, all 194new open attempts will be rejected. When No more open file descriptors 195reference the device, the first next open will load a new set of 196figures (including disklabel) for the drive. 197 198An ioctl to map out a bad block is planned. (the code is already present 199in the driver). 200 201.Sh FILES 202.Bl -tag -width /dev/rsd[0-9][a-h] -compact 203.It Pa /dev/sd[0-9][a-h] 204block mode scsi disks 205.It Pa /dev/rsd[0-9][a-h] 206raw scsi disks 207.El 208.Sh DIAGNOSTICS 209None. 210.Sh SEE ALSO 211.Xr disklabel 1 212.Xr disklabel 5 213.Xr fdisk 1 214.Xr wd 4 215.Xr dk 4 216(on other systems) 217.Sh HISTORY 218The 219.Nm 220driver appeared in MACH 2.5 . 221 222