217 lines
6.7 KiB
Groff
217 lines
6.7 KiB
Groff
.Dd August 27, 1993
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.Dt SD 4
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.Os FreeBSD
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.Sh NAME
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.Nm sd
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.Nd scsi disk driver
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.Sh SYNOPSIS
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.Nm device-driver sd
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.Op Ar count
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.Sh DESCRIPTION
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The
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.Xr sd
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driver provides support for a
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.Em scsi
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disk. It allows the disk
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to be divided up into a set of pseudo devices called
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.Em partitions.
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A Partition can have both a
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.Em raw
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interface
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and a
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.Em Block mode
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interface.
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In general the interfaces are similar to those described by
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.Xr wd 4
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or
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.Xr dk 4 .
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.Pp
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Where the
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.Xr wd 4
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device has a fairly low level interface to the system,
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.Em SCSI
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devices have a much higher level interface and talk to the system via
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a
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.Em SCSI Adapter
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and a
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.Em Scsi Adapter driver
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e.g.
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.Xr AHA1542 .
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A scsi adapter must also be separatly configured into the system
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before a scsi disk can be configured.
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.Pp
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As the scsi adapter is probed during boot, the
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.Em SCSI
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bus is scanned for devices. Any devices found which answer as 'Direct'
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type devices will be 'attached' to the
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.Nm
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driver. The first found will be attached as
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.Em sd0
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and the next,
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.Em sd1
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etc.
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.Pp
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.Sh PARTITIONING
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The
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.Nm
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driver allows the disk to have two levels of partitioning.
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One which allows it to have
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partitions for different Operating systems, (one of which is BSD unix),
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(see also for the 386 port,
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.Xr fdisk 1
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), and within a BSD partition, further partitions which are individually
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addressable as separate entries in the
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.Em /dev
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directory. The second level of partitioning is controlled by the program
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.Xr disklabel 1
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and is common in format across most BSD operating systems. In most of
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the original BSD ports, what is the
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BSD part here, is the entire disk, and the outer layer of partitionning
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does not exist.
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.Nm
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will also run in this manner if
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.Xr disklabel 1
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is run with a blank disk, without first partitioning it
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with
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.Xr fdisk 1
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(or similar).
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.Pp
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Apologies for the two conflicting usages of the word Partition, but
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it's a historical artifact, and the meaning must be judged from context
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in each case. The next paragraph will discuss partitions exclusively
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in the context of WITHIN a BSD partition on the disk.
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.Pp
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The first few blocks of the BSD section (maybe all) of the disk contain
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some boot code, and a structure, known as the
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.Xr disklabel 5
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which describes the disk's characteristics and partitioning for BSD.
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It is set up by the
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.Xr disklabel 1
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program, and read in by the kernel when the device is first initialised
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during boot. It describes how the drive is further divided. The
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.Xr disklabel 5
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structure contains room for 8 (usually) partitions. Usually these
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partitions are calculated so as to fall evenly on cylinder boundaries,
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however on a
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.Em SCSI
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disk this is sometimes not possible. The reason for doing this is historically
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to get better performance, however modern
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.Em SCSI
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disks often have a variable format, so that it is hard to know at any point
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in the disk, where the cylinder or track boundaries are. Added to this, the
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fact that
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.Em SCSI
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disk blocks are addressed soley by their 'block number' and not by
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any geometry, leads to the common occurance on
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.Em SCSI
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disks, of laying out partitions on arbitrary boundaries. Because
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modern disks often have large track caches, this often leads to only small
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degadations of performance, and is in fact sometimes unavoidable. The
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boot messages will suggest a geometry similar in heads and cylinders
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to the real geometry, but the disklable need not agree with this for the
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system to be able to successfully work with the disk.
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.Pp
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During booting
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with an uninitialised disk, the
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.Nm
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driver will initialise the 'in-core' copy of the disklabel to the suggested
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values, however they are not written to the disk.
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.Pp
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The fourth partition is special. No matter what the disklabel
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says, the fourth partition (partition d) reflectls the entire disk, including
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those areas OUTSIDE the BSD partitions. At some times it is suggested that
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the c partition might be used to represent the entire BSD partition, so these
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two partitions should be avoided when laying out filesystems. The fourth
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partition must be used for general
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.Xr scsi 4
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ioctls.
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.Pp
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While partitions are only theoretically valid within the BSD partition, they
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are specified in terms of absolute block numbers, so it is possible to
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specify a partition that lies outside of the BSD partition. This is useful
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if one wants to have a /dev entry that points to a partition belonging
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to another OS (e.g. DOS).
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.Pp
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.Sh KERNEL CONFIGURATION
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In configuring, if an optional
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.Ar count
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is given in
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the specification, that number of scsi disks are configured;
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Most storage for them is allocated only when found so a large number
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of configured devices is cheap. (once the first has included the driver).
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.Pp
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.Sh IOCTLS
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The following
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.Xr ioctl 2
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calls apply to scsi disks as well as to other disks. They are defined
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in the header file
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.Em disklabel.h.
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.Bl -tag -width DIOCSDINFO
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.It Dv DIOCSBAD
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Usually used to set up a bad-block mapping system on the disk. Scsi
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drive incorporate their own bad-block mapping so this is not implimented,
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however it MAY be implimented in the future as a 'kludged' interface to the
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scsi bad-block mapping.
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.It Dv DIOCGDINFO
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Read, from the kernel, the in-core copy of the disklabel for the
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drive. This may be a ficticious disklabel if the drive has never
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been initialised, in which case it will contain information read
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from the scsi inquiry commands, and should be the same as
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the information printed at boot.
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.It Dv DIOCSDINFO
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Give the driver a new disklabel to use. The driver will NOT try write the new
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disklabel to the disk.
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.It Dv DIOCWLABEL
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Enable or Disable the driver's software
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write protect of the disklabel on the disk.
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.It Dv DIOCWDINFO
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Give the driver a new disklabel to use. The driver WILL try write the new
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disklabel to the disk.
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.El
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.Pp
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In addition, the
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.Xr scsi 4
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general ioctls may be used with the
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.Nm
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driver, but only against the fourth (whole disk) partition.
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.Sh NOTES
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If a removable device is attached to the
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.Nm
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driver, then the act of changing the media will invalidate the
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disklabel and information held within the kernel. To stop corruption,
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All accesses to the device will be discarded until there are no more
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open file descriptors referencing the device. During this period, all
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new open attempts will be rejected. When No more open file descriptors
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reference the device, the first next open will load a new set of
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figures (including disklabel) for the drive.
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An ioctl to map out a bad block is planned. (the code is already present
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in the driver).
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.Sh FILES
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.Bl -tag -width /dev/rsd[0-9][a-h] -compact
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.It Pa /dev/sd[0-9][a-h]
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block mode scsi disks
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.It Pa /dev/rsd[0-9][a-h]
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raw scsi disks
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.El
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.Sh DIAGNOSTICS
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None.
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.Sh SEE ALSO
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.Xr disklabel 1
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.Xr disklabel 5
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.Xr fdisk 1
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.Xr wd 4
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.Xr dk 4
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(on other systems)
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.Sh HISTORY
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The
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.Nm
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driver appeared in MACH 2.5 .
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