disk-utils/raw.8

   1 .TH RAW 8 "August 1999" "util-linux" "System Administration"
   2 .SH NAME
   3 raw \- bind a Linux raw character device
   4 .SH SYNOPSIS
   5 .B raw
   6 .I /dev/raw/raw<N> <major> <minor>
   7 .PP
   8 .B raw
   9 .I /dev/raw/raw<N> /dev/<blockdev>
  10 .PP
  11 .B raw \-q
  12 .I /dev/raw/raw<N>
  13 .PP
  14 .B raw \-qa
  15 .SH DESCRIPTION
  16 .B raw
  17 is used to bind a Linux raw character device to a block device.  Any
  18 block device may be used: at the time of binding, the device driver does
  19 not even have to be accessible (it may be loaded on demand as a kernel
  20 module later).
  21 .PP
  22 .B raw
  23 is used in two modes: it either sets raw device bindings, or it queries
  24 existing bindings.  When setting a raw device,
  25 .I /dev/raw/raw<N>
  26 is the device name of an existing raw device node in the filesystem.
  27 The block device to which it is to be bound can be specified either in
  28 terms of its
  29 .I major
  30 and
  31 .I minor
  32 device numbers, or as a path name
  33 .I /dev/<blockdev>
  34 to an existing block device file.
  35 .PP
  36 The bindings already in existence can be queried with the
  37 .I \-q
  38 option, which is used either with a raw device filename to query that one
  39 device, or with the
  40 .I \-a
  41 option to query all bound raw devices.
  42 .PP
  43 Unbinding can be done by specifying major and minor 0.
  44 .PP
  45 Once bound to a block device, a raw device can be opened, read and
  46 written, just like the block device it is bound to.  However, the raw
  47 device does not behave exactly like the block device.  In particular,
  48 access to the raw device bypasses the kernel's block buffer cache
  49 entirely: all I/O is done directly to and from the address space of the
  50 process performing the I/O.  If the underlying block device driver can
  51 support DMA, then no data copying at all is required to complete the
  52 I/O.
  53 .PP
  54 Because raw I/O involves direct hardware access to a process's memory, a
  55 few extra restrictions must be observed.  All I/Os must be correctly
  56 aligned in memory and on disk: they must start at a sector offset on
  57 disk, they must be an exact number of sectors long, and the data buffer
  58 in virtual memory must also be aligned to a multiple of the sector
  59 size.  The sector size is 512 bytes for most devices.
  60 .SH OPTIONS
  61 .TP
  62 \fB\-q\fR, \fB\-\-query\fR
  63 Set query mode.
  64 .B raw
  65 will query an existing binding instead of setting a new one.
  66 .TP
  67 \fB\-a\fR, \fB\-\-all\fR
  68 With
  69 .B \-q
  70 , specify that all bound raw devices should be queried.
  71 .TP
  72 \fB\-h\fR, \fB\-\-help\fR
  73 Display help text and exit.
  74 .TP
  75 \fB\-V\fR, \fB\-\-version\fR
  76 Display version information and exit.
  77
  78 .SH NOTES
  79 Rather than using raw devices applications should prefer
  80 .BR open (2)
  81 devices, such as /dev/sda1, with the O_DIRECT flag.
  82 .SH BUGS
  83 The Linux
  84 .BR dd (1)
  85 command should be used without the \fBbs=\fR option, or the blocksize
  86 needs to be a multiple of the sector size of the device (512 bytes usually),
  87 otherwise it will fail with "Invalid Argument" messages (EINVAL).
  88
  89 .PP
  90 Raw I/O devices do not maintain cache coherency with the Linux block
  91 device buffer cache.  If you use raw I/O to overwrite data already in
  92 the buffer cache, the buffer cache will no longer correspond to the
  93 contents of the actual storage device underneath.  This is deliberate,
  94 but is regarded either a bug or a feature depending on who you ask!
  95 .SH AUTHORS
  96 Stephen Tweedie (sct@redhat.com)
  97 .SH AVAILABILITY
  98 The raw command is part of the util-linux package and is available from
  99 https://www.kernel.org/pub/linux/utils/util-linux/.