| 1 | '\" t
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| 2 | .\" Title: raw
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| 3 | .\" Author: [see the "AUTHORS" section]
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| 4 | .\" Generator: DocBook XSL-NS Stylesheets vsnapshot <http://docbook.sf.net/>
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| 5 | .\" Date: 01/25/2022
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| 6 | .\" Manual: System Administration
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| 7 | .\" Source: [FIXME: source]
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| 8 | .\" Language: English
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| 9 | .\"
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| 10 | .TH "RAW" "8" "01/25/2022" "[FIXME: source]" "System Administration"
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| 11 | .\" -----------------------------------------------------------------
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| 12 | .\" * Define some portability stuff
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| 13 | .\" -----------------------------------------------------------------
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| 14 | .\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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| 15 | .\" http://bugs.debian.org/507673
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| 16 | .\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html
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| 17 | .\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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| 18 | .ie \n(.g .ds Aq \(aq
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| 19 | .el .ds Aq '
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| 20 | .\" -----------------------------------------------------------------
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| 21 | .\" * set default formatting
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| 22 | .\" -----------------------------------------------------------------
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| 23 | .\" disable hyphenation
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| 24 | .nh
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| 25 | .\" disable justification (adjust text to left margin only)
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| 26 | .ad l
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| 27 | .\" -----------------------------------------------------------------
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| 28 | .\" * MAIN CONTENT STARTS HERE *
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| 29 | .\" -----------------------------------------------------------------
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| 30 | .SH "NAME"
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| 31 | raw \- bind a Linux raw character device
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| 32 | .SH "SYNOPSIS"
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| 33 | .sp
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| 34 | \fBraw\fR \fI/dev/raw/raw<N>\fR \fI<major>\fR \fI<minor>\fR
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| 35 | .sp
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| 36 | \fBraw\fR \fI/dev/raw/raw<N>\fR \fI/dev/<blockdev>\fR
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| 37 | .sp
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| 38 | \fBraw\fR \fB\-q\fR \fI/dev/raw/raw<N>\fR
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| 39 | .sp
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| 40 | \fBraw\fR \fB\-qa\fR
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| 41 | .SH "DESCRIPTION"
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| 42 | .sp
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| 43 | \fBraw\fR is used to bind a Linux raw character device to a block device\&. Any block device may be used: at the time of binding, the device driver does not even have to be accessible (it may be loaded on demand as a kernel module later)\&.
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| 44 | .sp
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| 45 | \fBraw\fR is used in two modes: it either sets raw device bindings, or it queries existing bindings\&. When setting a raw device, \fI/dev/raw/raw<N>\fR is the device name of an existing raw device node in the filesystem\&. The block device to which it is to be bound can be specified either in terms of its \fImajor\fR and \fIminor\fR device numbers, or as a path name \fI/dev/<blockdev>\fR to an existing block device file\&.
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| 46 | .sp
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| 47 | The bindings already in existence can be queried with the \fB\-q\fR option, which is used either with a raw device filename to query that one device, or with the \fB\-a\fR option to query all bound raw devices\&.
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| 48 | .sp
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| 49 | Unbinding can be done by specifying major and minor 0\&.
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| 50 | .sp
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| 51 | Once bound to a block device, a raw device can be opened, read and written, just like the block device it is bound to\&. However, the raw device does not behave exactly like the block device\&. In particular, access to the raw device bypasses the kernel\(cqs block buffer cache entirely: all I/O is done directly to and from the address space of the process performing the I/O\&. If the underlying block device driver can support DMA, then no data copying at all is required to complete the I/O\&.
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| 52 | .sp
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| 53 | Because raw I/O involves direct hardware access to a process\(cqs memory, a few extra restrictions must be observed\&. All I/Os must be correctly aligned in memory and on disk: they must start at a sector offset on disk, they must be an exact number of sectors long, and the data buffer in virtual memory must also be aligned to a multiple of the sector size\&. The sector size is 512 bytes for most devices\&.
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| 54 | .SH "OPTIONS"
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| 55 | .PP
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| 56 | \fB\-q\fR, \fB\-\-query\fR
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| 57 | .RS 4
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| 58 | Set query mode\&.
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| 59 | \fBraw\fR
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| 60 | will query an existing binding instead of setting a new one\&.
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| 61 | .RE
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| 62 | .PP
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| 63 | \fB\-a\fR, \fB\-\-all\fR
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| 64 | .RS 4
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| 65 | With
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| 66 | \fB\-q\fR, specify that all bound raw devices should be queried\&.
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| 67 | .RE
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| 68 | .PP
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| 69 | \fB\-h\fR, \fB\-\-help\fR
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| 70 | .RS 4
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| 71 | Display help text and exit\&.
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| 72 | .RE
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| 73 | .PP
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| 74 | \fB\-V\fR, \fB\-\-version\fR
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| 75 | .RS 4
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| 76 | Display version information and exit\&.
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| 77 | .RE
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| 78 | .SH "NOTES"
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| 79 | .sp
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| 80 | Rather than using raw devices applications should prefer open2 devices, such as \fI/dev/sda1\fR, with the \fBO_DIRECT\fR flag\&.
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| 81 | .SH "BUGS"
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| 82 | .sp
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| 83 | The Linux \fBdd\fR(1) command should be used without the \fBbs=\fR option, or the blocksize needs to be a multiple of the sector size of the device (512 bytes usually), otherwise it will fail with "Invalid Argument" messages (\fBEINVAL\fR)\&.
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| 84 | .sp
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| 85 | Raw I/O devices do not maintain cache coherency with the Linux block device buffer cache\&. If you use raw I/O to overwrite data already in the buffer cache, the buffer cache will no longer correspond to the contents of the actual storage device underneath\&. This is deliberate, but is regarded as either a bug or a feature, depending on who you ask!
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| 86 | .SH "AUTHORS"
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| 87 | .sp
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| 88 | \m[blue]\fBStephen Tweedie\fR\m[]\&\s-2\u[1]\d\s+2
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| 89 | .SH "NOTES"
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| 90 | .IP " 1." 4
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| 91 | Stephen Tweedie
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| 92 | .RS 4
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| 93 | \%mailto:sct@redhat.com
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| 94 | .RE
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