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diff --git a/disk-utils/raw.8 b/disk-utils/raw.8 new file mode 100644 index 0000000..ba9fd25 --- /dev/null +++ b/disk-utils/raw.8 @@ -0,0 +1,99 @@ +.TH RAW 8 "August 1999" "util-linux" "System Administration" +.SH NAME +raw \- bind a Linux raw character device +.SH SYNOPSIS +.B raw +.I /dev/raw/raw<N> <major> <minor> +.PP +.B raw +.I /dev/raw/raw<N> /dev/<blockdev> +.PP +.B raw \-q +.I /dev/raw/raw<N> +.PP +.B raw \-qa +.SH DESCRIPTION +.B raw +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). +.PP +.B raw +is used in two modes: it either sets raw device bindings, or it queries +existing bindings. When setting a raw device, +.I /dev/raw/raw<N> +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 +.I major +and +.I minor +device numbers, or as a path name +.I /dev/<blockdev> +to an existing block device file. +.PP +The bindings already in existence can be queried with the +.I \-q +option, which is used either with a raw device filename to query that one +device, or with the +.I \-a +option to query all bound raw devices. +.PP +Unbinding can be done by specifying major and minor 0. +.PP +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's 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. +.PP +Because raw I/O involves direct hardware access to a process's 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. +.SH OPTIONS +.TP +\fB\-q\fR, \fB\-\-query\fR +Set query mode. +.B raw +will query an existing binding instead of setting a new one. +.TP +\fB\-a\fR, \fB\-\-all\fR +With +.B \-q +, specify that all bound raw devices should be queried. +.TP +\fB\-h\fR, \fB\-\-help\fR +Display help text and exit. +.TP +\fB\-V\fR, \fB\-\-version\fR +Display version information and exit. + +.SH NOTES +Rather than using raw devices applications should prefer +.BR open (2) +devices, such as /dev/sda1, with the O_DIRECT flag. +.SH BUGS +The Linux +.BR dd (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 (EINVAL). + +.PP +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 either a bug or a feature depending on who you ask! +.SH AUTHORS +Stephen Tweedie (sct@redhat.com) +.SH AVAILABILITY +The raw command is part of the util-linux package and is available from +https://www.kernel.org/pub/linux/utils/util-linux/. |