BLOCKDEV(8) System Administration BLOCKDEV(8)NAME
blockdev - call block device ioctls from the command line
SYNOPSIS
blockdev [-q] [-v] command [command...] device [device...]
blockdev --report [device...]
blockdev -h|-V
DESCRIPTION
The utility blockdev allows one to call block device ioctls from the command line.
OPTIONS -q Be quiet.
-v Be verbose.
--report
Print a report for the specified device. It is possible to give multiple devices. If none is given, all devices which appear in
/proc/partitions are shown. Note that the partition StartSec is in 512-byte sectors.
-h, --help
Display help text and exit.
-V, --version
Print version and exit.
COMMANDS
It is possible to give multiple devices and multiple commands.
--flushbufs
Flush buffers.
--getalignoff
Get alignment offset.
--getbsz
Print blocksize in bytes.
--getdiscardzeroes
Get discard zeroes support status.
--getfra
Get filesystem readahead in 512-byte sectors.
--getiomin
Get minimum I/O size.
--getioopt
Get optimal I/O size.
--getmaxsect
Get max sectors per request
--getpbsz
Get physical block (sector) size.
--getra
Print readahead (in 512-byte sectors).
--getro
Get read-only. Print 1 if the device is read-only, 0 otherwise.
--getsize64
Print device size in bytes.
--getsize
Print device size (32-bit!) in sectors. Deprecated in favor of the --getsz option.
--getss
Print logical sector size in bytes - usually 512.
--getsz
Get size in 512-byte sectors.
--rereadpt
Reread partition table
--setbsz bytes
Set blocksize. Note that the block size is specific to the current file descriptor opening the block device, so the change of block
size only persists for as long as blockdev has the device open, and is lost once blockdev exits.
--setfra sectors
Set filesystem readahead (same like --setra on 2.6 kernels).
--setra sectors
Set readahead (in 512-byte sectors).
--setro
Set read-only. The currently active access to the device may not be affected by the change. For example filesystem already mounted
in read-write mode will not be affected. The change applies after remount.
--setrw
Set read-write.
AUTHOR
blockdev was written by Andries E. Brouwer and rewritten by Karel Zak.
AVAILABILITY
The blockdev command is part of the util-linux package and is available from https://www.kernel.org/pub/linux/utils/util-linux/.
util-linux August 2010 BLOCKDEV(8)
Check Out this Related Man Page
RAW(8) System Administration RAW(8)NAME
raw - bind a Linux raw character device
SYNOPSIS
raw /dev/raw/raw<N> <major> <minor>
raw /dev/raw/raw<N> /dev/<blockdev>
raw -q /dev/raw/raw<N>
raw -qa
DESCRIPTION
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).
raw is used in two modes: it either sets raw device bindings, or it queries existing bindings. When setting a raw device, /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 major and minor device numbers, or as a path name /dev/<blockdev> to an existing block device file.
The bindings already in existence can be queried with the -q option, which is used either with a raw device filename to query that one
device, or with the -a option to query all bound raw devices.
Unbinding can be done by specifying major and minor 0.
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.
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 cor-
rectly 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.
OPTIONS -q, --query
Set query mode. raw will query an existing binding instead of setting a new one.
-a, --all
With -q , specify that all bound raw devices should be queried.
-h, --help
Display help text and exit.
-V, --version
Display version information and exit.
BUGS
The Linux dd(1) command should be used without the bs= 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).
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!
NOTES
Rather than using raw devices applications should prefer open(2) devices, such as /dev/sda1, with the O_DIRECT flag.
AUTHOR
Stephen Tweedie (sct@redhat.com)
AVAILABILITY
The raw command is part of the util-linux package and is available from https://www.kernel.org/pub/linux/utils/util-linux/.
util-linux August 1999 RAW(8)