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Test Your Knowledge in Computers #592
Difficulty: Medium
Suppose we want to sort a huge collection of 1 trillion, 10^12, elements. Doing so with Selection Sort or Insertion Sort would require about n^2 = (10^12)^2 = 10^24 or 1 sextillion comparisons.
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blockdev(8) [linux man page]

BLOCKDEV(8)						      System Manager's Manual						       BLOCKDEV(8)

NAME
blockdev - call block device ioctls from the command line SYNOPSIS
blockdev [-q] [-v] command [command...] device [device...] blockdev --report [device...] DESCRIPTION
The utility blockdev allows one to call block device ioctls from the command line. OPTIONS
-V Print version and exit. -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. 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 sectorsize in bytes - usually 512. --getsz Get size in 512-byte sectors. --rereadpt Reread partition table --setbsz bytes Set blocksize. --setfra sectors Set filesystem readahead (same like --setra on 2.6 kernels). --setra sectors Set readahead (in 512-byte sectors). --setro Set read-only. --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 ftp://ftp.kernel.org/pub/linux/utils/util-linux/. Aug 2010 BLOCKDEV(8)

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RAW(8)							      System Manager's Manual							    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, with 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 Set query mode. raw will query an existing binding instead of setting a new one. -a With -q , specifies that all bound raw devices should be queried. -h provides a usage summary. BUGS
The Linux dd (1) command should be used without 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! AUTHOR
Stephen Tweedie (sct@redhat.com) AVAILABILITY
The raw command is part of the util-linux-ng package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/. Version 0.1 Aug 1999 RAW(8)

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