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Full Discussion: Maximum buffer size for read()
Top Forums Programming Maximum buffer size for read() Post 302601540 by jim mcnamara on Friday 24th of February 2012 12:57:21 AM
Old 02-24-2012
read and write both use (int file descriptor, memory buffer, size_t bytes) and they return ssize_t bytes read. You DO NOT WANT a 1 byte buffer for read except in very special cases. Modern disks have large fetch buffers, modern filesystems read and write really big chunks. All because it is a more efficient use of resources.

Example: Compellent SAN storage uses a preferred internal block size of 2 MB. The ufs filesystem has a default buffer size of 1MB.

So if you periodically read 1 byte at a time, after a while the rest of the data that came in with your original read request is flushed from the cache and has to be read again.

Look at it this way, you are asking the system to pitch 99.99999% of every read when you request 1 byte, then twiddle your vitrual thumbs for a while. Then come back for byte #2.
 

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LEARN ABOUT MOJAVE

blockdev

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)
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