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Full Discussion: Blockdev vs /sys/block/
Special Forums Hardware Filesystems, Disks and Memory Blockdev vs /sys/block/ Post 302905586 by Sergiu-IT on Thursday 12th of June 2014 08:53:05 AM
Old 06-12-2014
Blockdev vs /sys/block/

Hello,

I want to set/get the read-ahead for my disks and there is a little issue that confuses me. There are three ways to get/set the read-ahead buffer:

1. cat /sys/block/md0/bdi/read_ahead_kb
2. cat /sys/block/md0/queue/read_ahead_kb
3. blockdev --getra /dev/md0

Which one is the correct one? The first two gives me a value of 128. The last one gives me a value of 256. Where does this difference come from?
 

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

NAME
blockdev - call block device ioctls from the command line SYNOPSIS
blockdev [options] commands devices blockdev --report [devices] 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 devices. Note that the partition StartSec is in 512-byte sectors. COMMANDS
--setro Set read-only. --setrw Set read-write. --getro Get read-only. Print 1 if the device is read-only, 0 otherwise. --getss Print sectorsize in bytes - usually 512. --getbsz Print blocksize in bytes. --setbsz N Set blocksize to N bytes. --getsize Print device size in sectors (BLKGETSIZE). Deprecated in favor of the --getsz option. --getsize64 Print device size in bytes (BLKGETSIZE64) --getsz Get size in 512-byte sectors (BLKGETSIZE64 / 512). --setra N Set readahead to N 512-byte sectors. --getra Print readahead (in 512-byte sectors). --setfra N Set filesystem readahead (same like --setra on 2.6 kernels). --getfra Get filesystem readahead. --flushbufs Flush buffers. --rereadpt Reread partition table. AUTHOR
blockdev was written by Andries E. Brouwer. AVAILABILITY
The blockdev command is part of the util-linux-ng package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/. Jun 2007 BLOCKDEV(8)
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