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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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LEARN ABOUT DEBIAN

blktool

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

NAME

       blktool - Display or change block device settings

SYNOPSIS

       blktool [options] device [acoustic-mgmt nnn]

       blktool [options] device [block-sz nnn]

       blktool [options] device [bus-id]

       blktool [options] device [bus-state nnn]

       blktool [options] device [class]

       blktool [options] device [cd-speed nnn]

       blktool [options] device [defect-mgmt off|on]

       blktool [options] device [dev-keep-settings off|on]

       blktool [options] device [dev-read-ahead off|on]

       blktool [options] device [dma off|on]

       blktool [options] device [geometry]

       blktool [options] device [i2o-rcache nnn]

       blktool [options] device [i2o-wcache nnn]

       blktool [options] device [id]

       blktool [options] device [keep-settings off|on]

       blktool [options] device [media unlock|lock]

       blktool [options] device [multiple-count nnn]

       blktool [options] device [no-write-err off|on]

       blktool [options] device [pio-data 16-bit|32-bit]

       blktool [options] device [pio-mode nnn]

       blktool [options] device [pm-mode nnn]

       blktool [options] device [queue-depth nnn]

       blktool [options] device [read-ahead nnn]

       blktool [options] device [readonly off|on]

       blktool [options] device [reread-part]

       blktool [options] device [reset]

       blktool [options] device [sector-sz]

       blktool [options] device [sleep]

       blktool [options] device [standby]

       blktool [options] device [unmask-irq off|on]

       blktool [options] device [version]

       blktool [options] [version]

       blktool [options] device [wcache off|on]

DESCRIPTION

       blktool is used for querying and/or changing settings of a block device.

       device is the name of the block device to query or modify.

OPTIONS

       -f     For  a few commands, blktool will flush the buffers of the block device out to disk, before performing the requested command.  Spec-
	      ify this option, "force", disables any flushing blktool would attempt to do.

       -t class
	      where class is one of 'ide', 'ata', 'scsi', 'i2o', or 'auto' to override the default auto-detection of device class.

COMMANDS

       acoustic-mgmt
	      Query or set underlying device's acoustic level.

       block-sz
	      Query or set device block size.

       bus-id Query device bus id.

       bus-state
	      Query or set device bus state (0 off, 1 on, 2 tristate)

       class  Query the detected (or overridden, via -t) device class.	Typically this will result in 'ATA' or 'SCSI' for most devices.  Detection
	      is based on device major; thus your SATA device may appear as 'SCSI'.

       cd-speed
	      Set CD-ROM device speed (in Kbytes/sec).

       defect-mgmt
	      Toggle underlying device's defect management (obsolete).

       dev-keep-settings
	      Toggle underlying device's revert-to-defaults-on-reset feature.

       dev-read-ahead
	      Toggle underlying device's read-ahead feature.

       dma    Toggle DMA data transfer.  Omitting the on/off argument will print the current state.

       geometry
	      Query device geometry (cylinders / heads / sectors / etc.).

       i2o-rcache
	      Query or set an I2O block device's read cache.

       i2o-wcache
	      Query or set an I2O block device's write cache.

       id     Query IDENTIFY [PACKET] DEVICE data.

       keep-settings
	      Keep settings across device resets.  Omitting the on/off argument will print the current state.

       media  Lock in (or unlock) a removable device.

       multiple-count
	      Set multiple count for use in READ/WRITE MULTIPLE commands.

       no-write-err
	      Toggle ignore-write-errors flag.	Omitting the on/off argument will print the current state.

       pio-data
	      ATA PIO data transfer via 16-bit or 32-bit I/O.  Omitting the on/off argument will print the current state.

       pio-mode
	      Set underlying ATA device's PIO data transfer mode.

       pm-mode
	      Set underlying device's Advanced Power Management mode.

       queued-depth
	      Query or set underlying device's queue depth.

       read-ahead
	      Query or set block device read-ahead (in units of sectors).

       readonly
	      Toggle block device read-only setting.  Omitting the on/off argument will print the current state.

       reread-part
	      Re-read partition table.

       reset  Execute device reset.

       sector-sz
	      Query device sector size.

       sleep  Put device to sleep immediately.

       standby
	      Put device in standby mode.

       unmask-irq
	      Toggle unmasked irqs.  Omitting the on/off argument will print the current state.

       version
	      Print version number and exit.

       wcache Toggle underlying device's writeback cache.

BUGS

       Inevitably there are some.  Please report any you find.

AUTHOR

       blktool was written by Jeff Garzik.

AVAILABILITY

       blktool is available over the Web on the SourceForge site at http://sourceforge.net/projects/gkernel/

blktool version 4						    August 2004 							BLKTOOL(8)
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