I am trying to measure the speed of reading a given block size using the dd command. However depending on which input I use: a regular file (on the same device) or /dev/sdb1, I get some really different results.
Code:
$sudo sh -c 'echo 3 > /proc/sys/vm/drop_caches'; dd if=pirate of=/dev/null bs=512 count=1
1+0 records in
1+0 records out
512 bytes (512 B) copied, 0.0190755 s, 26.8 kB/s
$sudo sh -c 'echo 3 > /proc/sys/vm/drop_caches'; sudo dd if=/dev/sdb1 of=/dev/null bs=512 count=1
1+0 records in
1+0 records out
512 bytes (512 B) copied, 1.9649e-05 s, 26.1 MB/s
Note:
- Freshly formated ext4
ATA device, with non-removable media
Transport: Serial, SATA Rev 3.0
- I did the same experiment with varying blocksizes, and up to 128K the phenomena is the same.
- The file pirate is 100G and filefrag reports: 57 extents found
Questions:
- Is this normal?
- I am wondering if the speed of repositioning the disk head to the beginning of a device is fast comparing to any file offset movement. because reading 512B should be elementary ...
Thanks a lot
Last edited by methyl; 07-03-2012 at 07:36 PM..
Reason: please use code tags
Hi ......
I have file system problem when i try to mount get the following message :
mount: /dev/rdsk/c1t4d0s7 not a block device ....
I run fsck it's ok ,,,, after run
newfs -N /dev/dsk/c1t4d0s7 and
fsck -o b=535952 /dev/dsk/c1t4d0s7 .............it's ok ........
At the and when i try... (2 Replies)
Hi,
I am triying to make sure that there exists only one file with the pattern abc* in path /path/. This directory is having many huge files. If there is only one file then I have to take its complete name only to use furter in my script.
I am planning to do like this:
if ; then... (2 Replies)
We are intending to protect a set of user specified files using LVM mirroring where the protected space on which the user files are stored is mirrored on an LV on a different disk. Our problem is that for a user with a custom layout has installed linux with 2 partitons for swap and / and there is... (0 Replies)
I talked with this guy who seems to think loop mounting a fs ext3 image on a directory, as opposed to just using the underlying filesystem, will work better as far as IO conflicts. I have no idea why this would be better? I haven't been able to contact him.
Basically we have a daemon that... (0 Replies)
I use CentOS
I talked with this guy who seems to think loop mounting a fs ext3 image on a directory, as opposed to just using the underlying filesystem, will work better as far as IO conflicts. I have no idea why this would be better? I haven't been able to contact him.
Basically we have a... (1 Reply)
Hello everyone
I am finishing a script allowing me to purge logs on multiple servers, i have one last pb with the ssh command.........it is throwing me the following error :
tcgetattr: Inappropriate ioctl for device (full screen in attached file 1, full script in attached file 2)
It... (15 Replies)
Currently I am using this laborious command
lvdisplay | awk '/LV Path/ {p=$3} /LV Name/ {n=$3} /VG Name/ {v=$3} /Block device/ {d=$3; sub(".*:", "/dev/dm-", d); printf "%s\t%s\t%s\n", p, "/dev/mapper/"v"-"n, d}'
Would like to know if there is any shorter method to get this mapping of... (2 Replies)
Mac Version 10.15.2 (macOS Catalina)
Does anyone know how to change the name of a connected bluetooth device from the command line on macOS?
I am having trouble with various bluetooth devices which I cannot get the "rename" option in the GUI to "save" properly and so I cannot rename a few... (0 Replies)
Discussion started by: Neo
0 Replies
LEARN ABOUT V7
dd
DD(1) General Commands Manual DD(1)NAME
dd - convert and copy a file
SYNOPSIS
dd [option=value] ...
DESCRIPTION
Dd copies the specified input file to the specified output with possible conversions. The standard input and output are used by default.
The input and output block size may be specified to take advantage of raw physical I/O.
option values
if= input file name; standard input is default
of= output file name; standard output is default
ibs=n input block size n bytes (default 512)
obs=n output block size (default 512)
bs=n set both input and output block size, superseding ibs and obs; also, if no conversion is specified, it is particularly effi-
cient since no copy need be done
cbs=n conversion buffer size
skip=n skip n input records before starting copy
files=n copy n files from (tape) input
seek=n seek n records from beginning of output file before copying
count=n copy only n input records
conv=ascii convert EBCDIC to ASCII
ebcdic convert ASCII to EBCDIC
ibm slightly different map of ASCII to EBCDIC
lcase map alphabetics to lower case
ucase map alphabetics to upper case
swab swap every pair of bytes
noerror do not stop processing on an error
sync pad every input record to ibs
... , ... several comma-separated conversions
Where sizes are specified, a number of bytes is expected. A number may end with k, b or w to specify multiplication by 1024, 512, or 2
respectively; a pair of numbers may be separated by x to indicate a product.
Cbs is used only if ascii or ebcdic conversion is specified. In the former case cbs characters are placed into the conversion buffer, con-
verted to ASCII, and trailing blanks trimmed and new-line added before sending the line to the output. In the latter case ASCII characters
are read into the conversion buffer, converted to EBCDIC, and blanks added to make up an output record of size cbs.
After completion, dd reports the number of whole and partial input and output blocks.
For example, to read an EBCDIC tape blocked ten 80-byte EBCDIC card images per record into the ASCII file x:
dd if=/dev/rmt0 of=x ibs=800 cbs=80 conv=ascii,lcase
Note the use of raw magtape. Dd is especially suited to I/O on the raw physical devices because it allows reading and writing in arbitrary
record sizes.
To skip over a file before copying from magnetic tape do (dd of=/dev/null; dd of=x) </dev/rmt0
SEE ALSO cp(1), tr(1)DIAGNOSTICS
f+p records in(out): numbers of full and partial records read(written)
BUGS
The ASCII/EBCDIC conversion tables are taken from the 256 character standard in the CACM Nov, 1968. The `ibm' conversion, while less
blessed as a standard, corresponds better to certain IBM print train conventions. There is no universal solution.
Newlines are inserted only on conversion to ASCII; padding is done only on conversion to EBCDIC. These should be separate options.
DD(1)