Tag Archives: /dev/null

ftp to dev null to test bandwidth

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When testing bandwidth, and troubleshooting bottlenecks, I prefer to use iperf. If you insist on testing bandwidth with FTP, it’s important NOT to use regular files. If you transfer actual files, the transfer could be limited by disk i/o, due to reads and writes. To eliminate this you can FTP from /dev/zero to /dev/null. It sounds super easy, but you have to use FTP in a special way to get it to read and write to special devices.

Here’s a little script. Be sure to replace the destination IP address, username and password with actual values:

# cat ftp_dev_null.sh
#!/bin/bash
/usr/bin/ftp -n <IP address of machine> <<END
verbose on
user <usernanme> <password>
bin
put "|dd if=/dev/zero bs=32k" /dev/null
bye
END

# ftp_dev_null.sh
Verbose mode on.
331 Password required for fordodone
230 User fordodone logged in
Remote system type is UNIX.
Using binary mode to transfer files.
200 Type set to I
local: |dd if=/dev/zero bs=32k remote: /dev/null
200 PORT command successful
150 Opening BINARY mode data connection for /dev/null
^C
send aborted
waiting for remote to finish abort
129188+0 records in
129187+0 records out
4233199616 bytes (4.2 GB) copied, 145.851 s, 29.0 MB/s
226 Transfer complete
4233142272 bytes sent in 145.82 secs (28350.0 kB/s)
221 Goodbye.
#

In this case I was getting around 230Mbits per second (over an IPSec tunnel) between my client and the FTP server. Not too bad.

show progress for dd

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By default dd is silent. It just copies whatever blocks you want from in to out. In order to see progress, send it a USR1 signal using kill.

Start a useless dd:

# dd if=/dev/zero of=/dev/null

In another terminal find the pid:

# ps aux | grep dd | grep -v grep
root      7784 90.5  0.0   2884   560 pts/9    R+   10:01   0:06 dd if /dev/zero of /dev/null
#
# kill -USR1 7784

The original window will now show this:

# dd if=/dev/zero of=/dev/null
14501614+0 records in
14501614+0 records out
7424826368 bytes (7.4 GB) copied, 16.2149 seconds, 458 MB/s

Then you can ctrl+c it to get the final output:

# dd if=/dev/zero of=/dev/null
14501614+0 records in
14501614+0 records out
7424826368 bytes (7.4 GB) copied, 16.2149 seconds, 458 MB/s
16888077+0 records in
16888076+0 records out
8646694912 bytes (8.6 GB) copied, 19.3507 seconds, 447 MB/s

This one liner will start your dd, then monitor it and output progress every 20 seconds. Once the dd is finished it will stop and give your shell back.

dd if=/dev/zero of=/dev/null & pid=$! && sleep 20s && while true; do i=`ps aux | awk '{print $2}' | grep ^$pid$`; if [ "${i:-a}" !=  "$pid" ]; then break; fi; kill -USR1 $pid; sleep 20s; done;

time mysql select without output

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To see how long a query will take without actually outputting anything, change the pager in mysql client to /dev/null instead of STDOUT

mysql> pager;
Default pager wasn't set, using stdout.

mysql> pager >/dev/null
PAGER set to '>/dev/null'

mysql> select md5sum from table1;
5245618 rows in set (1 min 21.50 sec)

mysql> select md5sum from table1 where md5sum is not NULL and md5sum!='not calculated';
4832676 rows in set (1 min 11.52 sec)

mysql> select md5sum from table1 where md5sum not in ('NULL', '', 'not calculated');
4832078 rows in set (1 min 21.91 sec)