How to treat iostat?

sluge · 08-03-2012, 08:26 AM

Hello
I wanted to zero my USB key drive and I type folloing command:

Code:

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

So, after a while I call iostat

Code:

iostat  -m sdd
Linux 2.6.32.x86_64     08/01/2012      _x86_64_        (16 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.05    0.02    0.35    1.31    0.00   98.27

Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
sdj             175.25         0.66         0.66       3432       3430

I found that MB_read is 3412 but MB_wrtn is 3430.
Why MB_read is not zero? I write to device but not read from it

Beatriz85 · 08-03-2012, 09:48 AM

No answer?
C'mon guys! I also need this info, some help please!
I have only started my work with Linux, so I really need assistance

deadeyes · 08-04-2012, 04:53 PM

Quote:

Originally Posted by sluge

Hello
I wanted to zero my USB key drive and I type folloing command:

Code:

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

So, after a while I call iostat

Code:

iostat  -m sdd
Linux 2.6.32.x86_64     08/01/2012      _x86_64_        (16 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.05    0.02    0.35    1.31    0.00   98.27

Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
sdj             175.25         0.66         0.66       3432       3430

I found that MB_read is 3412 but MB_wrtn is 3430.
Why MB_read is not zero? I write to device but not read from it

Not sure if this is a typo in your output: iostat -m sdd and device: sdj.

So 2 different devices.

sluge · 08-06-2012, 12:49 AM

Quote:

Originally Posted by deadeyes

Not sure if this is a typo in your output: iostat -m sdd and device: sdj.

So 2 different devices.

Sorry, it's cut-paste error, but question is still actual

deadeyes · 08-06-2012, 01:48 AM

Quote:

Originally Posted by sluge

Sorry, it's cut-paste error, but question is still actual

I have to say this is a great find

I am really interested in the cause

The following is what dd does:

Code:

$ strace dd if=/dev/zero of=/dev/mapper/local-test
...

open("/dev/mapper/local-test", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
dup2(3, 1)                              = 1
close(3)                                = 0
rt_sigaction(SIGUSR1, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGINT, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGUSR1, {0x401c90, [INT USR1], SA_RESTORER, 0x7f3bef8d09e0}, NULL, 8) = 0
rt_sigaction(SIGINT, {0x401c80, [INT USR1], SA_RESTORER|SA_NODEFER|SA_RESETHAND, 0x7f3bef8d09e0}, NULL, 8) = 0
read(0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 512) = 512
write(1, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 512) = 512
close(0)                                = 0
close(1)

As you can see /dev/mapper/local-test is opened write only. So nothing can be read from it (at least not by dd at that moment).
/dev/zero is opened read-only (not listed).

Note that I could only test this with an LVM volume.

I wonder if this is an issue with IOstat.

From what I can see you are using a device directly so LVM doesn't seem to be the cause as well.

Then there is only one option left: the kernel. No idea why it would actually do this.

deadeyes · 08-06-2012, 09:10 AM

I found it

This has to do with improper block sizes.

Try to use the bs argument with 4k/512bytes.
dd if=/dev/zero of=/dev/sdd bs=512 count=1

Code:

lvcreate -L 1G -n test local
iostat
dd bs=4k if=/dev/zero of=/dev/mapper/local-test count=1
iostat

dm-9 0.00 0.00 0.00 544 16
dm-9 0.00 0.00 0.00 544 24

24-16 = 8
8*512 = 4k

Hopes this helps you