== Getting _dd_'s _skip_ and _seek_ straight once and for all

Earlier today I wanted to lightly damage a disk in a test ZFS pool in
order to make sure that some of our status monitoring code was working
right when ZFS was recovering from checksum failures.  The reason I
wanted to do *light* damage is that under normal circumstances, if you
do too much damage to a disk, ZFS declares the disk bad and ejects it
from your pool entirely; I didn't want this to happen.

So I did something like this:
.pn prewrap on

>  for i in $(seq 128 256 10240); do
>      dd if=/dev/urandom of=<disk> bs=128k count=4 skip=$i
>  done

The intent was to poke 512 KB of random data into the disk at
a number of different places, with the goal of both hopefully
overwriting space that was actually in use and not overwriting
too much of it.  This turned out to actually not do very much and
I spent some time scratching my head before [[the penny dropped
https://twitter.com/thatcks/status/633336883683172352]].

I've used _skip_ before and honestly, I wasn't thinking clearly here.
What I actually wanted to use was _seek_. The difference is this:

> ~~_skip_ skips over initial data in the *input*, while _seek_ skips
> over initial data in the *output*~~.

(Technically I think _skip_ usually silently consumes the initial input
data you asked it to skip over, although _dd_ may try to _lseek()_ on
inputs that seem to support it. _seek_ definitely must _lseek()_ and
_dd_ will error out if you ask it to _seek_ on something that doesn't
support _lseek()_, like a pipe.)

What I was really doing with my _dd_ command was throwing away
increasing amounts of data from _/dev/urandom_ and then repeatedly
writing 512 KB (of random data) over the start of the disk. This was
nowhere near what I intended and certainly didn't have the effects
on ZFS that I wanted.

I guess the way for me to remember this is 'skip initial data from the
input, seek over space in the output'. Hopefully it will stick after
this experience in [[toe stubbing ../sysadmin/SysadminAphorism]].

=== Sidebar: the other thing I initially did wrong

The test pool was full of test files, which I had created by copying
_/dev/zero_ into files. My initial _dd_ was also using _/dev/zero_
to overwrite disk blocks. It struck me that I was likely to be
mostly overwriting file data blocks full of zeroes with more zeroes,
which probably wasn't going to cause checksum failures.