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:
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.
skip before and honestly, I wasn't thinking clearly here.
What I actually wanted to use was
seek. The difference is this:
skipskips over initial data in the input, while
seekskips 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
inputs that seem to support it.
seek definitely must
dd will error out if you ask it to
seek on something that doesn't
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.
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
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.