Largest dd block size?

[wfamily]

Hi everyone!

I'm trying to figure out what is the largest 'dd' block size I can use to data wipe a HDD with random data?

So let's say I have a 1TB HDD and I wanna data wipe 260GB of space with random data.

I tried

dd if=/dev/urandom of=/dev/sdd count=130 bs=2G

but the error gave me

dd: invalid number '2G'

How do I go about finding the maximal dd block size allowable?

Thanks!

[Rubberman]

Trying to write a buffer of 2GB at a time is not a good idea as your kernel probably doesn't support that. It won't give you any performance benefit in any case. Writing 1M at a time is a good maximum. As it is, you don't need to specify the number of blocks to write. Finally, /dev/urandom is vulnerable to cryptographic attacks under certain conditions, especially as you are trying to use it. So, try this:

dd if=/dev/random of=/dev/sdd obs=2M

If you are trying to "shred" the drive so that data is totally unrecoverable, do a 7 pass overwrite, alternating between the use of /dev/zero and /dev/random for the input file. Recovery of any recognizable data will then be pretty much impossible with any current forensic tools.

[wfamily]

Thanks for the ideas!

a) Also, since the kernel probably doesn't support 2GB read/write at a time, is there a way to determine the maximum # of bytes it can read/write at a time?
b) And also, won't writing 1GB at a time be faster than writing 1M at a time?
c) And is there a reason you did "obs=2M" instead of "bs=2M"?
d) I know ibs = # of bytes to read at a time & obs = # of bytes to write at a time. So does this mean the command you gave would still read at the default 512 bytes at a time, but will only write until 2M have been read?

I don't quite understand where the speed-up is coming from between the 3:

1) dd if=/dev/random of=/dev/sdd obs=2M
2) dd if=/dev/urandom of=/dev/sdd bs=2G
3) dd if=/dev/urandom of=/dev/sdd

Thanks!

[Rubberman]

You can use bs=2M if you want, but I'm not sure how much data can be read from /dev/random at a time. This is one of those situations where experimenting with different parameters will help you find the "sweet spot" on your system. As for whether writing 1G vs 1M at a time is faster, I seriously doubt it. Even at 1M you will be saturating the drive hardware's capability.