For a conceptually simple program, dd has a number of dark corners. One of them (at least for me) is how it deals with input and output block sizes, and how the various blocking arguments change things around.

• ibs= sets the input block size, the size of the read() s that dd will make. Since you can get partial reads in various situations, this is really the maximum size that dd will ever read at once.
• obs= sets the output block size and makes dd 'reblock' output; dd will accumulate input until it can write a full sized output block (except at EOF, where it may write a final partial block).
• bs= sets the (maximum) IO block size for both reads and writes, but it turns off reblocking ; if dd gets a partial read, it will immediately write that partial block.

Because of the reblocking or lack thereof, ' ibs=N obs=N ' is subtly different from ' bs=N '. The former will accumulate multiple partial reads together in order to write N bytes, while the latter won't.

(On top of this is the ' conv=sync ' option, which pads partial reads.)

So if you're reading from a network or a pipe but want to write in large efficient blocks, you want to use obs , not bs (and you probably want to use ibs too, because otherwise you'll be doing a lot of 512 byte reads, which are kind of inefficient).