On 06/11/14 15:55, Christian Weisgerber wrote: > On 2014-06-11, Peter Fraser <[email protected]> wrote: ... >> Also for dd the block size has always been a puzzle. > > For accessing a raw device you want it to be a multiple of the > sector size of the device (512 bytes for most disks) and there is > usually no point in making it bigger than MAXPHYS (64k on OpenBSD), > i.e., the maximal size of a single I/O transfer the kernel handles; > larger reads or writes will be broken up into multiple transfers.
I've heard this a number of times...and yet my testing on hardware I've had in front of me (i.e., "your throughput may vary") has shown that bs=1M does give substantially better throughput when zeroing disks than 32k, and last time I did extensive testing in this, sizes larger than 1MB give even better throughput, though the return gets very small after around 1MB -- so I usually use 1MB so a "pkill -INFO dd" will give me an indication of the progress in easy to read terms, which I find more useful than a 1% reduction in time. I'm just reporting an observation, not explaining it. :) Nick.
