"The default block size is 512 bytes, but we can specify anything we want using the bs= flag. We can copy 1 byte at a time, or we can copy 1GB at a time. Any value will work, but some values will give you better performance than others. Most disks read a minimum of 512 bytes at a time and can easily read more at the same time. Using a value that is too small is wasteful because the disk will need to be frequently read, and time will be wasted in the copying process. If you choose a value that is too large, you will waste time filling up the buffer in dd before the copy is performed. I have found that values in the 2KB to 8KB range work well."

/nix | Oct 11, 2009