SCSI emulator

On 11/5/10, Alexandre Souza - Listas <pu1bzz.listas at gmail.com> wrote: As I mentioned in my recent reply to this thread, most of the time (nearly all of the time for stuff we on this list are likely to be working with these days), there is no strange formatting that's relevant. Long ago, we used to worry about what cylinder filesystems started on and optimizing things for head switching vs track stepping, but embedded SCSI (and IDE) drives hid so many of those details (and in many cases interposed a large on-board RAM cache) from the host that the art of tuning filesystems by position. The blocks themselves are nearly always formatted to 512 bytes per block (though in the earliest days, 256 bytes per block was not unknown), and the blocks can be just read out sequentially starting with 0000000 (which is something 'dd' is exceptionally good at). Another thing that changed was handling bad blocks on the disk - now, it's all hidden behind the scenes so that your OS "sees" a field of good blocks (previously, it wasn't unusual to have the OS-level driver participate in the sparing mechanism for using unallocated blocks/tracks to replace bad ones, or at least have a way to ignore bad spots - DOS floppies are a good example of that). Drives with analog cables to distinct read/write electronics are a different matter, but that's exactly what went away when the world turned to embedded-controller drives. You have a I/O-bus-like interface directly to the drive (digital data bus of 8 or 16 or more bits, handshaking, virtual I/O registers to read/write, a packet-like command structure, etc) that is from the view of the driver essentially identical to talking to an older-style controller card with detached drives. That's why 'dd' handles so many cases - you are just slurping sequential blocks from the media - you don't have to worry about head and track details, about vectoring off for bad blocks - a "perfect" disk image (until things start to go bad, then they go really bad, really fast). Hopefully this helps illuminate the arrangement some. -ethan