## 2 Basic concepts ¶

Block

  Any amount of data. A block is described by its starting position and its size. The starting position (or beginning position) is the lowest position in the block. The end of the block is its starting position plus its size.

Cluster

  Group of consecutive sectors read or written in one go.

Device

  Piece of hardware containing data. Hard disc drives, cdrom drives, USB pendrives, are devices. `/dev/hda`, `/dev/sdb`, are device names (file names associated to devices).

File

  Files are named units of data which are stored by the operating system for you to retrieve later by name. Devices and partitions are accessed by means of their associated file names.

Partition

  Every part in which a device is divided. A partition normally contains a file system. `/dev/hda1`, `/dev/sdb3`, are partition names (file names associated to partitions).

Recoverable formats

  As ddrescue uses standard library functions to read data from the device being rescued, only mountable device formats can be rescued with ddrescue. CD-ROMs and DVDs can be rescued, "compact disc digital audio" CDs can’t, "video CDs"[1] maybe.
  [1] http://en.wikipedia.org/wiki/Video_CD

Rescue domain

  Block or set of blocks to be acted upon (rescued, listed, etc). You may define it with the options `--input-position`, `--size`, and `--domain-mapfile`. The rescue domain defaults to the whole input file or mapfile. If ddrescue can’t determine the size of the input file, the rescue domain defaults to the maximum size of a block (at least 2^63 - 1 bytes, or 8 EiB minus 1 byte).

  Ddrescue never tries to read any data outside the rescue domain except when unaligned direct disc access is requested (see Direct disc access). If it does, please, report it as a bug.

  The data shown by ddrescue (amount of data rescued, number of bad areas, etc) may vary or even become zero if you limit the rescue domain. Don’t worry, they have not disappeared; they are simply out of the rescue domain specified.

Sector

  Hardware block. Smallest accessible amount of data on a block device.