File: ddrescue.info,  Node: Basic concepts,  Next: Important advice,  Prev: Introduction,  Up: Top

2 Basic concepts
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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 (*note 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.