Ubuntu Manpage: vdmfec - Block ECC for files

NAME

       vdmfec - Block ECC for files

SYNOPSIS

       vdmfec [-v] [-d] [-b blocksize] [-n N] [-k K] [file]
       vdm_encode [-v] [-b blocksize] [-n N] [-k K] [file]
       vdm_decode [-v] [-b blocksize] [-n N] [-k K] [file]

DESCRIPTION

       vdmfec  reads  an input stream and adds error correction blocks so that large consecutive sections of the
       output stream may be corrupted, and the data recovered.  For  example,  diskettes  typically  lose  whole
       sectors  at  once,  or related groups of sectors, or even entire tracks.  Data written to a diskette with
       this program may be recovered even with many read errors.

       The algorithm used is a Forward Error Correction (FEC)  code  based  on  Vandermonde  (VDM)  matrices  in
       GF(2^8)  due  to  Luigi  Rizzo.   Given  the  FEC parameters K and N, with N greater than K, N blocks are
       written for every K input blocks in such a way that any K blocks are sufficient to reconstruct the  data.
       That is, up to N - K blocks out of every group of N blocks may be lost without loss of data.

       The  amount  of overhead in the output stream is easily adjustable by varying K.  N and blocksize control
       the total amount of data written.  Depending on the types of errors you expect, different settings may be
       more or less useful.  For example, you may not expect to have two or three bad sectors on every track (if
       you do it's time to replace the diskette!), but you might expect  three  bad  sectors  on  two  or  three
       contiguous tracks (diskette errors tend to cluster).

OPTIONS

-v Print informative messages about the encoding or decoding process to stderr, including the
expansion factor. Use -vv to get information about each block being read or written.

-d Decode the input rather than encoding it. Using this option is equivalent to invoking the program
as vdm_decode.

-b blocksize
Set the FEC block size to blocksize bytes. The blocksize may have a 'k' or 'K' appended, in which
case the block size is set to blocksize * 1024 bytes. This should usually be a multiple of the
output media block or sector size (e.g., 512, 1024, 18K, etc.), and must be at least 26 bytes in
version 1. Default 1024.

-n N -k K
Set the FEC N and K parameters. N must be greater than 2 and less than or equal to 256. K must
be greater than 0 and less than N. Default N=18, K=14.

Note that the N, K, and blocksize parameters are NOT written to the output! You must specify the same
parameters when you run the decoder. (Actually, the decoder is capable of explicitly detecting an
invalid K value, but incorrect blocksize or N values will result in bad blocks and decode failure.)

The decoder is capable of reading from non-seekable media such as pipes, however, buffer underruns are
not detected and will result in failure. Also, when reading from a pipe the entire file must be read.
Reading from a seek-able stream can be faster because only K good blocks out of N need to be read.

The encoder stores chunk and block ids which are used to detect out of order blocks, but those errors
(which can sometimes happen due to filesystem corruption) are not corrected. The only type of error that
can be corrected is in place corruption of data. In particular, missing blocks (as from lost packets)
result in decode failure, even when K good blocks are available. This program is primarily intended to
recover data lost due to read errors on fixed media such as diskettes; several other programs and
libraries are available that use FEC to handle the other (typically network packet) errors.

EXAMPLES

       Consider  a  diskette  with  80  tracks,  2  heads,  and 18 512-byte sectors per track (a standard 1.44MB
       diskette).

              bzip2 < file | vdmfec > /dev/fd0

       uses the default parameters of -b1k -n18 -k14, stores up to ~1.1MB of compressed input data, written  one
       (double-sided) track (18K) at a time, and allows 4 bad clusters (8 sectors) per track.

              vdmfec -d /dev/fd0 | bunzip2 > file

       recovers the original file.

              vdm_encode -b18k -n80 -k70 file.img > /dev/fd0

       writes  the  entire  diskette,  stores  up to ~1.23MB of data, and allows multiple errors on 10 different
       double-sided tracks.

              vdm_decode -b18k -n80 -k70 /dev/fd0 > file.img

       recovers the data and reads no less than 70 tracks.  Note that vdm_decode exits with a non-zero status on
       failure, so that constructions such as

              vdm_decode /dev/fd0 > moo && tar -xvf moo

       are possible.

              dd if=/dev/tape ibs=1024 conv=noerror,sync | \
              vdm_decode > file

       This might be necessary to recover data from some non-seekable media.  You can of course simply write the
       encoded output to a normal file, as in

              tar -cf - dir | gzip | vdm_encode -v > dir.tgz.vdm

REPORTING BUGS