Provided by: vdmfec_1.0-2_amd64 
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
Report bugs to Tom Holroyd <tomh@po.crl.go.jp>.
SEE ALSO
fec(3)