pyjerasure

Python Wrapper library for libjerasure

Only basic encoding/decoding methods are implemented.

Usage

In this example we have blocks of data that we would like to protect against data loss.

data = [
  b"hello",
  b"world",
  b"data-123",
  b"data-0123456789A",
]

First we import the library and setup our matrix.

There are multiple matrix types available which can be shown with Matrix.TYPES

import pyjerasure

matrix_type = "rs_r6"
k = 4  # Number of data blocks
m = 2  # Number of coding blocks. This is also the maximum number of blocks that can be lost.
w = 8  # Word Size

matrix = pyjerasure.Matrix(matrix_type, k, m, w)

Next we encode our data using the matrix. This adds redundant coding blocks to our data.

coded_data = pyjerasure.encode_from_blocks(matrix, data)
print(coded_data)

#  Outputs:
#
#  [b'hello\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00',
#   b'world\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00',
#   b'data-123\x00\x00\x00\x00\x00\x00\x00\x00',
#   b'data-0123456789A',
#   b'\x1f\n\x1e\x00\x0b\x01\x03\x013456789A',
#   b'\x0c\r\xc2\x02fY]A\x85\xbd\xb5\xad\xa5\xdd\xd52']
#

Notice how some of our data has been padded with zeroed bytes. This is necessary for encoding our data. The *_from_blocks method automatically pads data to the correct length.

Now we're going to simulate a loss of data by deleting 'm' blocks.

missing = [1, 3]
for i in missing:
  coded_data[i] = b""
print(coded_data)

#  Outputs:
#
#  [b'hello\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00',
#   b'',
#   b'data-123\x00\x00\x00\x00\x00\x00\x00\x00',
#   b'',
#   b'\x1f\n\x1e\x00\x0b\x01\x03\x013456789A',
#   b'\x0c\r\xc2\x02fY]A\x85\xbd\xb5\xad\xa5\xdd\xd52']
#

Recovering the data can be done if we know the indexes of the missing blocks.

restored = pyjerasure.decode_from_blocks(matrix, coded_data, missing)
print(restored)

#  Outputs:
#
#  [b'hello\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00',
#   b'world\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00',
#   b'data-123\x00\x00\x00\x00\x00\x00\x00\x00',
#   b'data-0123456789A',
#   b'\x1f\n\x1e\x00\x0b\x01\x03\x013456789A',
#   b'\x0c\r\xc2\x02fY]A\x85\xbd\xb5\xad\xa5\xdd\xd52']
#

Blocks can be encoded and decoded as a byte string using the *_from_bytes methods. Each block needs to be padded in respect to the longest block. The align_size method returns the appropriate size for each block. The Matrix.align_block method pads each block.

matrix_type = "cauchy"
matrix = pyjerasure.Matrix(matrix_type, k, m, w)
size = max([len(block) for block in data])
padded = [matrix.align_block(block, size) for block in data]
padded_data = b"".join(padded)
encoded = pyjerasure.encode_from_bytes(matrix, padded_data, size)
print(encoded)

#  Outputs:
#
#  b'hello\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
#    world\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
#    data-123\x00\x00\x00\x00\x00\x00\x00\x00
#    data-0123456789A
#    M\x8a\xea\x01+\xb0\xdem\x0f]\xfa\x0e\xa9\xaa\r\x15
#    2\x89\xef\x01RP\xe3\x8d\xc5\rJ\x83\xc4\x0eIW'
#

block_size = len(encoded) // (matrix.k + matrix.m)
erased = bytearray(encoded)
erased[:block_size * 2] = bytes(block_size * 2) # Erase first two blocks.
erased = bytes(erased)
print(erased)

#  Outputs:
#
#  b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
#    \x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00 
#    data-123\x00\x00\x00\x00\x00\x00\x00\x00
#    data-0123456789A
#    M\x8a\xea\x01+\xb0\xdem\x0f]\xfa\x0e\xa9\xaa\r\x15
#    2\x89\xef\x01RP\xe3\x8d\xc5\rJ\x83\xc4\x0eIW'
#

erasures = [0, 1]
restored = pyjerasure.decode_from_bytes(matrix, erased, erasures, size)
print(restored)

#  Outputs:
#
#  b'hello\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
#    world\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00
#    data-123\x00\x00\x00\x00\x00\x00\x00\x00
#    data-0123456789A
#    M\x8a\xea\x01+\xb0\xdem\x0f]\xfa\x0e\xa9\xaa\r\x15
#    2\x89\xef\x01RP\xe3\x8d\xc5\rJ\x83\xc4\x0eIW'
#

Installing

Using pip

pip install pyjerasure

From Source

Installing from source requires libjerasure-dev to be installed. In addition, cython may need to be installed as well.

pip install cython
python setup.py install build_ext

Original License

Copyright (c) 2013, James S. Plank and Kevin Greenan All rights reserved.

Jerasure - A C/C++ Library for a Variety of Reed-Solomon and RAID-6 Erasure Coding Techniques

Revision 2.0: Galois Field backend now links to GF-Complete

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

• Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

• Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

• Neither the name of the University of Tennessee nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

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