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PyECLib 0.9.5

This library provides a simple Python interface for implementing erasure codes. To obtain the best possible performance, the underlying erasure code algorithms are written in C.

Latest Version: 1.0.7

This is v0.9 of PyECLib. This library provides a simple Python interface for implementing erasure codes and is known to work with Python v2.6, 2.7 and 3.x.

To obtain the best possible performance, the underlying erasure code algorithms are written in C. Please let us know if you have any other issues building or installing (email: kmgreen2@gmail.com or tusharsg@gmail.com).

This library makes use of Jerasure for Reed-Solomon and provides its own flat XOR-based erasure code encoder and decoder. Currently, it implements a specific class of HD Combination Codes (see “Flat XOR-based erasure codes in storage systems: Constructions, efficient recovery, and tradeoffs” in IEEE MSST 2010). These codes are well-suited to archival use-cases, have a simple construction and require a minimum number of participating disks during single-disk reconstruction (think XOR-based LRC code).

Examples of using this library are provided in “tools” directory:

Command-line encoder:

tools/pyeclib_encode.py

Command-line decoder:

tools/pyeclib_decode.py

Utility to determine what is needed to reconstruct missing fragments:

tools/pyeclib_fragments_needed.py

PyEClib initialization:

ec_driver = ECDriver(k=<num_encoded_data_fragments>,
                     m=<num_encoded_parity_fragments>,
                     ec_type=<ec_scheme>))

Supported ec_type values:

  • jerasure_rs_vand => Vandermonde Reed-Solomon encoding (Jerasure variant)
  • jerasure_rs_cauchy_orig => Cauchy Reed-Solomon encoding (Jerasure variant)
  • flat_xor_3, flat_xor_4 => Flat-XOR based HD combination codes

A configuration utility is provided to help compare available EC schemes in terms of performance and redundancy:: tools/pyeclib_conf_tool.py

The Python API supports the following functions:

  • EC Encode

    Encode N bytes of a data object into k (data) + m (parity) fragments:

    def encode(self, data_bytes)
    
    input:   data_bytes - input data object (bytes)
    returns: list of fragments (bytes)
    
  • EC Decode

    Decode between k and k+m fragments into original object:

    def decode(self, fragment_payloads)
    
    input:   list of fragment_payloads (bytes)
    returns: decoded object (bytes)
    

Note: bytes is a synonym to str in Python 2.6, 2.7. In Python 3.x, bytes and str types are non-interchangeable and care needs to be taken when handling input to and output from the encode() and decode() routines.

  • EC Reconstruct

    Reconstruct “missing_fragment_indexes” using “available_fragment_payloads”:

    def reconstruct(self, available_fragment_payloads, missing_fragment_indexes)
    
  • Fragments needed for EC Reconstruct

    Return the indexes of fragments needed to reconstruct “missing_fragment_indexes”:

    def fragments_needed(self, missing_fragment_indexes)
    
  • Get EC Metadata

    Return an opaque buffer known by the underlying library:

    def get_metadata(self, fragment)
    
  • Verify EC Stripe Consistency

    Use opaque buffers from get_metadata() to verify a the consistency of a stripe:

    def verify_stripe_metadata(self, fragment_metadata_list)
    
  • Get EC Segment Info

    Return a dict with the keys - segment_size, last_segment_size, fragment_size, last_fragment_size and num_segments:

    def get_segment_info(self, data_len, segment_size)
    

Quick Start:

Standard stuff to install:

``Python 2.6``, ``2.7`` or ``3.x`` (including development packages) and ``argparse``.

This package includes c_eclib, which contains a few C libraries used by PyECLib. These are built when running the PyECLib install (see below).

Install PyECLib:

$ sudo python setup.py install

Run test suite included:

$ (cd test && \
   python run_tests.py && \
   ./ec_pyeclib_file_test.sh)

If all of this works, then you should be good to go. If not, send us an email!

If the test suite fails because it cannot find any of the shared libraries, then you probably need to add /usr/local/lib to the path searched when loading libraries. The best way to do this (on Linux) is to add ‘/usr/local/lib’ to:

/etc/ld.so.conf

and then run:

$ ldconfig

– 0.9

 
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