LRC-Erasure-Code

LRC(Local Reconstruction Codes) Erasure Code based on Reed-Solomon with Vandermonde matrix.

Table of Contents generated with DocToc

• Status
• Description
  • LRC parameters and the differences from original Erasure Code
• Synopsis
• Install
• API
  • lrc_init_n
  • lrc_destroy
  • lrc_encode
  • lrc_decode
  • lrc_get_source
  • lrc_buf_init
  • lrc_buf_destroy
• Analysis
  • Reliability
  • IO bandwidth
• TODO
• Author
• Copyright and License

Status

This library is considered production ready.

And it is the core EC implementation in open.sinastorage.com, which has been protecting dozens of PB user data.

Description

LRC(Local Reconstruction Codes) Erasure Code supplies almost the same functionality and reliability as original Erasure Code does. And at the same time it reduces reconstruction IO consumption by 50% or more.

Erasure Code Algorithm makes it possible to achieve as high reliability(11 9s) as 3-copy replication provides, with highly reduced storage overhead(130% against 300%).

But one of the problems with Erasure Code is the high IO consumption during data reconstruction. Normally to reconstruct 1 chunk it is required to read n chunks.

LRC is a trade-off between storage cost and IO cost.

With several additional local Coding chunks calculated from subsets of Data chunks, average IO consumption for reconstruction would be reduced to 1 / number_of_local_sets(normally 10% ~ 50%), at the cost of only about 10%(depends on LRC policy) more space used.

LRC parameters and the differences from original Erasure Code

For a collection there are 5 data chunks in it. To create LRC Erasure Code with:

• 2 local EC codes;
• 2 global EC codes;

LRC should be initialized with:

lrc_init_n(lrc, 2, (uint8_t[]){3, 2}, 4)

Here in this example, the first local EC code will be created from the 1st 3 data chunks, and the 2nd local EC code will be created from the last 2 data chunks.

4 is the total number of codes, which includes:

• 2 of them are local EC codes, for data[0, 1, 2] and data[3, 4] respectively.
• 2 additional global EC codes.

The encoding matrix for this LRC parameter is:

1  1  1  0  0
0  0  0  1  1
1  2  4  8 16
1  3  9 27 81

LRC-EC (2,2)+4 is identical to original EC 5+3, except that it splits the first row into 2 rows(which makes it possible to use less data/code chunks to reconstruct one). The original EC 5+3 encoding matrix is:

1  1  1  1  1
1  2  4  8 16
1  3  9 27 81

If you prefer to use original EC 5+3 like above, lrc can be initialized with:

lrc_init_n(lrc, 1, (uint8_t[]){5}, 3)

Synopsis

#include "lrc.h"

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv) {

    int        size = 16;
    lrc_t     *lrc  = &(lrc_t) {0};
    lrc_buf_t *buf  = &(lrc_buf_t) {0};

    if (lrc_init_n(lrc, 2, (uint8_t[]) {2, 2}, 3) != 0) {
        exit(-1);
    }

    if (lrc_buf_init(buf, lrc, size) != 0) {
        exit(-1);
    }

    strcpy(buf->data[0], "hello");
    strcpy(buf->data[1], "world");
    strcpy(buf->data[2], "lrc");
    strcpy(buf->data[3], "ec");

    if (lrc_encode(lrc, buf) != 0) {
        exit(-1);
    }

    strcpy(buf->data[0], "*");

    printf("damaged: %s %s %s %s\n", buf->data[0], buf->data[1], buf->data[2], buf->data[3]);

    int8_t erased[2 + 2 + 3] = {
        1, 0,
        0, 0,
        0, 0, 0};

    if (lrc_decode(lrc, buf, erased) != 0) {
        exit(-1);
    }

    printf("reconstructed: %s %s %s %s\n", buf->data[0], buf->data[1], buf->data[2], buf->data[3]);

    lrc_destroy(lrc);
    lrc_buf_destroy(buf);

    return 0;
}

Install

./configure
make
sudo make install

# run a test
cd test
gcc example.c -o example -llrc
./example

API

lrc_init_n

int lrc_init_n(lrc_t *lrc, int n_local, uint8_t *local_arr, int m)

Initializes LRC descriptor lrc.

Parameters:

• lrc Pointer to a struct lrc_t. a lrc_t describes the parameters LRC to generate codes.

• n_local Specify the number of local EC to create.

• local_arr An array of length n_local of number of data chunks in each local EC.

• m Specifies the total number of codes. It must be equal or greater than n_local. Thus there are n_local local EC codes and m - n_local + 1 global EC codes. Because the first global EC code can be calculated by local-code-1 ^ local-code-2 ^ ...

Returns:

• 0 If Success.

• LRC_INIT_TWICE If lrc is already initialized.

• LRC_INVALID_M If m is less than n_local.

• LRC_OUT_OF_MEMORY If any malloc() fails during initializing.

lrc_destroy

void lrc_destroy(lrc_t *lrc);

Free memory allocated by lrc_init_n(). It does not free *lrc itself.

lrc_encode

int lrc_encode(lrc_t *lrc, lrc_buf_t *lrc_buf);

Generate m(from lrc_init_n()) code chunks from all k data chunks. k = sum(local_arr). lrc_buf_t is the container of all data chunks and code chunks. It must be initialized with lrc_buf_init() before use.

After lrc_encode(), your program should save lrc_buf->data[0..k-1] and lrc_buf->code[0..m-1] on persistent storage for later reconstruction.

Returns:

• 0 If Success.

• LRC_OUT_OF_MEMORY If any malloc() fails during initializing.

lrc_decode

int lrc_decode(lrc_t *lrc, lrc_buf_t *lrc_buf, int8_t *erased);

Reconstruct lost data and code chunks from existing data and code.

If too many data or code are lost, reconstruction

Parameters:

• lrc_buf Specifies data/code buffer for reconstruction and the buffer to store reconstructed data/code.

• erased Specifies which data / code are missing that needs to reconstruct. It is an array of length k + m. Array element erased[i] value 1 means the data(i<k) or code(i<=k<m) is missing, 0 means data/code presents in lrc_buf->data[i] or lrc_buf->code[i-k].

Returns:

• 0 If Success.

• LRC_OUT_OF_MEMORY If any malloc() fails during decoding.

• LRC_UNRECOVERABLE If there is not enough data / code to reconstruct the missing ones.

lrc_get_source

int lrc_get_source(lrc_t *lrc, int8_t *erased, int8_t *source);

If LRC is used(n_local passed to lrc_init_n() is greater than 1), not always all data/code are required. This function calculate which data/code is required.

For example if LRC parameter is 2, 2, 3, and erased = {1, 0, 0, 0, 0, 0, 0} which means only 0-th data is missing, source will be filled in with: {0, 1, 0, 0, 1, 0, 0} which means only data[1], code[0] are required to reconstruct the missing data[0].

Parameters:

• erased Specifies missing data/code. There must be at least k+m 0/1 elements in erased.

• source Specifies where to store the indexes of source data/code for reconstruction. There must be at least k+m available bytes in source.

Returns:

• 0 If Success.

• LRC_UNRECOVERABLE If there is not enough data / code to reconstruct the missing ones.

lrc_buf_init

int lrc_buf_init(lrc_buf_t *lrc_buf, lrc_t *lrc, int64_t chunk_size);

Allocate memory that will be used during reconstruction, which includes: k+m byte arrays and a matrix for reconstruction.

Parameters:

• lrc Specifies LRC parameters. It must have been initialized by lrc_init_n() first.

• chunk_size Specifies the size for each of k+m data/code buffers. Internally, actual memory allocated is 16 byte aligned in order to utilize SMID instructions.

Returns:

• 0 If Success.

• LRC_INIT_TWICE If lrc is already initialized.

• LRC_OUT_OF_MEMORY If any malloc() fails during initializing.

lrc_buf_destroy

void lrc_buf_destroy(lrc_buf_t *lrc_buf);

Free memory allocated by lrc_buf_init(). It does not free lrc_buf.

This is a specialized Erasure Code implementation for storage service. What matrix to choose does not matter. Because usually most CPU cycles are spent on matrix multiplication to decode lost data, but not on finding reversed matrix.

In this implementation Vandermonde matrix is used.

Analysis

Reliability

• If k(number of data chunks) is not very large, reliability of Erasure Code(LRC-EC or EC) with m code is similar with n-copy replication with m+1 copies.

• LRC-EC can always reconstruct m - n_local + 1 data loss. In a (6,6)+4 LRC-EC, 3 data loss is always reconstructible.

• LRC-EC with m codes can not always reconstruct m data loss. In a (6,6)+4 LRC, there are 1820 different combinations but only 1568 of them can be reconstructed(87%).

IO bandwidth

In calculation, each TB of storage requires k * 0.13G IO throughput(both for network and disk drive) each day to reconstruct lost data. Where k is the number of members in a Erasure Code group.

TODO

• Another local code that covers all global codes.

Author

Zhang Yanpo (张炎泼) drdr.xp@gmail.com

Copyright and License

The MIT License (MIT)

Copyright (c) 2015 Zhang Yanpo (张炎泼) drdr.xp@gmail.com