liberasurecode

liberasurecode is an Erasure Code API library written in C with pluggable Erasure Code backends.

---

Highlights

• Unified Erasure Coding interface for common storage workloads.

• Pluggable Erasure Code backends - As of v0.9.10, liberasurecode supports 'Jerasure' (Reed-Solomon, Cauchy), 'ISA-L' (Intel Storage Acceleration Library), 'Flat XOR HD' backends. A template 'NULL' backend is implemented to help future backend writers.

• True 'plugin' architecture - liberasurecode uses Dynamically Loaded (DL) libraries to realize a true 'plugin' architecture. This also allows one to build liberasurecode indepdendent of the Erasure Code backend libraries.

• Cross-platform - liberasurecode is known to work on Linux (Fedora/Debian flavors), Solaris, BSD and Darwin/Mac OS X.

• Community support - Developed alongside Erasure Code authority Kevin Greenan, liberasurecode is an actively maintained open-source project with growing community involvement (Openstack Swift, Ceph, PyECLib).

---

License

liberasurecode is distributed under the terms of the BSD license.

---

Active Users

• PyECLib - Python EC library: https://pypi.python.org/pypi/PyECLib
• Openstack Swift Object Store - https://wiki.openstack.org/wiki/Swift

---

liberasurecode API Definition

---

Backend Support

typedef enum {
    EC_BACKEND_NULL                 = 0, /* "null" */
    EC_BACKEND_JERASURE_RS_VAND     = 1, /* "jerasure_rs_vand" */
    EC_BACKEND_JERASURE_RS_CAUCHY   = 2, /* "jerasure_rs_cauchy" */
    EC_BACKEND_FLAT_XOR_HD          = 3, /* "flat_xor_hd */
    EC_BACKENDS_MAX,
} ec_backend_id_t;

---

User Arguments

/**
 * Common and backend-specific args
 * to be passed to liberasurecode_instance_create()
 */
struct ec_args {
    int k;                  /* number of data fragments */
    int m;                  /* number of parity fragments */
    int w;                  /* word size, in bits (optional) */
    int hd;                 /* hamming distance (=m for Reed-Solomon) */

    union {
        struct {
            uint64_t arg1;  /* sample arg */
        } null_args;        /* args specific to the null codes */
        struct {
            uint64_t x, y;  /* reserved for future expansion */
            uint64_t z, a;  /* reserved for future expansion */
        } reserved;
    } priv_args1;

    void *priv_args2;       /** flexible placeholder for
                              * future backend args */
    ec_checksum_type_t ct;  /* fragment checksum type */
};

---

User-facing API Functions

/* liberasurecode frontend API functions */

/**
 * Create a liberasurecode instance and return a descriptor 
 * for use with EC operations (encode, decode, reconstruct)
 *
 * @param id - one of the supported backends as
 *        defined by ec_backend_id_t
 * @param ec_args - arguments to the EC backend
 *        arguments common to all backends
 *          k - number of data fragments
 *          m - number of parity fragments
 *          w - word size, in bits
 *          hd - hamming distance (=m for Reed-Solomon)
 *          ct - fragment checksum type (stored with the fragment metadata)
 *        backend-specific arguments
 *          null_args - arguments for the null backend
 *          flat_xor_hd, jerasure do not require any special args
 *      
 * @return liberasurecode instance descriptor (int > 0)
 */
int liberasurecode_instance_create(const ec_backend_id_t id,
                                   struct ec_args *args);

/**
 * Close a liberasurecode instance
 *
 * @param desc - liberasurecode descriptor to close
 *
 * @return 0 on success, otherwise non-zero error code
 */
int liberasurecode_instance_destroy(int desc);


/**
 * Erasure encode a data buffer
 *
 * @param desc - liberasurecode descriptor/handle
 *        from liberasurecode_instance_create()
 * @param orig_data - data to encode
 * @param orig_data_size - length of data to encode
 * @param encoded_data - pointer to _output_ array (char **) of k data
 *        fragments (char *), allocated by the callee
 * @param encoded_parity - pointer to _output_ array (char **) of m parity
 *        fragments (char *), allocated by the callee
 * @param fragment_len - pointer to _output_ length of each fragment, assuming
 *        all fragments are the same length
 *
 * @return 0 on success, -error code otherwise
 */
int liberasurecode_encode(int desc,
        const char *orig_data, uint64_t orig_data_size, /* input */
        char ***encoded_data, char ***encoded_parity,   /* output */
        uint64_t *fragment_len);                        /* output */

/**
 * Cleanup structures allocated by librasurecode_encode
 *
 * The caller has no context, so cannot safely free memory
 * allocated by liberasurecode, so it must pass the
 * deallocation responsibility back to liberasurecode.
 *
 * @param desc - liberasurecode descriptor/handle
 *        from liberasurecode_instance_create()
 * @param encoded_data - (char **) array of k data
 *        fragments (char *), allocated by liberasurecode_encode
 * @param encoded_parity - (char **) array of m parity
 *        fragments (char *), allocated by liberasurecode_encode
 *
 * @return 0 in success; -error otherwise
 */
int liberasurecode_encode_cleanup(int desc, char **encoded_data,
        char **encoded_parity);

/**
 * Reconstruct original data from a set of k encoded fragments
 *
 * @param desc - liberasurecode descriptor/handle
 *        from liberasurecode_instance_create()
 * @param fragments - erasure encoded fragments (> = k)
 * @param num_fragments - number of fragments being passed in
 * @param fragment_len - length of each fragment (assume they are the same)
 * @param out_data - _output_ pointer to decoded data
 * @param out_data_len - _output_ length of decoded output
 *          (both output data pointers are allocated by liberasurecode,
 *           caller invokes liberasurecode_decode_clean() after it has
 *           read decoded data in 'out_data')
 *
 * @return 0 on success, -error code otherwise
 */
int liberasurecode_decode(int desc,
        char **available_fragments,                     /* input */
        int num_fragments, uint64_t fragment_len,       /* input */
        char **out_data, uint64_t *out_data_len);       /* output */

/**
 * Cleanup structures allocated by librasurecode_decode
 *
 * The caller has no context, so cannot safely free memory
 * allocated by liberasurecode, so it must pass the
 * deallocation responsibility back to liberasurecode.
 *
 * @param desc - liberasurecode descriptor/handle
 *        from liberasurecode_instance_create()
 * @param data - (char *) buffer of data decoded by librasurecode_decode
 *
 * @return 0 on success; -error otherwise
 */
int liberasurecode_decode_cleanup(int desc, char *data);

/**
 * Reconstruct a missing fragment from a subset of available fragments
 *
 * @param desc - liberasurecode descriptor/handle 
 *        from liberasurecode_instance_create()
 * @param available_fragments - erasure encoded fragments
 * @param num_fragments - number of fragments being passed in
 * @param fragment_len - size in bytes of the fragments
 * @param destination_idx - missing idx to reconstruct
 * @param out_fragment - output of reconstruct
 *
 * @return 0 on success, -error code otherwise
 */
int liberasurecode_reconstruct_fragment(int desc,
        char **available_fragments,                     /* input */
        int num_fragments, uint64_t fragment_len,       /* input */
        int destination_idx,                            /* input */
        char* out_fragment);                            /* output */

/**
 * Return a list of lists with valid rebuild indexes given
 * a list of missing indexes.
 *
 * @desc: liberasurecode instance descriptor (obtained with
 *        liberasurecode_instance_create)
 * @fragments_to_reconstruct list of indexes to reconstruct
 * @fragments_to_exclude list of indexes to exclude from 
 *        reconstruction equation
 * @fragments_needed list of fragments needed to reconstruct
 *        fragments in fragments_to_reconstruct
 *
 * @return 0 on success, non-zero on error
 */
int liberasurecode_fragments_needed(int desc,
        int *fragments_to_reconstruct, 
        int *fragments_to_exclude,
        int *fragments_needed);

Erasure Code Fragment Checksum Types Supported

/* Checksum types supported for fragment metadata stored in each fragment */
typedef enum {
    CHKSUM_NONE                     = 0, /* "none" (default) */
    CHKSUM_CRC32                    = 1, /* "crc32" */
    CHKSUM_MD5                      = 2, /* "md5" */
    CHKSUM_TYPES_MAX,
} ec_checksum_type_t;

Erasure Code Fragment Checksum API

struct
fragment_metadata
{
    uint32_t    idx;                /* 4 */
    uint32_t    size;               /* 4 */
    uint64_t    orig_data_size;     /* 8 */
    uint8_t     chksum_type;        /* 1 */
    uint32_t    chksum[LIBERASURECODE_MAX_CHECKSUM_LEN]; /* 32 */
    uint8_t     chksum_mismatch;    /* 1 */
} fragment_metadata_t;


/**
 * Get opaque metadata for a fragment.  The metadata is opaque to the
 * client, but meaningful to the underlying library.  It is used to verify
 * stripes in verify_stripe_metadata().
 *
 * @param desc - liberasurecode descriptor/handle
 *        from liberasurecode_instance_create()
 * @param fragment - fragment data pointer
 * @param fragment_metadata - pointer to allocated buffer of size at least
 *        sizeof(struct fragment_metadata) to hold fragment metadata struct
 *
 * @return 0 on success, non-zero on error
 */
int liberasurecode_get_fragment_metadata(int desc,
        char *fragment, fragment_metadata_t *fragment_metadata);

/**
 * Verify a subset of fragments generated by encode()
 *
 * @param desc - liberasurecode descriptor/handle
 *        from liberasurecode_instance_create()
 * @param fragments - fragments part of the EC stripe to verify
 * @param num_fragments - number of fragments part of the EC stripe
 *
 * @return 1 if stripe checksum verification is successful, 0 otherwise
 */
int liberasurecode_verify_stripe_metadata(int desc,
        char **fragments, int num_fragments);

/**
 * This computes the aligned size of a buffer passed into 
 * the encode function.  The encode function must pad fragments
 * to be algined with the word size (w) and the last fragment also
 * needs to be aligned.  This computes the sum of the algined fragment
 * sizes for a given buffer to encode.
 *
 * @param desc - liberasurecode descriptor/handle
 *        from liberasurecode_instance_create()
 * @param data_len - original data length in bytes
 *
 * @return aligned length, or -error code on error
 */
int liberasurecode_get_aligned_data_size(int desc, uint64_t data_len);
 
/**
 * This will return the minimum encode size, which is the minimum
 * buffer size that can be encoded.
 * 
 * @param desc - liberasurecode descriptor/handle
 *        from liberasurecode_instance_create()
 *
 * @return minimum data length length, or -error code on error
 */
int liberasurecode_get_minimum_encode_size(int desc);

---

Error Codes

/* Error codes */
typedef enum {
    EBACKENDNOTSUPP  = 200, /* EC Backend not supported by the library */
    EECMETHODNOTIMPL = 201, /* EC Backend method not implemented */
    EBACKENDINITERR  = 202, /* EC Backend could not be initialized */
    EBACKENDINUSE    = 203, /* EC Backend in use (locked) */
} LIBERASURECODE_ERROR_CODES;

---

Build and Install

To build the liberasurecode repository, perform the following from the top-level directory:

 $ ./autogen.sh
 $ ./configure
 $ make
 $ make test
 $ sudo make install

---

Code organization

 |-- include
 |   +-- erasurecode
 |   |   +-- erasurecode.h            --> liberasurecode frontend API header
 |   |   +-- erasurecode_backend.h    --> liberasurecode backend API header
 |   +-- xor_codes                    --> headers for the built-in XOR codes
 |
 |-- src
 |   |-- erasurecode.c                --> liberasurecode API implementation
 |   |                                    (frontend + backend)
 |   |-- backends
 |   |   +-- null
 |   |       +--- null.c              --> 'null' erasure code backend
 |   |   +-- xor
 |   |       +--- flat_xor_hd.c       --> 'flat_xor_hd' erasure code backend
 |   |   +-- jerasure                 
 |   |       +-- jerasure_rs_cauchy.c --> 'jerasure_rs_vand' erasure code backend
 |   |       +-- jerasure_rs_vand.c   --> 'jerasure_rs_cauchy' erasure code backend
 |   |
 |   |-- builtin
 |   |   +-- xor_codes                --> XOR HD code backend, built-in erasure
 |   |       |                            code implementation (shared library)
 |   |       +-- xor_code.c
 |   |       +-- xor_hd_code.c
 |   |
 |   +-- utils
 |       +-- chksum                   --> fragment checksum utils for erasure
 |           +-- alg_sig.c                coded fragments
 |           +-- crc32.c
 |
 |-- doc                              --> API Documentation
 |   +-- Doxyfile
 |   +-- html
 |
 |--- test                            --> Test routines
 |    +-- builtin
 |    |   +-- xor_codes
 |    +-- liberasurecode_test.c
 |    +-- utils
 |
 |-- autogen.sh
 |-- configure.ac
 |-- Makefile.am
 |-- README
 |-- NEWS
 |-- COPYING
 |-- AUTHORS
 |-- INSTALL
 +-- ChangeLog

---