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Add function to set EC based on recoverability percentage.

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This commit is contained in:
Natalia Portillo
2026-04-11 22:11:56 +01:00
parent ea28c65aa2
commit edc65f6b1c
2 changed files with 49 additions and 0 deletions
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1
include/aaruformat/decls.h
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@@ -294,5 +294,6 @@ AARU_EXPORT TARGET_WITH_SIMD uint64_t AARU_CALL aaruf_crc64_vmull(uint64_t previ
/* Erasure coding */
AARU_EXPORT int32_t AARU_CALL aaruf_set_erasure_coding(void *context, uint8_t algorithm, uint16_t K, uint16_t M);
AARU_EXPORT int32_t AARU_CALL aaruf_set_erasure_coding_auto(void *context, double recovery_fraction);
#endif // LIBAARUFORMAT_DECLS_H

48
src/erasure.c
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@@ -164,6 +164,54 @@ AARU_EXPORT int32_t AARU_CALL aaruf_set_erasure_coding(void *context, uint8_t al
return AARUF_STATUS_OK;
}
/**
* @brief Configure erasure coding from a desired recovery fraction.
*
* Computes K and M from the requested fraction of data that should be
* recoverable. Prefers M=2 (Reed-Solomon) for better burst-corruption
* resilience. Falls back to M=1 only when the requested fraction is so
* large that K would be 1 with M=2 (i.e., recovery_fraction >= 1.0).
*
* Strategy (M=2 preferred):
* K = floor(M / recovery_fraction), clamped to [1, 253].
* If K < 2 with M=2, try M=1 with K = floor(1 / recovery_fraction).
*
* Examples:
* 0.01 (1%) → M=2, K=200 (burst tolerance: 2 blocks per stripe)
* 0.05 (5%) → M=2, K=40
* 0.10 (10%) → M=2, K=20
* 0.25 (25%) → M=2, K=8
* 0.50 (50%) → M=2, K=4
* 1.00 (100%)→ M=1, K=1 (full mirror)
*
* @param context Opaque context from aaruf_create().
* @param recovery_fraction Desired recoverable fraction (0.0 < r <= 1.0).
* @return AARUF_STATUS_OK on success, error code otherwise.
*/
AARU_EXPORT int32_t AARU_CALL aaruf_set_erasure_coding_auto(void *context, double recovery_fraction)
{
TRACE("Entering aaruf_set_erasure_coding_auto(%p, %f)", context, recovery_fraction);
if(recovery_fraction <= 0.0 || recovery_fraction > 1.0)
return AARUF_ERROR_INCORRECT_DATA_SIZE;
uint16_t M = 2;
uint16_t K = (uint16_t)(M / recovery_fraction);
/* Clamp K to valid range */
if(K < 1) K = 1;
if(K > 253) K = 253; /* K + M <= 255 */
/* If K=1 with M=2 means >=200% overhead — fall back to M=1 for a full mirror */
if(K == 1 && recovery_fraction >= 1.0)
M = 1;
uint8_t algorithm = (M == 1) ? kErasureCodingXor : kErasureCodingRsVandermonde;
TRACE("Auto EC: recovery_fraction=%f -> K=%u M=%u algorithm=%u", recovery_fraction, K, M, algorithm);
return aaruf_set_erasure_coding(context, algorithm, K, M);
}
/**
* @brief Accumulate parity for a data block that was just written to disk.
*