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Fix RAID documentation accuracy

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Andrea Mazzoleni 2026-08-24 20:20:35 +02:00
parent 03d82a71f4
commit c1a78092c4
3 changed files with 17 additions and 14 deletions
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4
raid/internal.h
View file

@ -44,7 +44,9 @@
#include <string.h>
/*
* Inverse assert.
* Inverse assert used to check internal invariants and API preconditions.
* It is intentionally based on assert() so that all checks can be disabled
* with NDEBUG. When disabled, callers must satisfy all documented preconditions.
*/
#define BUG_ON(a) assert(!(a))

16
raid/raid.c
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@ -26,12 +26,10 @@
* and divisions by 2 in GF(2^8) are very fast. It is similar to ZFS RAIDZ3,
* which uses powers of 4 instead of 2^-1.
*
* Unfortunately, this method does not extend beyond triple parity because
* no choice of power coefficients guarantees solvable equations for all
* combinations of missing disks. This is expected: a Vandermonde matrix,
* used in this method, does not ensure that all submatrices are nonsingular
* [2, Chap. 11, Problem 7], which is required for an MDS (Maximum Distance
* Separable) code [2, Chap. 11, Theorem 8].
* Unfortunately, this method does not extend reliably beyond triple parity.
* Vandermonde matrices do not guarantee that all square submatrices are
* nonsingular, which is required to recover every possible combination of
* missing disks [2, Chap. 11, Problem 7][2, Chap. 11, Theorem 8].
*
* To overcome this limitation, a Cauchy matrix [3][4] is used for parity
* computation. A Cauchy matrix ensures that all square submatrices are
@ -121,9 +119,9 @@
* data disks, one for each column. All the 377,342,351,231 square submatrices
* are nonsingular, verified also by brute-force testing.
*
* This matrix can be extended to support any number of parities by simply
* adding additional rows and removing one column for each new row to maintain
* the condition i + j < 255.
* The construction can be extended to additional parity rows while reducing
* the maximum number of data columns accordingly. In GF(2^8), the Extended
* Cauchy construction requires data + parity <= 257
* (See mktables.c for more details on how the matrix is generated.)
*
* The downside is that generic multiplications are required to compute the

11
raid/raid.h
View file

@ -20,8 +20,8 @@
*
* The polynomial used by the original RAID-6 specification and the
* Linux kernel RAID implementation, used with primitive generator g=2.
* Choosing this polynomial ensures on-disk compatibility with existing
* RAID arrays and interchange with other RAID implementations.
* For identically ordered data blocks, the generated P/Q syndrome bytes
* match the Linux RAID6 implementation.
*
* On CPUs supporting Intel GFNI (Galois Field New Instructions),
* generation and recovery are accelerated with vgf2p8affineqb.
@ -63,7 +63,7 @@
*
* Provides high performance on modern CPUs with SSSE3, AVX2, AVX-512, GFNI, or NEON support.
* On GFNI CPUs, the standard RAID field is accelerated using affine transformations.
* Ensures maximum compatibility with standard Linux RAID arrays.
* Uses Linux RAID6-compatible P/Q coefficients.
*
* This is the default mode set after calling raid_init().
*/
@ -331,7 +331,10 @@ int raid_check(int nr, int *ir, int nd, int np, size_t size, void **v);
* first one encountered is returned. This function does not check that the
* returned combination is unique.
*
* It uses a brute force method, and then the call can be expensive.
* This function is primarily intended as a reference implementation and to
* demonstrate how raid_check() can be used to locate failed blocks. It uses
* an exhaustive brute force search and is not intended for production use.
*
* The expected execution time is proportional to the binomial coefficient
* @np + @nd choose @np - 1, usually written as:
*