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Diffstat (limited to 'doc/rados/operations/erasure-code-jerasure.rst')
-rw-r--r-- | doc/rados/operations/erasure-code-jerasure.rst | 18 |
1 files changed, 18 insertions, 0 deletions
diff --git a/doc/rados/operations/erasure-code-jerasure.rst b/doc/rados/operations/erasure-code-jerasure.rst index 8a0207748ae..c3717750754 100644 --- a/doc/rados/operations/erasure-code-jerasure.rst +++ b/doc/rados/operations/erasure-code-jerasure.rst @@ -60,6 +60,24 @@ Where: *blaum_roth*, *liber8tion* are *RAID6* equivalents in the sense that they can only be configured with *m=2*. + .. note:: When using ``blaum_roth`` coding, the default + word size of ``w=7`` is suboptimal because ``blaum_roth`` + works best when ``w+1`` is prime. When creating a new + erasure-code profile with ``technique=blaum_roth``, + set ``w`` to a number that is one integer less than a prime + number (for example, ``6``). See `Loic Dachary's + commit f51d21b to ceph/ceph <https://github.com/ceph/ceph/commit/f51d21b53d26d4f27c950cb1ba3f989e713ab325>`_ for information about + why this default cannot be changed easily in the + source code, and see `the second bullet point on + page 29 of Plank and Greenan's "Jerasure: A Library + in C Facilitating Erasure Coding for Storage + Applications" <https://github.com/ceph/jerasure/blob/master/Manual.pdf>`_ for an unequivocal statement of the restriction that applies + to ``w`` when using Blaum-Roth coding. + (Information about the proper value of ``w`` when + using ``blaum_roth`` coding was provided to the + Ceph upstream in September of 2024 by Benjamin + Mare.) + :Type: String :Required: No. :Default: reed_sol_van |