-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 10/26/2011 11:23 PM, NeilBrown wrote: > On Wed, 26 Oct 2011 16:01:19 -0500 Chris Pearson > <kermit4@xxxxxxxxx> wrote: > >> In 2.6.39.1, any writes to a raid-6 array cause all disks to be >> accessed. Though I don't understand the math behind raid-6, I >> have tested on LSI cards that it is possible to only access 3 >> disks. > > You are correct. md/raid6 doesn't do the required maths. > > i.e. it always adds all data together to calculate the parity. It > never subtracts old data from the parity, then add new data. > > This was a decision made by the original implementer (hpa) and > no-one has offered code to change it. > > (yes, I review and accept patches :-) > This was based on benchmarks at the time that indicated that performance suffered more than it helped. However, since then CPUs have gotten much faster whereas disks haven't. There was also the issue of getting something working reliably first. Getting a set of hardware acceleration routines for arbitrary GF multiplies (as is possible with SSSE3) might change that tradeoff dramatically. -hpa -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iQIcBAEBAgAGBQJOqIn1AAoJEL2gYIVJO6zkKX0QAKCNUaxXnA22+e/Xas2G1M2z 5EXKvMxJqFeDojGkQ5H0cFW+CoU5h93EY3WaVGWz9IW9fiWnMGaIoPszYcKzw3Hr OkCyzytvLpeJK/sgPX3+/o2K8ZbNFWIJopdUnZivhWvPqyWFDwvquIv1Pgyj0RQc 74nPs03m4EV1zMrCgv34/JpQatiabBBSvMXzNs1kWLbQfUHr2SLMMGML02utakMb w74FdBfLaZpWlXC+Lu3G6i97+Xv8LZ9+4Z5Iqj2jVf3JjriBLOT02nChuauJ1c5y DL6vLmPedxJ3GTKUNb4fcwLOwDW1a1TqQl4QU+kytL25Ico1uXvXgjwgmO/JssfV z26+NEPOeMfth7+f6tIIOKxnIYTrCvs3p5L66TJ3h7LaHg8M3f6wtq6nmXdavc5P rwTdYKe5WlO3e8vooWExts+yWcjMufQA3gopkrLJF3gPFVYZaAlEeIIudxU11ofa b+akxIvUSze7Dqvt6GmggIP+AaAwBGNSMMEYx7XyHJqFjm6rCaUjeo1eUmDUpVbX o9rSQT3x8lgOs2yh9jNuBZtD1unXEbhXCK+nhB+UKMcKE43rxP+WztJNVG4Dkn6k pjnO3MnivlrU3WjUlZ6tEzzoxsKbYr2oSzWBM8dKGFKg9eMr+v81sSfF9HETh2ia lBcAujjR/lgnSpdKk93U =GHON -----END PGP SIGNATURE----- -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
