[ ... ] >> * Can Linux MD do "abbreviated" read-modify-write RAID6 >> updates like for RAID5? That is where not the whole stripe is >> read in, modified and written, but just the block to be >> updated and the parity wblocks. > No. (patches welcome). >> * When reading or writing part of RAID[456] stripe for >> example smaller than a sector, what is the minimum unit of >> transfer with Linux MD? The full stripe, the chunk containing >> the sector, or just the sector containing the bytes to be >> written or updated (and potentially the parity sectors)? I >> would expect reads to always read just the sector, but not so >> sure about writing. > 1 "PAGE" - normally 4K. In retrospect I should have expected this, the "page cache" seems to matter here :-). Recently I was thinking again about the above, please let know some more details. To make some questions unambiguous, let's imagine a setup with a disk with 512B sectors, an architecture page size of 4KiB, an MD chunk size of 16KiB, and an array with 6 drives. * Does MD consider a chunk*width "stripe" (96KiB) as made of a set of page*width "stripelets" (24KiB) *in general*? In other words, is the "fundamental" unit (if any) of MD array-wide work the "stripe" or "stripelet"? * With RAID6, are the two parities computed per "stripelet" or per "stripe" (that is, are parity blocks 4KiB or 16Kib)? Does this make a difference? * With RAID6, does MD perform RMW on a "stripe" of a "stripelet"? -- 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
