No, but of course for a RAID with additional parity (error correction) bits the controller needs to read and write more. So if, for example, n+2 sub-blocks per stripe = n data and 2 error correction... Then the smallest update requires read-compute-write on 1 data and 2 ecc. = 3 reads and 3 writes.
The calculation each parity block of the requires "subtracting" out the contribution of the old data value and adding in the contribution of the new data value Ref: http://igoro.com/archive/how-raid-6-dual-parity-calculation-works/ Look at it this way: The k'th parity value is Parityk= Ak*(data1) + Bk*(data2) + Ck*(data3) + ... (Ak, Bk, Ck, ... are coefficients for the computation of the k'th parity value) When updating data2 to data2x we update Parityk to Paritykx with Paritykx = Pariktyk - Bk*(data2) + Bk*(data2x) (Arithmetic done in a Galois Field chosen to make error correction practical.) From: "Uwe Falke" <[email protected]> Hi, Marc, I was clearly unaware of that function. If my understanding of parity-based redundancy is about correct, then that method would only work with RAID 5, because that is a simple XOR-based hash, but RAID 6, if used, would not allow that stripped-down RMW. Is that correct?
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