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**To**:**tech-userlevel%netbsd.org@localhost****Subject**:**Re: swap-on-raidframe vs raidctl -P****From**:**Greg Oster <oster%cs.usask.ca@localhost>**- Date: Tue, 22 Jan 2008 11:18:21 -0600

der Mouse writes: > >> One would be a way to configure raidframe for uses (like swap) that > >> don't care about data preservation when the partition is not in use; > >> parity rewrite at boot would be dummied out for such partitions. > > Consider a RAID 5 set, used for swap, where there is a stripe with > > bad parity. Let the blocks of that stripe be A, B, and P. A "small > > write" to A will read P, compute P', and write out A' and P'. > > How can it compute P' without having B on hand? I must be missing > something. Sorry.. in the description above I neglected to mention that A will have to be read as well.... In the normal case, P is equal to A XOR B. With a change from A to A', P' is computed as P' = P XOR A XOR A'. After this, P' is equivalent to A' XOR B, and any of P', A', or B can be reconstructed via an XOR of all the others. The same arguments can be made for a stripe of blocks A, B, C, D, and P. P equals A XOR B XOR C XOR D. When changing D to D', P' = P XOR D XOR D'. After this, P' is equal to A XOR B XOR C XOR D', and any of A, B, C, D' or P' can be computed via an XOR of all the others. Later... Greg Oster

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