As I understand it, there are 2 valid algoritms for writing in raid5. 1. calculate the parity data by XOR'ing all data of the relevant data chunks.
2. calculate the parity data by kind of XOR-subtracting the old data to be changed, and then XOR-adding the new data. (XOR-subtract and XOR-add is actually the same). There are situations where method 1 is the fastest, and situations where method 2 is the fastest. My idea is then that the raid5 code in the kernel can calculate which method is the faster. method 1 is faster, if all data is already available. I understand that this method is employed in the current kernel. This would eg be the case with sequential writes. Method 2 is faster, if no data is available in core. It would require 2 reads and two writes, which always will be faster than n reads and 1 write, possibly except for n=2. method 2 is thus faster normally for random writes. I think that method 2 is not used in the kernel today. Mayby I am wrong, but I did have a look in the kernel code. So I hereby give the idea for inspiration to kernel hackers. Yoyr kernel hacker wannabe keld - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
