If D1, D2, D3 or D4 change, then P must also change. If all D1, D2, D3 and D4 change, then md can compute P without any reads. Then write D1, D2, D3, D4 and P. If less than all D1, D2, D3 and D4 change, then md has 2 options, md does whichever requires less disk I/O. 1) md must read the unchanging data and read P, compute a new P and write the changes. 2) Read the old D1, D2, D3 and/or D4 (whichever is changing) and read P. Then factor out the old data from P, then factor in the new data, write all changes. Example 1: D1, D2 and D3 are changing. md will read D4, using D1, D2, D3 and D4 (from memory) compute new P, write D1, D2, D3 and P. Example 2: D1 is changing. md will read old D1 and P. Factor out old D1 from P. Factor new D1 into new P, write D1 and P. Guy -----Original Message----- From: linux-raid-owner@xxxxxxxxxxxxxxx [mailto:linux-raid-owner@xxxxxxxxxxxxxxx] On Behalf Of Yinan Liu Sent: Thursday, October 14, 2004 2:31 PM To: linux-raid@xxxxxxxxxxxxxxx Subject: RAID-5 Parity calculation Hi, I am a new guy for a Linux programming. I got a question for a software RAID-5 parity calculation. If we suppose for data block D1, D2, D3, D4 with a parity block P, When some data block is changed, P will be changed. I am not so clear how does Linux md handle the parity re calculating, when data changed . Suppose there will be three cases, 1. sequential write, is that get new P' directly with new data D1', D2', D3' and D4' or need read D1-4 out from disk doing the normal operation. 2. if we changed there blocks, D1',D2' and D3', does md read D1, D2, D3 out to rebuild P or just read D4 out from disk and rebuild P with D1', D2', D3' and D4. 3. is the simple case, just change one block. Thanks a lot, if some can help me get understand it. - 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 - 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
