I) Can you give the complete mdadm command used to create it ? Normally it should create a RAID5 without spares. (unless instructed otherwise/you passed the wrong options) Also giving us the output of mdadm --detail /dev/mdXXX could help II) ***Disclaimer*** following information below might not be accurate, but such a system could work. If it's incorrect it should help you understand when someone corrects me. mdadm --examine /dev/sdXX shows me "Internal Bitmap : 8 sectors from superblock" This would indicate there is a bitmap on each drive (although I'm not sure, theoretically you could RAID it, but why increase complexity). However the RAID only need 1 write indent map. But in the worst case scenario only 1 disk is left, so a copy is maintained on each drive. Example: Write indent map for 512K disk using 64K chunks Bit 1: Synchronized Bit 0: Not synced | Chunk | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | | Bit | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | When you write in Chunk 1, the bit is set to 0. Now assume 1 of the disk power connector flies of, and the write to the chunk fails. | Chunk | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | | Bit | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | Meanwhile another write is done to Chunk 2, new bitmap: | Chunk | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | | Bit | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | Now when you plug the disk back in it looks for unwritten chunks, and it find 1 and 2, now it nows it can start from this. (Note it reject the bitmap of the disk you plugged back in.) In case you are building a new raid something simular occurs: This would be the start bitmap: | Chunk | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | | Bit | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | As each chunk is sycned the bit is set to 1: C1234578 B0000000 Later it becomes: B1000000 Then later it becomes B1100000 ... So at any point you can reboot, and the raid will know where to continue by looking at the non-sycned bitmaps. Also see the wiki: https://raid.wiki.kernel.org/index.php/Write-intent_bitmap Killian De Volder On 18-07-14 16:21, Henry Cai wrote: > Hi, > > Here, I got two confusing questions about Linux MD: > > I. Why when initial create RAID5, mdadm marks a physical disk as "spare"? > > Is this for random write with RMW, or for "sync" speed? > > > II. The write intent bitmap, each disk in RAID with a "write intent > bitmap", or the whole RAID with one "write intent bitmap"? > > If the whole RAID with one "write intent bitmap", how to know > which disk's data need reconstruct, or just use the data disks' > > data to calculate the P data, and write to the P disk? If the only > one "write intent bitmap", how to decide which disk to save > > the "write intent bitmap"? > > And is there has any MD design architecture document? > > Thanks a lot > -- > 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
