On 08/05/17 16:50, Nix wrote: > > I wonder... scrubbing is not very useful with md, particularly with RAID > 6, because it does no writes unless something mismatches, and on failure > there is no attempt to determine which of the N disks is bad and rewrite > its contents from the other devices (nor, as I understand it, does it > clearly say which drive gave the error, so even failing it out and > resyncing it is hard). > Please read Neil Brown's article on this: "Smart or simple RAID recovery?" <http://neil.brown.name/blog/20100211050355> > If there was a way to get md to *rewrite* everything during scrub, > rather than just checking, this might help (in addition to letting the > drive refresh the magnetization of absolutely everything). "repair" mode > appears to do no writes until an error is found, whereupon (on RAID 6) > it proceeds to make a "repair" that is more likely than not to overwrite > good data with bad. Optionally writing what's already there on non-error > seems like it might be a worthwhile (and fairly simple) change. > Scrubbing /does/ rewrite disk blocks - when necessary. It does not do it explicitly, but the disks handle this themselves. To the processor, a disk block is 4K of data. But to the disk and its controllers, it is 4K plus a sizeable amount of error checking and correcting bits. Some are spread out within the block, some are collected together at the end of the block. The ECC system can handle a large number of failed bits, either in lumps caused by a physical defect on the disk surface, or spread out due to the slow decay of the magnetic orientation, or hits by cosmic rays. When the disk is asked to read a block, it pulls up the data and the ECC bits, and uses this to check and re-construct the 4K of data, and a measure of how many errors were corrected. On modern high-capacity drives, it is normal that some errors are corrected on a read. But if more than a certain level occur, then the firmware will trigger a re-write automatically to the same sector. This will then be re-read. If the error rate is low, fine. If it is high, then the sector will be remapped by the disk. So simply /reading/ the data, as far as the processor is concerned, will cause re-writes as and when needed. -- 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
