On 03/17/2011 06:36 PM, Chris Mason wrote: > Excerpts from Jan Schmidt's message of 2011-03-17 10:46:43 -0400: >> Hello everyone, >> >> Currently, btrfs has its own raid1 but no repair mechanism for bad >> checksums or EIOs. While trying to implement such a repair mechanism, >> several more or less general questions came up. >> >> There are two different retry paths for data and metadata. (If you know >> or don't care how btrfs handles read errors: goto questions) > > You should talk with Ilya, who is working on replacing failed raid drives as > well. Thanks, I'll do that. >> >> The data path: btrfs_io_failed_hook is called for each failed bio (EIO >> or checksum error). Currently, it does not know which mirror failed at >> first, because normally btrfs_map_block is called with mirror_num=0, >> leading to a path where find_live_mirror picks one of them. The error >> recovery strategy is then to explicitly read available mirrors one after >> the other until one succeeds. In case the very first read picked mirror >> 1 and failed, the retry code will most likely fail at mirror 1 as well. >> It would be nice to know which mirror was picked formerly and directly >> try the other. > > Agree with Josef here, change the code to record which one was used. > The current bio submission stuff only keeps the btrfs_multi_bio struct > around when a given IO spans more than one disk. But you can easily > change it to keep the struct around for all IOs. I was about to write the same in reply to Josefs mail :-) >> The metadata path: there is no failure hook, instead there is a loop in >> btree_read_extent_buffer_pages, also starting off at mirror_num=0, which >> again leaves the decision to find_live_mirror. If there is an error for >> any page to be read, the same retry strategy is used as is in the data >> path. This obviously might leave you alone with unreadable data >> (consider page x is bad on mirror 1 and page x+1 is bad on mirror 2, >> both belonging to the same extent, you lose). It would be nice to have a >> mechanism at a lower level issuing page-sized retries. Of course, >> knowing which mirror is bad before trying mirror 1 again is desirable as >> well. > > Currently the block size is always smaller than the stripe size. But > you have a good point. > >> >> questions: >> I have a raid1 repair solution in mind (partially coded) for btrfs that >> can be implemented quite easily. However, I have some misgivings. All of >> the following questions would need a "yes" for my solution to stand: >> >> - Is it acceptable to retry reading a block immediately after the disk >> said it won't work? Or in case of a successful read followed by a >> checksum error? (Which is already being done right now in btrfs.) > > In the initial implementation sure, but long term it's not the best. > >> >> - Is it acceptable to always write both mirrors if one is found to be >> bad (also consider ssds)? > > Sorry, I'd rather not overwrite the copy we know to be good. > >> >> If either of the answers is "no", tracking where the initial read came >> from seems inevitable. Tracking would be very easy if bios came back >> with unmodified values in bd_bdev and bd_sector, which is not the case. First off, I'll go for the tracking part then. I'll see whether implementing the retry mechanism somewhere near end_bio_multi_stripe is a good thing or else, how to fiddle the mirror_num further up so the code calling the hooks can care for the retries and repairs. Thanks! -Jan -- 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
