On Friday February 23, tytso@xxxxxxx wrote: > On Fri, Feb 23, 2007 at 05:37:23PM -0700, Andreas Dilger wrote: > > > Probably the only sane thing to do is to remember the bad sectors and > > > avoid attempting reading them; that would mean marking "automatic" > > > versus "explicitly requested" requests to determine whether or not to > > > filter them against a list of discovered bad blocks. > > > > And clearing this list when the sector is overwritten, as it will almost > > certainly be relocated at the disk level. For that matter, a huge win > > would be to have the MD RAID layer rewrite only the bad sector (in hopes > > of the disk relocating it) instead of failing the whiole disk. Otherwise, > > a few read errors on different disks in a RAID set can take the whole > > system offline. Apologies if this is already done in recent kernels... Yes, current md does this. > > And having a way of making this list available to both the filesystem > and to a userspace utility, so they can more easily deal with doing a > forced rewrite of the bad sector, after determining which file is > involved and perhaps doing something intelligent (up to and including > automatically requesting a backup system to fetch a backup version of > the file, and if it can be determined that the file shouldn't have > been changed since the last backup, automatically fixing up the > corrupted data block :-). > > - Ted So we want a clear path for media read errors from the device up to user-space. Stacked devices (like md) would do appropriate mappings maybe (for raid0/linear at least. Other levels wouldn't tolerate errors). There would need to be a limit on the number of 'bad blocks' that is recorded. Maybe a mechanism to clear old bad blocks from the list is needed. Maybe if generic make request gets a request for a block which overlaps a 'bad-block' it returns an error immediately. Do we want a path in the other direction to handle write errors? The file system could say "Don't worry to much if this block cannot be written, just return an error and I will write it somewhere else"? This might allow md not to fail a whole drive if there is a single write error. Or is that completely un-necessary as all modern devices do bad-block relocation for us? Is there any need for a bad-block-relocating layer in md or dm? What about corrected-error counts? Drives provide them with SMART. The SCSI layer could provide some as well. Md can do a similar thing to some extent. Where these are actually useful predictors of pending failure is unclear, but there could be some value. e.g. after a certain number of recovered errors raid5 could trigger a background consistency check, or a filesystem could trigger a background fsck should it support that. Lots of interesting questions... not so many answers. NeilBrown - 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
