Eric Sandeen wrote: > On 8/15/13 12:55 PM, Michael Maier wrote: >> Eric Sandeen wrote: >>> On 8/14/13 11:20 AM, Michael Maier wrote: >>>> Dave Chinner wrote: >>> >>> ... >>> >>>>> If it makes you feel any better, the bug that caused this had been >>>>> in the code for 15+ years and you are the first person I know of to >>>>> have ever hit it.... >>>> >>>> Probably the second one :-) See >>>> http://thread.gmane.org/gmane.comp.file-systems.xfs.general/54428 >>>> >>>>> xfs_repair doesn't appear to have any checks in it to detect this >>>>> situation or repair it - there are some conditions for zeroing the >>>>> unused parts of a superblock, but they are focussed around detecting >>>>> and correcting damage caused by a buggy Irix 6.5-beta mkfs from 15 >>>>> years ago. >>>> >>>> The _big problem_ is: xfs_repair not just doesn't repair it, but it >>>> _causes data loss_ in some situations! >>>> >>> >>> So as far as I can tell at this point, a few things have happened to >>> result in this unfortunate situation. Congratulations, you hit a >>> perfect storm. :( >> >> I can appease you - as it "only" hit my backup device and because I >> noticed the problem before I really needed it: I didn't hit any data >> loss over all, because the original data is ok and I repeated the backup >> w/ the fixed FS now! >> >>> 1) prior resize operations populated unused portions of backup sbs w/ junk >>> 2) newer kernels fail to verify superblocks in this state >>> 3) during your growfs under 3.10, that verification failure aborted >>> backup superblock updates, leaving many unmodified >>> 4a) xfs_repair doesn't find or fix the junk in the backup sbs, and >>> 4b) when running, it looks for the superblocks which are "most matching" >>> other superblocks on the disk, and takes that version as correct. >>> >>> So you had 16 superblocks (0-15) which were correct after the growfs. >>> But 16 didn't verify and was aborted, so nothing was updated after that. >>> This means that 16 onward have the wrong number of AGs and disk blocks; >>> i.e. they are the pre-growfs size, and there are 26 of them. >>> >>> Today, xfs_repair sees this 26-to-16 vote, and decides that the 26 >>> matching superblocks "win," rewrites the first superblock with this >>> geometry, and uses that to verify the rest of the filesytem. Hence >>> anything post-growfs looks out of bounds, and gets nuked. >>> >>> So right now, I'm thinking that the "proper geometry" heuristic should >>> be adjusted, but how to do that in general, I'm not sure. Weighting >>> sb 0 heavily, especially if it matches many subsequent superblocks, >>> seems somewhat reasonable. >> >> This would have been my next question! I repaired it w/ the git >> xfs_repair on the already reduced to original size FS. I think, if I >> would have done the same w/ the grown FS, the FS most probably would be >> reduced to the size before the growing. >> >> Wouldn't it be better to not grow at all if there are problems detected? >> Means: Don't do the check after the growing, but before? Ok, I could >> have done it myself ... . From now on, I will do it like this! > > well, see the next couple patches I'm about to send to the list ... ;) Cool! > but a check prior wouldn't have helped you, because repair didn't detect > the problem that growfs choked on. The old xfs_repair! Your patched one would have detected the problem if I got it right. But globally speaking: you're right - it's impossible to get 100% security. But couldn't xfs_repair -n find other problems which therefore could be repaired before growing the FS? Thanks, regards, Michael _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
