On Fri, Mar 25, 2016 at 4:31 PM, John Marrett <jo...@zioncluster.ca> wrote: > Continuing with my recovery efforts I've built overlay mounts of each > of the block devices supporting my btrfs filesystem as well as the new > disk I'm trying to introduce. I have patched the kernel to disable the > check for multiple missing devices. I then exported the overlayed > devices using iSCSI to a second system to attempt the recovery. > > I am able to mount the device rw, then I can remove missing devices > which removes the missing empty disk. I can add in a new device to the > filesystem and then attempt to remove the second missing disk (which > has 2.7 TB of content on it). > > Unfortunately this removal fails as follows: > > ubuntu@btrfs-recovery:~$ sudo btrfs device delete missing /mnt > ERROR: error removing the device 'missing' - Input/output error
Quite honestly I don't understand how Btrfs raid1 volume with two missing devices even permits you to mount it degraded,rw in the first place. That's rather mystifying considering the other thread where there's a 4 disk raid10 with one missing device, and rw,degraded mount is allowed only once, after that it disallows further attempts to rw,degraded mount it. Anyway, maybe it's possible there's no dual missing metadata chunks, although I find it hard to believe. But OK, maybe it works for a while and you can copy some stuff off the drives where there's at least one data copy. If there's dual missing data copies but there's still at least 1 metadata copy, then file system will just spit out noisy error messages. But if there ends up being dual missing metadata, I expect a crash, or the file system goes read only, or maybe even unmounts. I'm not sure. But once there's 0 copies of metadata I don't see how the file system can correct for that. Because there are two devices missing, I doubt this matters, but I think you're better off using 'btrfs replace' for this rather than 'device add' followed by 'device remove'. The two catches with replace: the replacement device must be as big or bigger than the one being replaced; you have to do a resize on the replacement device, using 'fi resize devid:max' to use all the space if the new one is bigger than the old device. But I suspect either the first or second replacement will fail also, it's too many missing devices. So what can happen, if there's 0 copies of metadata, is that you might not get everything off the drives before you hit the 0 copies problem and the ensuing face plant. In that case you might have to depend on btrfs restore. It could be really tedious to find out what can be scraped. But I still think you're better off than any other file system in this case, because they wouldn't even mount if there were two mirrors lost. -- Chris Murphy -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
