Phillip Susi wrote on 02.12.2014 at 20:19: > On 12/1/2014 4:45 PM, Konstantin wrote: > > The bug appears also when using mdadm RAID1 - when one of the > > drives is detached from the array then the OS discovers it and > > after a while (not directly, it takes several minutes) it appears > > under /proc/mounts: instead of /dev/md0p1 I see there /dev/sdb1. > > And usually after some hour or so (depending on system workload) > > the PC completely freezes. So discussion about the uniqueness of > > UUIDs or not, a crashing kernel is telling me that there is a > > serious bug. > > I'm guessing you are using metadata format 0.9 or 1.0, which put the > metadata at the end of the drive and the filesystem still starts in > sector zero. 1.2 is now the default and would not have this problem > as its metadata is at the start of the disk ( well, 4k from the start > ) and the fs starts further down. I know this and I'm using 0.9 on purpose. I need to boot from these disks so I can't use 1.2 format as the BIOS wouldn't recognize the partitions. Having an additional non-RAID disk for booting introduces a single point of failure which contrary to the idea of RAID>0.
Anyway, to avoid a futile discussion, mdraid and its format is not the problem, it is just an example of the problem. Using dm-raid would do the same trouble, LVM apparently, too. I could think of a bunch of other cases including the use of hardware based RAID controllers. OK, it's not the majority's problem, but that's not the argument to keep a bug/flaw capable of crashing your system. As it is a nice feature that the kernel apparently scans for drives and automatically identifies BTRFS ones, it seems to me that this feature is useless. When in a live system a BTRFS RAID disk fails, it is not sufficient to hot-replace it, the kernel will not automatically rebalance. Commands are still needed for the task as are with mdraid. So the only point I can see at the moment where this auto-detect feature makes sense is when mounting the device for the first time. If I remember the documentation correctly, you mount one of the RAID devices and the others are automagically attached as well. But outside of the mount process, what is this auto-detect used for? So here a couple of rather simple solutions which, as far as I can see, could solve the problem: 1. Limit the auto-detect to the mount process and don't do it when devices are appearing. 2. When a BTRFS device is detected and its metadata is identical to one already mounted, just ignore it. -- 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
