On 10/11/17 2:20 PM, Ian Kumlien wrote: > > > On Wed, Oct 11, 2017 at 2:10 PM Jeff Mahoney <je...@suse.com > <mailto:je...@suse.com>> wrote: > > On 10/11/17 12:41 PM, Ian Kumlien wrote: > > [--8<--] > > > Eventually the filesystem becomes read-only and everything is odd... > > Are you still able to mount it? I'd be surprised if you could if check > can't open the file system. > > > Nope, it's like there never was a filesystem in the first place... > > But since metadata should be duplicated all over, i'd assume that it > would be able to mount it and survive.... =)
If you'd been using RAID1 or something instead, you'd be able to mount the file system and replace the disk. > > Trying to run btrfs check on the disks results in: > > btrfs check -b /dev/disk/by-uuid/8d431da9-dad4-481c-a5ad-5e6844f31da0 > > bytenr mismatch, want=912228352, have=0 > > Couldn't read tree root > > ERROR: cannot open file system > > > > (For backup and normal) > > > > So even if the data is duplicated on all disks, something in the above > > errors seemed to cause it to abort > > (These disks are seagate sshd disks, never ever buying them again) > > If you have metadata: dup, that doesn't mean the metadata is duplicated > on every disk. It means that there are two copies of the metadata on a > single disk. If that disk is going bad and returning failures for both > copies of the metadata, you may be out of luck. It's really intended > for single spinning disks to get a little bit more resiliency in the > face of bad sectors. > > > Oh? it looks like it would be 2 per 1 device, but ok - Then i could have > had a issue where the drive that keeps the metadata is gone... I > suspected that I did do DUP on multiple devices > > from the man page: > Note 1: DUP may exist on more than 1 device if it starts on a > single device and another one is added. Since version 4.5.1, > mkfs.btrfs will let you create DUP on multiple devices. I can see how you'd reach that conclusion. The wording is somewhat confusing. We allocate space in chunks that are usually about 1GB in size. When DUP is used, we allocate two chunks on the same device and that is presented as a single usable chunk. The constituent chunks will be allocated on the same device, but which device is used can change with each allocation. Say you have 5 disks and 8 metadata chunks. They can be allocated like so: sda: A A D D sdb: B B sdc: C C sde: F F G G sdf: H H I I There is no redundancy in the case of a disk failure, only for sector failures. To spread metadata across disks for redundancy you'll need to use a raid mode instead. If one of those disks is failing and it contains a critical part of the metadata, the file system won't be mountable. > The check error above means that it wasn't able to map a logical address > to a physical address. Typically that means that the mapping was lost. > > > I was more reporting that it happened and if there was any useful data > that we could extract from this if it's a failure that shouldn't happen :) > > I haven't wiped anything yet - preparing to replace the disks though Thanks for reporting it, but in this context, it's somewhat of an expected failure mode. -Jeff -- Jeff Mahoney SUSE Labs
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