Read-only mounts (was mdadm RAID6 "active" with spares and failed disks; need help)

[Phil Turmel <philip@turmel.org>]

On 03/28/2015 06:11 AM, Roman Mamedov wrote:
> On Fri, 27 Mar 2015 21:59:38 -0400
> Phil Turmel <philip@turmel.org> wrote:
> 
>> I read through this.  Given all of the destructive actions you took, I
>> am doubtful you will ever get your data.  Like mounting "readonly".
>> That gives you a readonly filesystem, but it writes to the device.
>> Possibly a great deal if there's a journal to replay.
> 
> Are you sure, which FS does that? I remember some discussion on FS lists
> (Btrfs?), and IIRC the consensus and the implemented behavior was that the
> device shouldn't ever be touched with writes on RO mounts, no matter what.

I remember people being burned by it with ext3/4 a couple years ago.
Which is why all of the array disaster recoveries I've helped with
called for fsck -n to verify a reconstruction attempt, not a mount -o ro.

I will set up a small VM and see what recent kernels do.

Phil