LVM data corruption

[Robie Basak]

On Mon, Jan 23, 2017 at 12:42:37PM +0100, Xen wrote:
> Perhaps you could say the intent of my message is to urge a newer version
> for 16.04 as well, but that is up to you. I just wanted to state the issues
> here.

This won't happen unless someone interested in the problem files a bug.
If you want to urge us to fix it in 16.04, you should file a bug.
Otherwise, you should not expect that anything will happen at all.

> Steps to reproduce would be simple:

> 1. DD your entire disk including partition table to another drive.

What sort of LVM? Exactly one LV in one VG on one PV? Or something else?
What commands can a developer use to create this exactly? Please, save
developers from having to look everything up and second guess exactly
what you did. The closer you can get to giving developers a script to
run (inside a KVM, probably) to reproduce the problem, the more chance
you have of getting some developer attention.

> 2. Run a single LVM command.

Which command? If one of many will do, please pick one and specify it
exactly. Then others can follow the same thing and be confident that
they are doing what you're doing. Otherwise you risk wasting developers'
time and this makes it less likely they'll give you any attention.

> 3. LVM will now -- sometimes -- -- often times -- start replacing the
> backing device of your earlier partitions.
> 
> 4. If this actually happens (it might also choose to stick to the old, but
> maybe it always chooses the "higher" number (sdb over sda) so if you are
> backing up to a "later attached" drive this will probably happen) -- you
> will get the data corruption I mention.

Is there a command that will demonstrate the corruption immediately?

> But I don't see much the point of writing a detailed bug report, sorry. I
> also do not have the logs of the resulting fsck etc.

If you do not write a detailed bug report, then it is unlikely that
anything will happen as a result of your report.

I can't say that this can easily be fixed in 16.04. A backport may be
non-trivial, and bumping a major version of LVM seems also quite risky
in terms of inadvertent changes in user-visible behaviour.

I can say that by not filing a detailed bug report, you are ensuring
that it will not be fixed in 16.04. Unless somebody else takes this on
and files the bug.

Robie