Re: LVM on NetBSD/i386 v6.1.4

[Michael van Elst <mlelstv%serpens.de@localhost>]

On Sun, May 18, 2014 at 09:11:45AM +0200, Ib-Michael Martinsen wrote:
> Michael van Elst writes:
>  > 
>  > LVM dm-based mirroring (on Linux) has a usability problem as it doesn't
>  > handle errors gracefully. A failed mirror is almost silently converted into
>  > a non-mirror. AFAIK more recent LVM versions try to use md-based mirroring
>  > instead.
> 
> I am not sure I understand this point. I thought a failed mirror would
> always be converted to a non-mirror. You just have to detect it and
> fix it. And it is not that hard to fix/recreate a broken LVM mirror
> (a non-mirror).

The only way to find out about the issue is to scan the system logs,
you cannot distinguish between a broken mirror and a volume that hasn't
been mirrored in the first place.

Fixing the mirror is obviously simply a lvconvert command, but LVM
doesn't know the old configuration, you have to specify again, what
disks are to be used how for the mirror.
Also, since that's basically setting up a mirror from scratch, it
needs to copy the whole volume and not just the information on
the lost disk (think about volumes that span multiple disks).


> Are you saying that you can encounter data inconsistency on the healthy
> disk in a Linux LVM, if a disk in a mirrored LVM volume breaks down?

No. That would make LVM completely useless.


> (I have never experienced that with the AIX LVM, but of curse it is
> another LVM).

AIX is completely different and much easier to use (you can even
move and resize busy filesystems), but then, who would want to
run AIX?  :)

HPUX has a LVM that looks and operates similar to the Linux one.


Greetings,
-- 
                                Michael van Elst
Internet: mlelstv%serpens.de@localhost
                                "A potential Snark may lurk in every tree."