[linux-lvm] Re: LVM writes on raw disk of ATA RAID Mirror

[Ewen McNeill]

In message <1051724087.19901.287.camel at cinshrlnxws01.shermfin.com>, Andrew Reche
nberg writes:
>>[LVM 1.0.x; vgscan finds parts of RAID arrays and uses those instead]
>vgscan has similar problems with MD devices in a software RAID10 (but
>not RAID1) configuration. [...]

Interesting.  I've been told, and found in the source, that there were
some hacks in LVM to make LVM and md work in some common situations
(basically there's a loop after the first device scan which tries to
eliminate some of the duplicates; and there's a /* FIXME */ comment
immediately before that loop).  It seems that you've found one of
the situations where these hacks are too limited.

>I was trying to hack vgscan to get my setup working but I needed the box
>working ASAP so time dictated that I use the KludgeMethod(tm).  Someone
>on the list recommended that I move /dev/md0 before I run my vgscan. 
>[....]

That is really quite ugly :-)  I suspect something like that might work
for the ataraid case too, right up to the point that someone runs vgscan
for some reason without going through the "hide things from vgscan so it
doesn't get it wrong" ritual -- at which point (with a little prompting)
it silently swaps over to using part of the RAID array, and the RAID
array gets out of sync, and then on next reboot it swaps back to using
the RAID array, and the partitions are corrupt.

I'm afraid that's a little too much potential excitement for my liking!

I've been told that LVM 2 has some support where you can say what
devices to scan (and what devices not to scan).  I've not looked at LVM
2 yet, so I don't know how fine grained it is, but it might suit your
situation.

However after all this investigation I can't help thinking that the LVM
vgscan approach is broken by design, particularly to be run automatically
on startup.  The idea that one can somehow look through all the connected
devices and guess which one to use, and then automatically use those
guesses on the assumption they'll always be right, just seems to be asking
for trouble.  Static configuration files, and having these things under
manual control, seems a far more reliable way to approach the situation.

For now this ataraid system will be built without LVM, as it seems the
only way to be sure that the RAID array will actually always be used.

Thanks for your comments,

Ewen