Re: Bug?: Minor numbers in true evms volumes are *not* persitant!

Am Mittwoch, 3. Januar 2007 20:56 schrieben Sie:

Hello Steve,

and thanks for yor explanations.

> > idea what might happen or how the minor numbers of existing EVMS volumes 
are
> > assigned and when they might be reassigned?
>
> Device Mapper assigns a minor number to the device when it is created.
> Assuming the disk configuration doesn't change between reboots, EVMS will
> discover and create the devices in the same order and Device Mapper will
> assign the minor numbers in the same order.  If something causes EVMS to
> discover and create the devices in a different order, then the Device
> Mapper devices may not get the same minor numbers as they did on the
> previous system boot.

Ok, this is an explanation. I would have expected that this integral part of 
the volume is part of its metadata. So I wonder why this situation really 
exists from a users point of view. In an enterprise volume management system 
I wouldn't have expected such a problem. After all using evms or lvm2 you can 
dynamically create, resize etc volumes but you may not delete them else 
things can terribly go wrong in a NFS environment since minor numbers change.

I made a simple test, by creating 4 EVMS volumes based on the lvm2  region 
manager. They got assigned the minor numbers 3, 5, 7, 9.  Then I deleted the 
volume with minor id 5. So far everything is fine. After a reboot however the 
minor numbers did change to 3, 5, 7 where  two volumes changed their minor 
numbers:  one from 7->5 and the second changed from 9->7.  This could cause a 
lot of unexpected trouble in a NFS environment.  And I think that while disk 
space is getting larger and cheaper such a scenario where volumes are created 
and others  are deleted is very realistic in our days.

I also checked native lvm2 but here the behaviour is the same.

> > the boot script boot.evms then later boot.lvm is started. So evms scannes
>> all
>
> > disks and then a little later lvm does the same again. Could this be the
> > cause of this strange trouble?
>
> Yes, this could be a problem.  If you are using EVMS to discover the LVM
> devices, then you should not let the native LVM discover the devices.  The
> native LVM will find the same regions/LVM volumes and will also use Device
> Mapper to create devices for them.  Try disabling boot.lvm and see if the
> problem goes away.

It helps a little in that a fresh created EVMS volume does not change its 
minor after a reboot if no other volumes was deleted at the same time.

Thanks
Rainer
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Rainer Krienke, Universitaet Koblenz, Rechenzentrum, Raum A022
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