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Re: [linux-lvm] LVM2, NFS and random device (major:minor) numbers



On Fri, 14 Apr 2006, Sander Steffann wrote:
NFS depends on what the server says the underlying device id is of the
exported volume and if that changes, then NFS mounts are going to change
out from under themselves.  And everytime I add a volume and reboot the
device ids change.

Unfortunate, but true. One of the side-effects of having dynamic volumes.
Also note that you would get similar trouble if, for example, you added a
SCSI device with a lower SCSI-id than an existing SCSI device. /dev/sdb1
would become /dev/sdc1, etc.

True, but that is a lot more obvious to an admin when it happens because the ARE adding a new device to the system. Also, ironically, lvm1 actually protected a user from having the device id change on them due to just adding a new disk. In fact making such physical device issues not a factor seems to be a feature of LVM the way PVs have unique UUIDs

I finally come upon this old post to this list:
http://www.redhat.com/archives/linux-lvm/2005-May/msg00029.html

The part that can solve everything for you is:
These days, that's configurable.
See 'man exports' fsid=num.

With an explicit fsid=n for every export, you tell the NFS server to ignore
the device-id, and use the specified fsid instead. That way you get
consistent NFS exports, without being dependent on the underlying
device-ids.


I have reasons besides NFS I need the device id to be persistant.
So I want to use the -My option and would like to know if it is safe
to do on volumes in use.

I have had the same trouble as you have, and this solution works great.

I think this speaks poorly of LVM2 that this issue which is such a big change from LVM1 (which was explictly patched at one point to "fix" this changing device ID issue) is not more well documented. Sure, docs about LVM2 do say the big change is the use of the device-mapper. But the final implications of this difference such as this NFS fsid issue are by no means obvious.


One thing I don't understand about not being able to set the major number in the 2.6 kernel is that since 253 will be the major number for all volumes, then is 256 the maximum number of LVM volumes one can have on a single system? Or will the major number increment to 254 randomly
and there is no way to control it?


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