[Linux-cluster] CLVM, iSCSI and naming rules

[Pierre-Alain RIVIERE]

After reading your response I've proceed some tests (disconnecting and 
reconnecting iSCSI volumes in a random order) and you're right LVM 
doesn't care that much about which device a iSCSI volume is really 
connected to.

But I will still keep my symlinks for admin purpose, much more simple 
for me.


And by the way thanks for your answer.


Graham Wood wrote:
> On Wed, Oct 24, 2007 at 12:09:04PM +0200, Pierre-Alain RIVIERE wrote:
>   
>> I've also added some udev custom rules to have the same device name on 
>> each box
>>     
> This isn't actually necessary.  The same name makes things easier from the user/admin 
> perspective, but the LVM side of things uses the ID of the pv, rather than the device name.
>
>   
>> This is a normal behavior? And when I will deploy CMAN, CLVM and iSCSI 
>> on another box what will be LVM behaviours? I can't guarantee that 
>> /dev/sdj will be accessible on the other box, only the symlink will be 
>> the same.
>>     
> LVM is using /etc/lvm/lvm.conf (on this RH machine anyway) to choose what devices to scan.  
> The default on this RHEL5 machine is:
>
> devices {
>     dir = "/dev"
>     scan = [ "/dev" ]
>     filter = [ "a/.*/" ]
>     cache = "/etc/lvm/.cache"
>     write_cache_state = 1
>     sysfs_scan = 1      
>     md_component_detection = 1
> }
>
> which means that it'll detect anything in /dev and use that.  This has bit me in the past 
> (using drbd) but shouldn't be a problem here.  If it is (and you're not using LVM for the 
> OS) you could change this to only scan your links instead.
>
> The reason it caused a problem with DRBD (it's a way of doing mirroring between 2 nodes, if 
> you've never seen it) is that the PV is visible as the local device as well as the mirrored 
> one, and writing changes to the local copy doesn't work (they don't get replicated to the 
> other node).
>
> So as long as the /dev/sd is a valid device to use, don't worry about it.
>
> Indeed - if your iSCSI stuff is as straight forward as it sounds, I'd be tempted to stop 
> doing the symlinks and just let the LVM stuff do its "thing".
>
> Graham
>
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