[linux-lvm] multipath_component_detection

[Ken Goods]

Thanks so much Peter, exactly what I was looking for. Also thanks for the heads up on external_device_info_source, I'll use it. 

----- Original Message -----

From: "Peter Rajnoha" <prajnoha at redhat.com> 
To: kgoods at cropusainsurance.com 
Cc: "LVM general discussion and development" <linux-lvm at redhat.com> 
Sent: Tuesday, January 17, 2017 3:19:44 AM 
Subject: Re: [linux-lvm] multipath_component_detection 

On 01/16/2017 08:46 PM, Ken Goods wrote: 
> Hi all, 
> Just a quick question. I've been setting up a (iscsi) LUN and want to 
> create an LVM PV on it. 
> 
> The docs I have read are somewhat conflicted. Some say that the 
> underlying devices should be filtered, however according to comments in 
> lvm.conf it says that if multipath_component_detection is enabled, "LVM2 
> will ignore devices used as component paths of device-mapper multipath 
> devices". 
> 
> So filtering is no longer necessary with LVM2? 
> 

The multipath component detection is an automatic filter in LVM2 (if 
enabled with devices/multipath_component_detection=1 in LVM 
configuration - that is used by default). So in this case you don't need 
to set up devices/global_filter (or devices/filter) manually. LVM will 
filter multipath components automatically by looking at the device stack 
and if it detects that there's a multipath device on top of certain set 
of devices, such devices are assumed as multipath components and they're 
filtered automatically. 

Of course, for this automatic filter to work correctly, you also need to 
have your multipath correctly configured - LVM doesn't know that the 
devices are multipath components unless there's multipath device already 
set up and running on top of those components. 

However, there's one improvement in this detection if you use 
devices/external_device_info_source="udev" in your LVM configuration 
(this is not used by default yet). In this case, LVM reads information 
from udev database which in turn has this information about multipath 
components directly from multipath and that one takes this information 
from its own configuration (the component WWID list in multipath's 
configuraton) - so in this case you don't even need to have multipath 
device set up yet on top of multipath components for LVM to detect 
multipath components properly. 

But usually, if your system is properly configured, you always have 
multipath device set up and running on top of multipath components. 

-- 
Peter 

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