[lvm-devel] [PATCH] config: set external_device_info_source=none if udev isn't running

[heming.zhao at suse.com]

On 1/29/21 6:56 AM, Zdenek Kabelac wrote:
> Dne 28. 01. 21 v 11:27 Martin Wilck napsal(a):
>> On Thu, 2021-01-28 at 11:10 +0100, Zdenek Kabelac wrote:
>>> Dne 27. 01. 21 v 18:28 mwilck at suse.com napsal(a):
>>>> From: Martin Wilck <mwilck at suse.com>
>>>>
>>>> LVM2 has several configuration options related to device detection
>>>> and udev. In particular, we have obtain_device_list_from_udev=(0|1)
>>>> and external_device_info_source=("none"|"udev"). The two options
>>>> are
>>>> obviously semantically related, but it's rather unclear if and how
>>>> they interact.
>>>>
>>>> If udev is unavailable, e.g. in containers,
>>>> obtain_device_list_from_udev
>>>> (which defaults to 1) will be automatically reset to 0. However,
>>>> if external_device_info_source="udev" is set, this setting is not
>>>> reset to "none", leading to error messages like
>>>>
>>>>     Udev database has incomplete information about device /dev/vda.
>>>>     /dev/vda: Failed to get external handle [udev].
>>>>
>>>> This patch changes that, treating external_device_info_source the
>>>> same way as obtain_device_list_from_udev, thereby making LVM2's
>>>> device detection more consistent.
>>>>
>>>> The default for external_device_info_source is "none", but I
>>>> believe
>>>> there are very good reasons to change this setting to "udev",
>>>> because
>>>> LVM will get detection of multipath and md devices wrong most of
>>>> the
>>>> time otherwise. LVM should follow the same logic as systemd and
>>>> other
>>>
>>>
>>> Hi
>>>
>>> I'm afraid there is no such simple fix for this as you might think.
>>>
>>>
>> But does that mean my patch is wrong? Don't you agree that the
>> different handling of obtain_device_list_from_udev and
>> external_device_info_source in the current code is inconsistent?
>>
> 
> Hi
> 
> One of the main point probably is  -
> 
> if udev is not working on your main system - and should - you should
> get it working first.
> 
> Side case can be - you run lvm2 command - and someone 'restarts' udev
> (i.e. via upgrade...)
> 
> So in general - this fallback should be only like new configurable
> option - since normally you do not want lvm2 to ever touch /dev
> dir which is under udev control.
> 
> Adding your 'fallback' adds some level of randomness and diggers
> possibly bigger hole of troubles in system.
> 

 From above logic, lvm2 should set 'external_device_info_source = "udev"' as default,
but real world is not.
why? I guess it's for compatible with old systems which can't run udev.
Whether or not we would change this item from "none" to "udev" in main branch?

>> The background of this patch is not that I want to enable use of LVM in
>> containers. It is that we (openSUSE/SUSE) have recently made
> 
> Yep - everyone thinks running lvm2 in Container is 'smart' :)
> 
>> Still, I think the patch is not a hack, but generally correct, and has
>> the pleasant side effect to fix our issue.
> 
> It would need some 'new' mode - but IMHO that's then equivalent
> to setting correct mode directly.
> 
> What can likely work better is to add some 'detection' of being executed
> in container -  scream at user and do maybe that 'udev' hack fallback ;)
> 

Regards
Heming