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

[Martin Wilck]

On Fri, 2021-01-29 at 11:34 -0600, David Teigland wrote:
> On Fri, Jan 29, 2021 at 02:02:11PM +0100, Martin Wilck wrote:
> > LVM filters are hard to get right, even for experienced admins. Not
> > because of the regexes, but because of the many different names
> > under
> > which devices appear under /dev. Telling admins that they need to
> > fiddle with filter rules just to make multipath work sounds like
> > something I'd have said 10 years ago, but not in 2021.
> 
> I agree.
> 
> > > There will be also a new 'filtering' introduced in a form a basic
> > > acceptance 
> > > list of devices - that may be seen in some cases as more simple
> > > to
> > > use.
> > 
> > Perhaps this will improve matters, perhaps it'll add more
> > confusion. 
> > I don't know enough to tell.
> 
> The filter problems, among many others, were the motivation to create
> a
> new feature "devices file" which I'm planning to merge into lvm soon.
> 
> Latest devel branch (see latest -N suffix):
>  
> https://sourceware.org/git/?p=lvm2.git;a=shortlog;h=refs/heads/dev-dct-devicesid-22
> 

Interesting, thanks.

> The basic idea is that a new lvm config file (the devices file) lists
> the
> devices that lvm can use, and lvm will not even look at devices
> outside
> that list.  So the user tells lvm which devices it should use, and
> that's
> all lvm sees.  (With an optional allowance for pvcreate to implicitly
> mean
> "use this device", to make it easier for people to begin using this.)
> 
> The devices are primarily identified by a WWID, or other similar
> identifier.  lvm commands are used manage that file, but it can be
> manually edited also.  It replaces filters, and will make a number of
> hard
> problems in lvm just go away:  the problem of unstable dev names in
> filters goes away, mpath/md component detection goes away (you
> wouldn't
> include component devs in the device file), the system mistakenly
> activating PVs belonging to VMs goes away.

If there are both SCSI devices and multipath devices with the given
WWID, multipath would be preferred, I suppose? What would happen during
device detection, when a SCSI device with the given WWID pops up? How
would lvm "know" that this device was going to be added to a multipath
map without asking udev?

Regards
Martin