[linux-lvm] devices/dir configuration option in /etc/lvm/lvm.conf can be edited?

[Gang He]

Hi Zdenek,

Thanks for explaining.
That means we should tell the user, you should not edit this option (dir = "/dev") in the lvm.conf, right?

Thanks
Gang

> -----Original Message-----
> From: Zdenek Kabelac [mailto:zkabelac at redhat.com]
> Sent: 2019年10月14日 18:40
> To: LVM general discussion and development <linux-lvm at redhat.com>; Gang
> He <GHe at suse.com>
> Subject: Re: devices/dir configuration option in /etc/lvm/lvm.conf can be
> edited?
> 
> Dne 14. 10. 19 v 10:00 Gang He napsal(a):
> > Hello List,
> >
> > By default, devices/dir configuration option in /etc/lvm/lvm.conf is "=/dev".
> > But, if I edit this configuration option, e.g. dir = "/dev/lvm", then lvcreate
> command will fail.
> >
> > sles12sp4-node:/dev # lvcreate -L2G -ay -n testlv vgdata
> >    /dev/lvm/vgdata/testlv: not found: device not cleared
> >    Aborting. Failed to wipe start of new LV.
> >
> > I am using lvm 2.02.183(or 180), this option can be edit individually?
> > or any other option will affect this option, then lead to lvcreate failure.
> 
> Hi
> 
> The option is not so easy to explain:
> 
> In the first place on a today's system you shall never ever need to change this
> setting - as majority of systems runs 'udev' or something similar in a fixed
> position /dev.
> 
> So symlinks & devices appears at this directory (and without 'lvm2' direct
> work) only as a consequence of configured udev rules.
> 
> So now surely comes the obvious question -  why the 'setting' even exists
> when you should always use '/dev' anyway right ;) ?
> 
> And here the answer is longer -  lvm2 is very 'oldish' project from 'dark' era
> before udev took control over devices - and in this old era you could have
> configured different device directory for devices created by lvm2 - since it's
> been lvm2 physically creating these devices.
> 
> The usability for normal users is relatively questionable since almost every
> user wants his devices in /dev dir anyway, but couple wanted to maintain
> separate dir for lvm2 devices.
> 
> The 'other' use-case is for testing - where i.e. lvm2 test suite is/(or was) able to
> run its tests in completely isolated device directory.
> 
> But to be able to use this 'capability' - one has to enable other lvm.conf
> setting:  'activation/verify_udev_operations=1'  - when enabled lvm2 will
> ensure devices are in give directory.
> 
> But !!!! (and it's BIG BUT) this shall never be enabled on a system with running
> udevd and /dev  dir set - as basically nothing else then udevd is supposed to
> be creating anything in /dev dir.
> 
> So hopefully this explains most of the question you may have about this
> setting.
> 
> Regards
> 
> Zdenek
> 
>