[linux-lvm] Discussion: performance issue on event activation mode

[Zdenek Kabelac]

Dne 08. 06. 21 v 17:47 Martin Wilck napsal(a):
> On Di, 2021-06-08 at 10:39 -0500, David Teigland wrote:
>> . Use both native md/mpath detection *and* udev info when it's
>> readily
>>    available (don't wait for it), instead of limiting ourselves to one
>>    source of info.  If either source indicates an md/mpath component,
>>    then we consider it true.
> Hm. You can boot with "multipath=off" which udev would take into
> account. What would you do in that case? Native mpath detection would
> probably not figure it out.
>
> multipath-tools itself follows the "try udev and fall back to native if
> it fails" approach, which isn't always perfect, either.
>
>> A third related improvement that could follow is to add stronger
>> native
>> mpath detection, in which lvm uses uses /etc/multipath/wwids,
>> directly or
>> through a multipath library, to identify mpath components.  This
>> would
>> supplement the existing sysfs and udev sources, and address the
>> difficult
>> case where the mpath device is not yet set up.
>>
> Please don't. Use libmpathvalid if you want to improve in this area.
> That's what it was made for.

Problem is addition of another dependency here.

We may probably think about using  'dlopen' and if library is present use it, 
but IMHO  libmpathvalid should be integrated into libblkid  in some way  - 
linking another library to many other projects that needs to detect MP devices 
really complicates this a lot.  libblkid should be able to decode this and 
make things much cleaner.

I'd also vote for lvm2 plugin for blkid as forking thousands of process simply 
always will take a lot of time (but this would require quite some code 
shufling withing lvm codebase).

Zdenek