Re: [linux-lvm] system boot time regression when using lvm2-2.03.05

[Zdenek Kabelac <zdenek.kabelac@gmail.com>]

Dne 10. 09. 19 v 17:20 David Teigland napsal(a):
>>>> _pvscan_aa
>>>>    vgchange_activate
>>>>     _activate_lvs_in_vg
>>>>      sync_local_dev_names
>>>>       fs_unlock
>>>>        dm_udev_wait <=== this point!
>>>> ```
> 
>> Could you explain to us what's happening in this code? IIUC, an
>> incoming uevent triggers pvscan, which then possibly triggers VG
>> activation. That in turn would create more uevents. The pvscan process
>> then waits for uevents for the tree "root" of the activated LVs to be
>> processed.
>>
>> Can't we move this waiting logic out of the uevent handling? It seems
>> weird to me that a process that acts on a uevent waits for the
>> completion of another, later uevent. This is almost guaranteed to cause
>> delays during "uevent storms". Is it really necessary?
>>
>> Maybe we could create a separate service that would be responsible for
>> waiting for all these outstanding udev cookies?
> 
> Peter Rajnoha walked me through the details of this, and explained that a
> timeout as you describe looks quite possible given default timeouts, and
> that lvm doesn't really require that udev wait.
> 
> So, I pushed out this patch to allow pvscan with --noudevsync:
> https://sourceware.org/git/?p=lvm2.git;a=commitdiff;h=3e5e7fd6c93517278b2451a08f47e16d052babbb
> 
> You'll want to add that option to lvm2-pvscan.service; we can hopefully
> update the service to use that if things look good from testing.

This is certainly a bug.

lvm2 surely does need to communication with udev for any activation.

We can't let running activation 'on-the-fly' without control on system with 
udev (so we do not issue 'remove' while there is still 'add' in progress)

Also any more complex target like thin-pool need to wait till metadata LV gets 
ready for thin-check.

Regards

Zdenek