[linux-lvm] system boot time regression when using lvm2-2.03.05
Zdenek Kabelac
zdenek.kabelac at gmail.com
Tue Sep 10 20:38:28 UTC 2019
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
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