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

["heming.zhao@suse.com" <heming.zhao@suse.com>]

On 9/11/21 1:38 AM, Martin Wilck wrote:
> On Thu, 2021-09-09 at 14:44 -0500, David Teigland wrote:
>> On Tue, Jun 08, 2021 at 01:23:33PM +0000, Martin Wilck wrote:
>>> On Di, 2021-06-08 at 14:29 +0200, Peter Rajnoha wrote:
>>>> On Mon 07 Jun 2021 16:48, David Teigland wrote:
>>>>>
>>>>> If there are say 1000 PVs already present on the system, there
>>>>> could be
>>>>> real savings in having one lvm command process all 1000, and
>>>>> then
>>>>> switch
>>>>> over to processing uevents for any further devices afterward.
>>>>> The
>>>>> switch
>>>>> over would be delicate because of the obvious races involved
>>>>> with
>>>>> new devs
>>>>> appearing, but probably feasible.
>>>>
>>>> Maybe to avoid the race, we could possibly write the proposed
>>>> "/run/lvm2/boot-finished" right before we initiate scanning in
>>>> "vgchange
>>>> -aay" that is a part of the lvm2-activation-net.service (the last
>>>> service to do the direct activation).
>>>>
>>>> A few event-based pvscans could fire during the window between
>>>> "scan initiated phase" in lvm2-activation-net.service's
>>>> "ExecStart=vgchange -aay..."
>>>> and the originally proposed "ExecStartPost=/bin/touch
>>>> /run/lvm2/boot-
>>>> finished",
>>>> but I think still better than missing important uevents
>>>> completely in
>>>> this window.
>>>
>>> That sounds reasonable. I was thinking along similar lines. Note
>>> that
>>> in the case where we had problems lately, all actual activation
>>> (and
>>> slowness) happened in lvm2-activation-early.service.
>>
>> I've implemented a solution like this and would like any thoughts,
>> improvements, or testing to verify it can help:
>> https://sourceware.org/git/?p=lvm2.git;a=shortlog;h=refs/heads/dev-dct-activation-switch-1
>>
>> I've taken some direction from the lvm activation generator, but
>> there are
>> details of that I'm not too familiar with, so I may be missing
>> something
>> (in particular it has three activation points but I'm showing two
>> below.)
>> This new method would probably let us drop the activation-generator,
>> since
>> we could easily configure an equivalent using this new method.
>>
>> Here's how it works:
>>
>> uevents for PVs run pvscan with the new option --eventactivation
>> check.
>> This makes pvscan check if the /run/lvm/event-activation-on file
>> exists.
>> If not, pvscan does nothing.
>>
>> lvm-activate-vgs-main.service
>> . always runs (not generated)
>> . does not wait for other virtual block device systems to start
>> . runs vgchange -aay to activate any VGs already present
>>
>> lvm-activate-vgs-last.service
>> . always runs (not generated)
>> . runs after other systems, like multipathd, have started (we want it
>>    to find as many VGs to activate as possible)
>> . runs vgchange -aay --eventactivation enable
>> . the --eventactivation enable creates /run/lvm/event-activation-on,
>>    which enables the traditional pvscan activations from uevents.
>> . this vgchange also creates pv online files for existing PVs.
>>    (Future pvscans will need the online files to know when VGs are
>>    completed, i.e. for VGs that are partially complete at the point
>>    of switching to event based actvivation.)
>>
>> uevents for PVs continue to run pvscan with the new option
>> --eventactivation check, but the check now sees the event-activation-
>> on
>> temp file, so they will do activation as they have before.
>>
>> Notes:
>>
>> - To avoid missing VGs during the transition to event-based, the
>> vgchange
>> in lvm-activate-vgs-last will create event-activation-on before doing
>> anything else.  This means for a period of time both vgchange and
>> pvscan
>> may attempt to activate the same VG.  These commits use the existing
>> mechanism to resolve this (the --vgonline option and
>> /run/lvm/vgs_online).
>>
>> - We could use the new lvm-activate-* services to replace the
>> activation
>> generator when lvm.conf event_activation=0.  This would be done by
>> simply
>> not creating the event-activation-on file when event_activation=0.
>>
>> - To do the reverse, and use only event based activation without any
>> lvm-activate-vgs services, a new lvm.conf setting could be used, e.g.
>> event_activation_switch=0 and disabling lvm-activate-vgs services.
> 
> This last idea sounds awkward to me. But the rest is very nice.
> Heming, do you agree we should give it a try?
> 

the last note is do the compatible things. we can't image & can't test all
the use cases, create a switch is a good idea.
but believe me, except lvm2 developers, no one understand event/direct activation
story. the new cfg item (event_activation_switch) is related with another item
(event_activation) will make users confuse.

We should help users to do the best performance job/selection. So we could reuse
the item "event_activation", current value is 0 and 1, we can add new value '2'.
i.e.:
0 - disable event_activation (use direct activation)
1 - new behaviour
2 - old/legacy mode/behavior

default value is 1 but the lvm behavior is changed.
if anyone want to use/reset, to assign '2' to this item.

-------
I had verified this new feature in my env. this feature make a great progress.

new feature with lvm config:
obtain_device_list_from_udev = 1
event_activation = 1
udev_sync = 1

systemd-analyze blame: (top 9 items)
          20.809s lvm2-pvscan@134:544.service
          20.808s lvm2-pvscan@134:656.service
          20.808s lvm2-pvscan@134:528.service
          20.807s lvm2-pvscan@133:640.service
          20.806s lvm2-pvscan@133:672.service
          20.785s lvm2-pvscan@134:672.service
          20.784s lvm2-pvscan@134:624.service
          20.784s lvm2-pvscan@128:1008.service
          20.783s lvm2-pvscan@128:832.service

the same lvm config costed 2min 6.736s (could find this result from my previous mail).

and the shortest time in previous mail is 17.552s, under cfg:
obtain_device_list_from_udev=1, event_activation=0, udev_sync=1

the result (20.809s) is very close to direct activation, which is also reasonable.
(lvm first uses direct mode then switch to event mode)

Thanks,
Heming


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