[Pacemaker] Migration of "lower" resource causes dependent resources to restart

[Florian Haas]

On Thu, Mar 29, 2012 at 8:35 AM, Andrew Beekhof <andrew at beekhof.net> wrote:
> On Thu, Mar 29, 2012 at 5:28 PM, Vladislav Bogdanov
> <bubble at hoster-ok.com> wrote:
>> Hi Andrew, all,
>>
>> Pacemaker restarts resources when resource they depend on (ordering
>> only, no colocation) is migrated.
>>
>> I mean that when I do crm resource migrate lustre, I get
>>
>> LogActions: Migrate lustre#011(Started lustre03-left -> lustre04-left)
>> LogActions: Restart mgs#011(Started lustre01-left)
>>
>> I only have one ordering constraint for these two resources:
>>
>> order mgs-after-lustre inf: lustre:start mgs:start
>>
>> This reminds me what have been with reload in a past (dependent resource
>> restart when "lower" resource is reloaded).
>>
>> Shouldn't this be changed? Migration usually means that service is not
>> interrupted...
>
> Is that strictly true?  Always?

No. Few things are always true. :) However, see below.

> My understanding was although A thinks the migration happens
> instantaneously, it is in fact more likely to be pause+migrate+resume
> and during that time anyone trying to talk to A during that time is
> going to be disappointed.

I tend to be with Vladislav on this one. The thing that most people
would expect from a "live migration" is that it's interruption free.
And what allow-migrate was first implemented for (iirc), live
migrations for Xen, does fulfill that expectation. Same thing is true
for live migrations in libvirt/KVM, and I think anyone would expect
essentially the same thing from checkpoint/restore migrations where
they're available.

So I guess it's reasonable to assume that if one resource migrates,
dependent resources need not be restarted. But since Pacemaker now
does restart them, you might need to figure out a way to preserve the
existing functionality for users who rely on that. Not sure if any do,
though.

Cheers,
Florian

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