Re: [RFC] Reliable Multicast on top of RTRS

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On 2020-09-09 04:42, Danil Kipnis wrote:
> On Fri, Sep 4, 2020 at 5:33 PM Bart Van Assche <bvanassche@xxxxxxx> wrote:
>> On 2020-09-04 04:35, Danil Kipnis wrote:
>>> On Thu, Sep 3, 2020 at 1:07 AM Bart Van Assche <bvanassche@xxxxxxx> wrote:
>>>> How will it be guaranteed that the resulting software does
>>>> not suffer from the problems that have been solved by the introduction
>>>> of the DRBD activity log
>>>> (https://www.linbit.com/drbd-user-guide/users-guide-drbd-8-4/#s-activity-log)?
>>>
>>> The above would require some kind of activity log also, I'm afraid.
>>
>> How about collaborating with the DRBD team? My concern is that otherwise
>> we will end up with two drivers in the kernel that implement block device
>> replication between servers connected over a network.
> 
> I have two general understanding questions:
> - What is the conceptual difference between DRBD and an md-raid1 with
> one local leg and one remote (imported over srp/nvmeof/rnbd)?

I'm not sure there is a conceptual difference. But there will be a big
difference in recovery speed after a temporary network outage (assuming that
the md-raid write intent bitmap has been disabled).

> - Is this possible to setup an md-raid1 on a client sitting on top of
> two remote DRBD devices, which are configured in "active-active" mode?

I don't think that DRBD supports this. From the DRBD source code:
"this code path is to recover from a situation that "should not happen":
concurrent writes in multi-primary setup."

Bart.



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