[DRBD-user] DRBD Dual Primary (writable/writeble) setup over VDSL WAN links

Digimer lists at alteeve.ca
Fri Oct 23 08:23:20 CEST 2015

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On 22/10/15 11:41 AM, Gionatan Danti wrote:
> Hi list,
> I have a question about the feasibility of a dual primary setup over two
> VDSL WAN links. I already searched the list and I found similar
> question, but without having a definitive conclusion.

No, it is not feasible.

Your cluster will block every time the network faults and will stay
blocked because the only path to fence the peer will go down with the WAN.

This is to say nothing about performance issues.

> My goal is to have a shared fileserver over two distant offices, using
> DRBD to create the illusion of a single block devices. I plan to use
> GFS2 (or OCFS) filesystem, exporting it to users (on both side) via
> Samaba shares.
> Please note that:
> 1) both server are CentOS 6.x x86_64 machines, with DRBD packages
> provided by ElRepo (DRBD 8.4.x)
> 2) the main and remote offices are connected with relatively fast (~100
> Mb/s) but high latency (+30 ms RTT) VDSL links
> 3) write speed should be as high as possible
> My current understanding (correct me if wrong) is that:
> 1) to not impair write speed, I should use protocol A
> 2) GFS2 (or any cluster filesystem) is quite sensitive to latency, so
> some operation (eg: metadata mangling, locking, reading an alredy
> remote-opened files) will be slowed down anyway
> 3) all will work (more or less) fine unless the WAN/VPN goes down.
> It is the "WAN failed" scenario that puzzle me. Let me do some questions:
> 1) at the block/device level, will both hosts (being both primary nodes)
> continue to accept writes? If so, will they end in a "split brain"
> scenario, right?
> 2) will the higher level filesystem (eg: GFS2) will block until the
> remote connection is re-established?
> 3) what will happens at connection recover?
> I understand that I can try myself (and I am going to do it, indeed),
> however I can only test on a fast LAN, not on a (relatively) slow WAN link.
> Thank you all.

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