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

Gionatan Danti g.danti at assyoma.it
Thu Oct 22 17:41:31 CEST 2015

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

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.

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.

Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
GPG public key ID: FF5F32A8

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