[DRBD-user] Dual primary and LVM

Gionatan Danti g.danti at assyoma.it
Thu Jul 27 10:11:48 CEST 2017

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

Il 27-07-2017 09:38 Gionatan Danti ha scritto:
> Thanks for your input. I also read your excellent suggestions on link
> Igor posted.

To clarify: the main reason I am asking about the feasibility of a 
dual-primary DRBD setup with LVs on top of it is about cache coherency. 
Let me do a step back: the given explaination for deny even read access 
on a secondary node is of broken cache coherency/consistency: if the 
read/write node writes something the secondary node had previously read, 
the latter will not recognize the changes done by the first node. The 
canonical solution to this problem is to use a dual-primary setup with a 
clustered filesystem (eg: GFS2) which not only arbitrates write access, 
but maintains read cache consistency also.

Now, let's remove the clustered filesystem layer, leaving "naked" LVs 
only. How read cache coherency is mantained in this case? As no 
filesystem is layered on top of the raw LVs, there is not real pagecache 
at work, but the kernel's buffers remains - and they need to be made 
coherents. How DRBD achieves this? Does it update the receiving kernel 
I/O buffers each time the other node writes something?


Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
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