[DRBD-user] Dual primary and LVM

Igor Cicimov igorc at encompasscorporation.com
Thu Jul 27 10:23:42 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.

On 27 Jul 2017 6:11 pm, "Gionatan Danti" <g.danti at assyoma.it> wrote:

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.

When in cluster mode LVM will not use local cache that's part of the
configuration you need to do during setup.

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
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Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
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