[DRBD-user] DRBD Dual Primary + GFS2 for redundant KVM hosts

Gionatan Danti g.danti at assyoma.it
Fri Aug 25 08:49:45 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.


Hi list,
in my endless search for information :) I was testing DRBD in dual 
primary mode + GFS2. The goal is "the same": running a replicated, 
2-node KVM setup where disk images are located on the GFS2 filesystem.

I know how to integrate DRBD with corosync/pacemaker, but I wonder how 
the system will perform under load. More specifically, it is my 
understanding that GFS2 inevitably has some significan overhead compared 
to a traditional, one-node filesystem, and this can lead to decreased 
performance.

This should be especially true when restarting (or live-migrate) a 
virtual machine on the other host: as the first node has cached 
significan portion of the vm disk, GFS2 will, on every first read on the 
new host, fire its coherency protocol to be sure to update the first's 
node in-memory cached data. The overhead should be lowered by not using 
cache at all (ie: cache=none, which implies O_DIRECT), but this will 
also case degraded peformance in the common case (ie: when all is 
working correctly and VMs run on the first node only).

So I ask: has some of you direct experience with a similar setup? How do 
you feel about? Any other suggestions?
Thanks.

-- 
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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