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

Digimer lists at alteeve.ca
Fri Aug 25 14:34:50 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 2017-08-25 02:49 AM, Gionatan Danti wrote:
> 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.

Our Anvil! project (https://www.alteeve.com/w/Build_an_m2_Anvil!) is
basically this, except we put the VMs on clustered LVs and use gfs2 to
store install media and the server XML files.

I would NOT put the image files on gfs2, as the distributed locking
overhead would hurt performance a fair bit.


-- 
Digimer
Papers and Projects: https://alteeve.com/w/
"I am, somehow, less interested in the weight and convolutions of
Einstein’s brain than in the near certainty that people of equal talent
have lived and died in cotton fields and sweatshops." - Stephen Jay Gould



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