[Linux-cluster] GFS2 as virtual machine disk store

[Gionatan Danti]

Hi list,
I am evaluating how to refresh my "standard" cluster configuration and 
GFS2 clearly is on the table ;)

GOAL: to have a 2-node HA cluster running DRBD (active/active), GFS2 (to 
store disk image) and KVM (as hypervisor). The cluster had to support 
live migration, but manual failover is sufficient (ie: if something goes 
wrong, is ok to require a sysadmin to take action to restore services).

The idea is to, by default, always run VMs on the first host (using 
virtlock or sanlock to deny the starting of the same virtual machine 
from the second host). Should anything bad happen, or should the first 
host be in maintenance mode, the VMs can be migrated/restarted on the 
second host.

I have a few questions:

- other peoples told me GFS2 is not well suited for such a tasks and 
that I am going to see much lower performance than running on a local 
filesystem (replicated via other means). This advice stems from the 
requirement to maintain proper write ordering, but strict cache 
coherency also between the hosts. However, from what I understand 
reading GFS2 documentation, when operating mostly on a single host (ie: 
not running anything on the second node), the overhead should be 
negligible. I am right, or orribly wrong?

- reading RedHat documentation here[1], I see that it is strongly 
advised to set cache=none for any virtual disk. Is this required from 
proper operation, or it is "only" a performance optimization to avoid 
what stated above (ie: two host sharing the same data in pagecache, thus 
requiring coherency traffic)? As I really like the improved performance 
with cache=writeback (which, by the virtue of barrier passing, comes 
without data loss concerns), you think it is safe to use writeback in 
production?

- I plan to have a volume of about 8 or 16 TB. I understand that GFS2 is 
tested with much bigger volumes (ie: 100 TB), but I would ask: do you 
would trust a TB-sized volume on GFS2? What about fsck? It works 
well/reliably?

- I plan to put GFS2 on top of LVM (for backup snapshot) and replicate 
the volume with DRBD2. Do you see any drawback in this approach?

- finally, how do you feel about running your production virtual 
machines on DRBD + GFS2?

Thank you all.

[1] 
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html-single/Global_File_System_2/index.html#s1-VMsGFS2-gfs2

-- 
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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