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
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