Hi,
On 26/08/17 07:11, Gionatan Danti wrote:
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?
Yes, there is some additional overhead due to the clustering. You can
however usually organise things so that the overheads are minimised as
you mentioned above by being careful about the workload.
- 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?
No. You want to use the default data=ordered for the most part. It is
less a question of data loss and more a question of whether in case of a
power outage it is possible for a file being written to, to land up with
incorrect content. That can happen in the data=writeback case (where
block allocation has succeeded, but the new data has not yet been
written to disk) but not in the data=ordered case.
- 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?
Yes, it works well. The size limit was based on fsck time, rather than
any reliability issues. It will work reliably at much larger sizes, but
it will take longer and use more memory.
I hope that answers a few more of your questions,
Steve.
- 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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