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Hi, From my point of view the problem is not so
related to what does the “bad node” is down but what happen when
communications are restored. Let me explain it. 1. Let’s start in a 2 nodes clean cluster,
each in a different site. Data duplication is done from the host using md or
lvm mirror. There is a service running on each node. Qdisk or third node in a
third site for quorum. 2. Communications are lost in site B (where
node B runs.). What happens?, not sure, but as my understanding: I-
Node B will continue working for some time until it realizes it’s not
quorated (depending on timeouts, let’s say 1 minute). Data writes on this
time are only written to Disks on site B, modifications not written to disks in
site A. II-
Finally, Node B detects it lost qdisk and detects it’s inquorate and
rgmanager stops all services running in node B. III-
In node A, time some time to detect A is dead and never will become inquorate. Services
running in node A will continue working, but writes will only be done to disks
in disks in site A. Mirror is lost. IV-
Finally, Node A detects node B is dead and will try to fence it (probably it
will need to use manual fence for confirmation). V-
Until fence is successful, services running originally in node B will not be
transferred to node A, so service will be never running simultaneously on both
nodes. VI-
After fence is successful, service starts in node A using disks in site A,
without any modification done since the outage until failure is deteced by node
B (from I to II). Data modification done from node A are only done to these
disks. 3. Communications are restored in site B. At
this time node B will join the cluster again. Acces to disks in site B is
recovered by node A. At this time mirror should be synchronized from disks in
site A to site B always, so that, we have a coherent view of the data in both
disks, and changes done from node B in the qdisk timeout (from I to II) will be
definitively lost. I think this the expected behavior for a
multisite cluster in this scenario. Best regards, Alfredo From:
linux-cluster-bounces@xxxxxxxxxx [mailto:linux-cluster-bounces@xxxxxxxxxx] On Behalf Of brem belguebli Hi, No comments on this RHCS gurus ? Am I trying to setup (multisite
cluster) something that 'll never be supported ? Or is the qdiskd reboot action considered as sufficient? (Reboot
action should be a dirty power reset to prevent data syncing) If so, all IO's on the wrong nodes (at the isolated site) should be
frozen untill quorum is eventually regained. If not it'll end up with a (dirty)
reboot. Brem 2009/8/21 brem belguebli <brem.belguebli@xxxxxxxxx> Hi, I'm trying to find out what best fencing solution could fit a dual
sites cluster. Cluster is equally sized on each site (2 nodes/site), each site hosting
a SAN array so that each node from any site can see the 2 arrays. Quorum disk (iscsi LUN) is hosted on a 3rd site. SAN and LAN using the same telco infrastructure (2 redundant
DWDM loops). In case something happens at Telco level (both DWDM loops are broken)
that makes 1 of the 2 sites completely isolated from the rest of the world, the nodes at the good site (the one still operationnal) won't be able
to fence any node from the wrong site (the one that is isolated) as there is no
way for them to reach their ILO's or do any SAN fencing as the switches at
the wrong site are no more reachable. As qdiskd is not reachable from the wrong nodes, they end up being
rebooted by qdisk, but there is a short time (a few seconds) during which
the wrong nodes are still seing their local SAN array storage and may
potentially have written data on it. Any ideas or comments on how to ensure data integrity in such setup ? Regards Brem |
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