[pve-devel] [PATCH v2 docs] Update pvecm doc regarding IP vs hostname as corosync address

[Stefan Reiter]

Signed-off-by: Stefan Reiter <s.reiter at proxmox.com>
---

v2:
 * Removed mentions of ring addresses to avoid confusion
   (removed in Corosync 3 - though the rest of the file should be
   updated for coro 3 too)
 * Changed section title
 * Added CAUTION: about hostnames, removed NOTE: about PVE tooling
 * Reworded according to feedback

Left the <<>> references as is, as Thomas mentioned about Aaron's rework.

 pvecm.adoc | 36 ++++++++++++++++++++++++++++++++++--
 1 file changed, 34 insertions(+), 2 deletions(-)

diff --git a/pvecm.adoc b/pvecm.adoc
index 61453f7..7525bb5 100644
--- a/pvecm.adoc
+++ b/pvecm.adoc
@@ -149,7 +149,8 @@ Login via `ssh` to the node you want to add.
  hp2# pvecm add IP-ADDRESS-CLUSTER
 ----
 
-For `IP-ADDRESS-CLUSTER` use the IP from an existing cluster node.
+For `IP-ADDRESS-CLUSTER` use the IP or hostname of an existing cluster node.
+An IP address is recommended (see <<corosync-addresses,Ring Address Types>>).
 
 CAUTION: A new node cannot hold any VMs, because you would get
 conflicts about identical VM IDs. Also, all existing configuration in
@@ -572,7 +573,8 @@ you do not see them already. Those *must* match the node name.
 
 Then replace the address from the 'ring0_addr' properties with the new
 addresses.  You may use plain IP addresses or also hostnames here. If you use
-hostnames ensure that they are resolvable from all nodes.
+hostnames ensure that they are resolvable from all nodes. (see also
+<<corosync-addresses,Ring Address Types>>)
 
 In my example I want to switch my cluster communication to the 10.10.10.1/25
 network. So I replace all 'ring0_addr' respectively. I also set the bindnetaddr
@@ -654,6 +656,36 @@ systemctl status corosync
 If corosync runs again correct restart corosync also on all other nodes.
 They will then join the cluster membership one by one on the new network.
 
+[[corosync-addresses]]
+Corosync addresses
+~~~~~~~~~~~~~~~~~~
+
+A corosync link or ring address can be specified in two ways:
+
+* **IPv4/v6 addresses** will be used directly. They are recommended, since they
+are static and usually not changed carelessly.
+
+* **Hostnames** will be resolved using `getaddrinfo`, which means that per
+default, IPv6 addresses will be used first, if available (see also
+`man gai.conf`). Keep this in mind, especially when upgrading an existing
+cluster to IPv6.
+
+CAUTION: Hostnames should be used with care, since the address they
+resolve to can be changed without touching corosync or the node it runs on -
+which may lead to a situation where an address is changed without thinking
+about implications for corosync.
+
+A seperate, static hostname specifically for corosync is recommended, if
+hostnames are preferred. Also, make sure that every node in the cluster can
+resolve all hostnames correctly.
+
+Since {pve} 5.1, while supported, hostnames will be resolved at the time of
+entry. Only the resolved IP is then saved to the configuration.
+
+Nodes that joined the cluster on earlier versions likely still use their
+unresolved hostname in `corosync.conf`. It might be a good idea to replace
+them with IPs or a seperate hostname, as mentioned above.
+
 [[pvecm_rrp]]
 Redundant Ring Protocol
 ~~~~~~~~~~~~~~~~~~~~~~~
-- 
2.20.1