* Pacemaker can be monitored via SNMP; https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/high_availability_add-on_reference/s1-snmpandpacemaker-HAAR
Changes made using tools such as nmcli do not require a reload but do require the associated interface to be put down and then up again. That can be done by using commands in the following format:
* nmcli dev disconnect interface-name
Followed by:
* nmcli con up interface-name
NOTE: RHEL doesn't support direct-cabled bonds - https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/networking_guide/ch-configure_network_bonding
ifcfg-X config Notes - /usr/share/doc/initscripts-*/sysconfig.txt (Look for the sections describing files /etc/sysconfig/network and /etc/sysconfig/network-scripts/ifcfg-<interface-name>);
* PREFIXx overrules NETMASKx. Use PREFIXx, not NETMASKx.
* The 'x' suffice for PREFIX, NETMASK, etc start at 0 and must count up by 1 at a time.
* ZONE will be useful for the firewall stuff later.
* ETHTOOL_OPTS is deprecated, replaced by using udev rules
* initscripts interpret PEERDNS=no to mean "never touch resolv.conf". NetworkManager interprets it to say "never add automatic (DHCP, PPP, VPN, etc.) nameservers to resolv.conf".
Bond
* resend_igmp & num_unsol_na={1~255} may help if a switch is slow to notice traffic has moved to the new interface. default is 1. Each update is send 200ms apart.
* Bridged interfaces should use BRIDGE_UUID="", _not_ BRIDGE="". The former causes the later to be ignored and the later is only used for possible compatibility reasons.
* resources can contain an US-ASCII character, except for spaces
* A resource is a single replication stream for 1 or more resources, max 65.535 vols per resource
* DRBD does, however, ship with an LVM integration facility that automates the creation of LVM snapshots immediately before synchronization. This ensures that a consistent copy of the data is always available on the peer, even while synchronization is running. See Using automated LVM snapshots during DRBD synchronization for details on using this facility.
* Checksum-based synchronization computes a block's hash on source and target and skips if matching, possibly making resync much faster for blocks rewritten with the same data, but at the cost of CPU. Make this a user-configurable option under the advanced tab.
* Suspended replication allows congested replication links to suspend replication, leaving the peer in a consistent state, but allowing the primary to "pull ahead". When the congestion passes, the delta resyncs. Make this a user-configurable option with scary warnings.
* Online verification can (should?) be run periodically on the server host (verification source will overwrite deltas on the verification target). Perhaps schedule to run once/month? Do resource sequentially as this places a CPU load on the nodes.
* Replication traffic integrity checking uses a given available kernel crypto to verify data integrity on transmission to the peer. If the replicated block can not be verified against the digest, the connection is dropped and immediately re-established; because of the bitmap the typical result is a retransmission.
** Make an option in the advanced tab. Test to see overhead this adds. Choose the lowest overhead algo (within reason)
* Support for disk flushes might be something we want to disable, as it seems to force write-through even with a function FBWC/BBU. Need to test.
* Note; "Inconsistent" is almost always useless. "Consistent" and "Outdated" are able to be used safely, just without whatever happened on the peer after.
* Truck based replication, also known as disk shipping, is a means of preseeding a remote site with data to be replicated, by physically shipping storage media to the remote site.
* Make sure that selinux doesn't block DRBD comms over the SN
* See "5.15.1. Growing on-line" for growing a DRBD resource
** Shrinking online is ONLY possible if the metadata is external. Worth creating *_md LVs? Offline requires backing up and restoring the MD
Provisioning a server will need to:
* Open up a DRBD port (or more, if multiple resources are created).
* Create the DRBD resource(s); Find the lowest free rX.res, create it locally and on the peer (if up),
* A resource agent receives all configuration information about the resource it manages via environment variables. The names of these environment variables are always the name of the resource parameter, prefixed with OCF_RESKEY_. For example, if the resource has an ip parameter set to 192.168.1.1, then the resource agent will have access to an environment variable OCF_RESKEY_ip holding that value.
