Thanks so much, @avayax, that is very helpful.
To summarize and confirm, we should expect two outage windows, both caused by HA failover between master and secondary nodes: first at the end of the distro upgrade, and second halfway through the incremental upgrade.
Just two questions remain then:
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The wiki says we must upgrade to FreePBX 13 before running the distro upgrade to 10.13.66, does that incur additional downtime? The wiki isn’t explicit about this. (@GSnover, it sounds like the FreePBX upgrade must be performed before the distro upgrade.)
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We’re upgrading in part to mitigate the glibc vulnerability described in CVE-2015-7547, for which FreePBX hasn’t released a patch for the 6.5 distro, only for 6.6. Does anyone (perhaps @tonyclewis) know whether we’ll receive the patched glibc package during the distro upgrade to 10.13.66-1, or whether the non-vulnerable glibc is packaged with one of the later incremental upgrades on the 10.13.66 track? If the latter case is true, I think another reboot may be required after the incremental upgrades to ensure full protection against CVE-2015-7547. RHEL describes a way to restart processes still using the old glibc libraries without fully rebooting, but this is labeled a “temporary workaround” that is “not supported”. I’m inclined to heed their advice, as FreePBX is ultimately RHEL under the hood!