Arg: no mdadm on Ubuntu 18.04.5 Desktop install disk?

[Robert Heller]

Arg! I want to install Ubuntu 18.04.5 on a LVM volume in the LVM container on
my *existing* CentOS 6.10 system. The LVM container is on a software RAID disk
(simple mirror set). This *appears* not to be possible (?) since the Ubuntu
installer is not starting/seeing the RAID array and in live mode, mdadm
appears absent (question: what happens if I try to run apt-get install mdadm
on a live ISO system?).

WTF? Does Ubuntu 18.04 in fact not support software RAID during installation?

How am I supposed to procede?

My goal is to *migrate* the system, which is a combo desktop / server for my
home LAN from (now EOL) CentOS 6.10 to Ubuntu 18.04, but I really don't want
to completely wipe the disk which is what the Ubuntu installer would like to
do, because it thinks I have a pair of nearly empty 1.5TB disks, when really,
there are three partitions on each: a VFAT one set aside for a UEFI partition
(now empty/not used since my current MB is old school BIOS, but I plan on
upgrading the MB at some time, so I'm guessing I will need a UEFI partition),
a small EXT4 RAID pair for /boot, and the rest for a large LVM RAID pair for
everything else. I want to keep the CentOS 6.10 system functional for awhile
while I get the Ubuntu 18.04 system configured with all of the bits and pieces
I need (bind9, dhcpd, OpenLDAP, KVM, and more). The disks, being 1.5TB, have
GPT partition tables.

PS: I also have the Ubuntu 18.04.5 live server/install disk -- should I try 
that instead?  Eg, does it have the RAID modules and mdadm in its init ramdisk 
(appearently, the stock Ubuntu 18.04.5 disktop/install disk does not)?

-- 
Robert Heller             -- Cell: 413-658-7953 GV: 978-633-5364
Deepwoods Software        -- Custom Software Services
http://www.deepsoft.com/  -- Linux Administration Services
heller at deepsoft.com       -- Webhosting Services