20G is probably too small. I run mine with a 50Gb LV for root, and seperate LV's for /home and /var/log, and you can see what I'm using. LMCE is virtualized using KVM, with LVM's passed through as block devices. /var/lib/mysql is an LV from an SSD, and the large log directory is for centralized syslogging, and verbose logging for dev/debugging work.
Filesystem Size Used Avail Use% Mounted on
rootfs 50G 36G 12G 76% /
none 787M 316K 787M 1% /dev
50G 36G 12G 76% /
/dev/vdc1 20G 1.8G 17G 10% /var/log
/dev/vde1 296G 2.9G 278G 2% /home
/dev/vdb1 12G 2.7G 8.6G 25% /var/lib/mysql
You have to remember, each MD is almost another install itself, averaging 2.5 G apiece.
Hope that helps!