SailfishX 2.1.3.5 uses Linux kernel version 3.10.84 compiled without btrfs support.

Both root and home partitions are formatted with ext4 and are respectively 2488624 blocks (aka less than 2.5 Gb for root, leaving about one gigabyte free for system stuff like /var/log and for installed apps binaries and libraries) and 18853612 blocks (aka less than 19 Gb for your personal data and Android apps+storage).

This is different from the original Jolla phone where both root and home were on the same btrfs partition, sized 14415852 blocks (aka about 10-11 Gb user space including system files, apps, etc).

I don't plan to install a bazillion apps; once I have working maps, socials and a very few utilities, I'm OK (multimedia will go on a 32 Gb microSD); I think my XperiaX will reach its end of life without needing to expand the root partition.

SailfishX is working surprisingly well in spite of all those "you're paying 50 bucks for an incomplete beta quality system" rants. After a few months of Android 7 I feel really better.

For example, backupping the entire home directory in a single Linux command (needs ssh enable in SailfishX developer settings, I also set up a fixed IP address to have "xperiax" hostname in /etc/hosts):
Clearly Android doesn't have such a feature.

Note, I used /home/xperiax as target because my /home/nemo already contains Jolla 1 backup using basically the same rsync command and excluding the "privileged" directories.