Thanks Chris Murphy for noting that btrfs can create and mount 8 EB (approx 263 byte) filesystems effortlessly:
$ nbdkit -fv memory size=$(( 2**63-1 ))
# modprobe nbd
# nbd-client -b 512 localhost /dev/nbd0
# blockdev --getss /dev/nbd0
512
# gdisk /dev/nbd0
Number Start (sector) End (sector) Size Code Name
1 2048 18014398509481948 8.0 EiB 8300 Linux filesystem
# mkfs.btrfs -K /dev/nbd0p1
btrfs-progs v4.16
See http://btrfs.wiki.kernel.org for more information.
Detected a SSD, turning off metadata duplication. Mkfs with -m dup if you want to force metadata duplication.
Label: (null)
UUID: 770e5592-9055-4551-8416-b6376802a2ad
Node size: 16384
Sector size: 4096
Filesystem size: 8.00EiB
Block group profiles:
Data: single 8.00MiB
Metadata: single 8.00MiB
System: single 4.00MiB
SSD detected: yes
Incompat features: extref, skinny-metadata
Number of devices: 1
Devices:
ID SIZE PATH
1 8.00EiB /dev/nbd0p1
# mount /dev/nbd0p1 /tmp/mnt
# df -h /tmp/mnt
Filesystem Size Used Avail Use% Mounted on
/dev/nbd0p1 8.0E 17M 8.0E 1% /tmp/mnt
I created a few files in there and it all seemed to work although I didn’t do any extensive testing. Good job btrfs!
Richard WM Jones 
You can also mount -o compress=zstd to get read-write mounted compressed file systems, just tested this. Soon we’ll have a compression level switch for zstd, like for zlib in recent kernels. Separately compressing with zstd (or xz) does do a better job because everything including fs metadata is compressed, whereas doing it with Btrfs compression support only compresses data extents. Also, Btrfs tends to give up easily and not compress some extents, unless using compress-force= option. But yeah, might be one way to gain some compression savings without adding extra layers.
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