Raid zim

# I couldn’t fit this 37GB Wikipedia dump (English with no images)
# <magnet:?xt=urn:btih:267d148412ad50361e0c0384c905fabe35e8539a&dn=wikipedia%5Fen%5Fall%5Fnopic%5F2018-09.zim>
# onto my flash disk because VFAT doesn’t support large files.  So, I
# thought, I’d stick it on an ext4 filesystem in a disk image file on
# the VFAT, thus circumventing all of VFAT’s restrictions.  Not so
# fast — that file can’t be 4GB or over either.  So what to do?  We
# can take advantage of Linux software RAID:

set -e
DANGER() {
         :
}

: ${usb?} ${user?} # ensure the $usb and $user variables are set
# This first part takes about 40 minutes because VFAT doesn’t support
# sparse files either:
DANGER time truncate -s 2g "$usb"/wikipedia-zim.img.{1..20}
for i in {1..20}; do sudo losetup "/dev/loop$i" "$usb"/wikipedia-zim.img."$i"; done
DANGER sudo time mdadm --create /dev/md0 --level 5 --force --raid-devices=20 /dev/loop{1..20}
# 0.00user 0.02system 0:33.79elapsed 0%CPU (0avgtext+0avgdata 2592maxresident)k
# This seems to work for subsequent mounting but never the first time:
sudo time mdadm --assemble /dev/md0 /dev/loop{1..20}
DANGER time sudo mkfs -t ext4 /dev/md0
# real	2m24.296s
DANGER sudo tune2fs -r 0 /dev/md0   # no root-reserved blocks; frees up about 2GB
sudo mount /dev/md0 /wikipedia-zim/
df -h
# Filesystem                 Size  Used Avail Use% Mounted on
# /dev/md0                    38G   48M   38G   1% /wikipedia-zim
# However, Transmission reports 39.9 GB.
DANGER sudo chown "$user"."$user" /wikipedia-zim/


remove_raid() {
    sudo time umount /wikipedia-zim/
    sudo mdadm --stop /dev/md0
    sudo losetup -D
    umount "$usb"
}