When life gives you temp space - make lemonade !

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Some VPS providers, e.g. Azure (I know..) provide you with 2 disks for your VPSes. One, of very limited size, system disk, and the other one, spacy but with not guarantees that the data survives reboot. Basically it means that you can have a small VPS, with a small amount of RAM but large temp disk space. Why this could be useful ? Imagine tasks with lots of mem requirements but that not need to be extra fast, where swapping is allowed. Like complex nightly builds. Here is a set of super simple scripts I've come up with to quickly boot up a system, and then in the background add a new swap file on the temp drive there. The temp drive is assumed to be under /mnt.

root@someazurehost:~# cat /etc/rc.local
#!/bin/sh -e
set -v

# do not wait for swap to become online,
# proceed with the boot further,
# with swap being created in the background
/etc/make_and_enable_swap &

exit 0
root@someazurehost:~# cat /etc/make_and_enable_swap
set -e
set -v
# create new 2GB swap file
dd if=/dev/zero of=/mnt/swap bs=1M count=2048
chmod 0600 /mnt/swap
mkswap /mnt/swap
swapon /mnt/swap

Don't forget to make /etc/make_and_enable_swap executable ! Do not add this swap file to fstab, as it is being read before rc.local, and this may certainly result in a boot failure, as the swap file would not be ready yet.

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