Ubuntu – How to zerofill the /swapfile without wiping out what makes it a swapfile

swapUbuntu

I have a script that I run in order to optimize the disk space prior to backing up a virtual guest (really doesn't matter too much which hypervisor is used).

Modern Ubuntu versions (e.g. 18.04) seem to default to using a /swapfile rather than a partition for the purpose. That's fine, too.

However, I'd like to zerofill the swap file along with the root partition containing it.

With swap partitions this was relatively painless as there was always a way to extract the existing $UUID from the swap partition and – after zerofilling the partition – run mkswap -U $UUID to re-create said swap partition.

However, with the /swapfile I don't see how to do that. While I realize that you can't mount a /swapfile by UUID in /etc/fstab, I'd still want to retain the UUID.

So I reckon need one of the following to proceed:

I need a method that can zerofill the swap file without also wiping those structures that make it a swap partition (i.e. avoid having to run mkswap -U $UUID)
I need a method to retrieve the existing UUID from a swap file (as opposed to a partition where I can use anything from blkid to looking at /dev/disk/by-uuid/$UUID …)

Best Answer

This won’t clear the swap file as thoroughly as re-creating it, but if you clear it after skipping the first 4KiB (strictly speaking, the first page, which depends on your architecture), you won’t touch any of the structures which identify a swap file.

blkid works fine on swap files too:

$ mkswap swap
Setting up swapspace version 1, size = 512 MiB (536866816 bytes)
no label, UUID=7916b81f-1faa-4b7d-84ef-b0bf2f75dbbc

$ blkid swap
swap: UUID="7916b81f-1faa-4b7d-84ef-b0bf2f75dbbc" TYPE="swap"

The header format is defined in the kernel: the old format has a magic value at the end of the first page, the new format combines that with a 1KiB free area, then a number of fields (version, size, bad pages, UUID, label) which all fit comfortably inside the first page.

Related Solutions

Linux Encryption – How to Set Up Encrypted Swap File in Linux

Indeed, the page describes setting up a partition, but it's similar for a swapfile:

dd if=/dev/urandom of=swapfile.crypt bs=1M count=64
loop=$(losetup -f)
losetup ${loop} swapfile.crypt
cryptsetup open --type plain --key-file /dev/urandom ${loop} swapfile
mkswap /dev/mapper/swapfile
swapon /dev/mapper/swapfile

The result:

# swapon -s
Filename                                Type            Size    Used    Priority
/dev/mapper/swap0                       partition       4000176 0       -1
/dev/mapper/swap1                       partition       2000084 0       -2
/dev/mapper/swapfile                    partition       65528   0       -3

swap0 and swap1 are real partitions.

Debian – How to Safely Turn Off Swap Permanently and Reclaim Space

If you have GParted open, close it. Its Swapoff feature does not appear to to be permanent.
Open terminal and become root (su); if you have sudo enabled, you may also do for example sudo -i; see man sudo for all options):
```
 sudo -i
```
Turn off the particular swap partition and / or all of the swaps:
```
 swapoff --all
```
Make 100% sure the particular swap partition partition is off:
```
 cat /proc/swaps
```
Open a text editor you are skilled in with this file, e.g. nano if unsure:
```
 nano /etc/fstab
```

Comment out / remove the swap partition's UUID, e.g.:

 # UUID=1d3c29bb-d730-4ad0-a659-45b25f60c37d    none    swap    sw    0    0

Open a text editor you are skilled in with this file, e.g. nano if unsure:
```
 nano /etc/initramfs-tools/conf.d/resume
```
Comment out / remove the previously identified swap partition's UUID, e.g.:
```
 # RESUME=UUID=1d3c29bb-d730-4ad0-a659-45b25f60c37d
```
Don't close the terminal as you will need it later anyway.

Note: The next steps differ depending on, whether you rely on CLI or GUI.

GUI:

Open up GParted, either from menu, or more conveniently from the terminal we have opened:
```
 gparted
```
If you don't have it installed, you may do so; afterwards run the previous command again:
```
 apt-get install gparted
```
Choose your drive from top-right menu.
As the GParted reactivates the swap partition upon launch, you will have to right-click the particular swap partition and click Swapoff -> This will be applied immediately.
Delete the swap partition with right click -> Delete. You must apply the change now.
Resize your main / other partition with right click -> Resize/Move. You must apply the change now.
Back to the terminal, let's recreate the boot images:
```
 update-initramfs -u -k all
```
Update GRUB:
```
 update-grub
```
You may reboot now if you wish to test that the machine boots up.

Encryption note: If your swap partition is encrypted, then you also need to comment out the related line in /etc/crypttab, otherwise CryptSetup will keep you waiting for 90 seconds during boot time. Thanks frank for this addition.

CLI:

I will check in VMs if my solution works, then I will share it. In the meantime, see this answer.

Best Answer

Related Solutions

Linux Encryption – How to Set Up Encrypted Swap File in Linux

Debian – How to Safely Turn Off Swap Permanently and Reclaim Space

Related Question