# Rootfs Compression
Compressing the root file system represents using an algorithm like `gzip`, `zstd`, etc. to reduce the size of the rootfs on disk.
This has the added benefit of reducing the time it takes to push the image to the Unikraft Cloud platform, as you transfer less data.
It also means that the size on disk on the host side reduces, which can be beneficial when dealing with many images or large images.
This size can drop depending on the contents of the rootfs and the compression algorithm used.
For example, text files often compress by up to 90% and more, whilst already compressed files like images or videos don't compress well at all.
## Getting started
The Unikraft Platform supports both compressed and uncompressed rootfses, but it's highly recommended to use uncompressed ones if size allows.
Decompressing is a step done by the kernel at boot time, resulting in slower access times across the board.
That's why the first attempt at using the platform should always be with uncompressed rootfses.
Both formats, [`CPIO` and `EROFS`](/tutorials/rootfs-formats), support compression.
`CPIO` does full file system compression (like running `zip` on the file).
`EROFS` does per-block compression meaning that the system decompresses only the accessed blocks on-the-fly.
Both of them support most compression algorithms, but the recommended ones are those that have fast decompression speeds like `lz4hc`.
As presented in the [rootfs formats tutorial](/tutorials/rootfs-formats) and in the [FAQ](/faq), the performance characteristics of the two formats differ.
When using compression, the system amplifies these differences further.
For `CPIO`, the system must decompress the entire archive first before using it.
This means that boot times increase further, and memory usage during boot is also higher negating the benefit of using compression in the first place.
`EROFS` is better, as the system decompresses only the accessed blocks on-the-fly, meaning that boot times are less affected, and memory usage is also kept lower.
This has the added downside that Direct Access (`DAX`) isn't possible when using compression, as the host side can't map a compressed image into guest memory.
This caveat results in having to load the entire compressed rootfs into guest memory, increasing memory usage by at least the size of the compressed rootfs.
Still, if many files aren't accessed in boot, the cold boot time might decrease compared to an uncompressed `EROFS` rootfs.
## Packaging a Compressed rootfs
Packaging a compressed rootfs is different depending on the format used.
The legacy [`kraft` CLI](https://unikraft.org/docs/cli/install) implements `CPIO` compression directly, while `EROFS` compression must happen externally using the `mkfs.erofs` tool.
### CPIO Compression
To package a compressed `CPIO` rootfs using the legacy CLI, you can use the `--compress` flag when running the `kraft pkg` or `kraft cloud deploy` command.
This uses `gzip` compression to compress the initrd file before packaging it into a `OCI` image and uploading it to the Unikraft Cloud platform.
```bash title="kraft"
kraft cloud deploy \
--rootfs-type cpio \
--compress \
...
```
### EROFS Compression
Currently, the legacy CLI has no direct support for compressing `EROFS` rootfses.
To package a compressed `EROFS` rootfs, you need to use the `mkfs.erofs` tool before packaging it with `kraft`.
To do this you will also have to first export the image built from the `Dockerfile` into a `tar` archive:
```bash
docker build -o type=tar,dest=rootfs -t my-docker-image -f Dockerfile .
```
You can then use the resulting `tar` archive to create a compressed `EROFS` rootfs using the following command.
Note that you can change the compression algorithm and level by modifying the `-z` option.
In this example the level is `9` (maximum) and the algorithm is `lz4hc` (high compression lz4):
```bash
mkfs.erofs --all-root -b 4096 -d2 -E noinline_data -z lz4hc,9 rootfs.erofs --tar=rootfs rootfs.tar
```
Finally, you can package and deploy the compressed EROFS rootfs using the legacy CLI:
```bash title="kraft"
kraft cloud deploy \
--rootfs-type erofs \
--rootfs ./rootfs.erofs \
...
```
## Performance of Compressed rootfs vs. Uncompressed rootfs Images
To illustrate the performance differences between compressed and uncompressed rootfses, below you can see the same [`node-22` image](https://github.com/unikraft-cloud/examples/tree/main/node-playwright-chromium) packaged and deployed four times:
1. Uncompressed `CPIO`
```ansi title="Uncompressed CPIO"
[90m[[0m[92m●[0m[90m][0m Deployed successfully!
[90m│[0m
[90m├[0m[90m───────[0m [90mname[0m: node-playwright-chromium-gjv31
[90m├[0m[90m───────[0m [90muuid[0m: 7ef181a3-8264-4418-82e9-4690ed75a030
[90m├[0m[90m──────[0m [90mmetro[0m: fra
[90m├[0m[90m──────[0m [90mstate[0m: [92mrunning[0m
[90m├[0m[90m─────[0m [90mdomain[0m: https://dark-feather-7drf2jvp.fra.unikraft.app
[90m├[0m[90m──────[0m [90mimage[0m: demo/node-playwright-chromium@sha256:eafb34e104efe139234f5aa753f3d380afea2c5b78a3011ccd2017a25a5172c0
[90m├[0m[90m──[0m [90mboot time[0m: 621.28 ms
[90m├[0m[90m─────[0m [90mmemory[0m: 2400 MiB
[90m├[0m[90m────[0m [90mservice[0m: dark-feather-7drf2jvp
[90m├[0m[90m─[0m [90mprivate ip[0m: 10.0.1.233
[90m└[0m[90m───────[0m [90margs[0m: /usr/bin/wrapper.sh /usr/bin/node /app/server.js
```
1. Compressed `CPIO`
```ansi title="Compressed CPIO"
[90m[[0m[92m●[0m[90m][0m Deployed successfully!
[90m│[0m
[90m├[0m[90m───────[0m [90mname[0m: node-playwright-chromium-bt6r9
[90m├[0m[90m───────[0m [90muuid[0m: 1ebc4ee0-2fb1-4272-81d5-7adc76bfb566
[90m├[0m[90m──────[0m [90mmetro[0m: fra
[90m├[0m[90m──────[0m [90mstate[0m: [92mrunning[0m
[90m├[0m[90m─────[0m [90mdomain[0m: https://old-cloud-em09h6ue.fra.unikraft.app
[90m├[0m[90m──────[0m [90mimage[0m: demo/node-playwright-chromium@sha256:ac6cde1c766162a3c1bc376985c2442eafacd5b18b26b599a25d3095c0a5f5ed
[90m├[0m[90m──[0m [90mboot time[0m: 2079.74 ms
[90m├[0m[90m─────[0m [90mmemory[0m: 2400 MiB
[90m├[0m[90m────[0m [90mservice[0m: old-cloud-em09h6ue
[90m├[0m[90m─[0m [90mprivate ip[0m: 10.0.1.233
[90m└[0m[90m───────[0m [90margs[0m: /usr/bin/wrapper.sh /usr/bin/node /app/server.js
```
1. Uncompressed `EROFS`
```ansi title="Uncompressed EROFS"
[90m[[0m[92m●[0m[90m][0m Deployed successfully!
[90m│[0m
[90m├[0m[90m───────[0m [90mname[0m: node-playwright-chromium-o4ds4
[90m├[0m[90m───────[0m [90muuid[0m: 967b66fd-f71c-4b2b-8982-88e4c0373d02
[90m├[0m[90m──────[0m [90mmetro[0m: fra
[90m├[0m[90m──────[0m [90mstate[0m: [92mrunning[0m
[90m├[0m[90m─────[0m [90mdomain[0m: https://damp-water-8ubacykw.fra.unikraft.app
[90m├[0m[90m──────[0m [90mimage[0m: cezar.unikraft.io/node-playwright-chromium@sha256:65bf231a557a0dd938592d0fd67f4d5f5e83c0df1ca27b8f8d8675253138ae4b
[90m├[0m[90m──[0m [90mboot time[0m: 121.75 ms
[90m├[0m[90m─────[0m [90mmemory[0m: 900 MiB
[90m├[0m[90m────[0m [90mservice[0m: damp-water-8ubacykw
[90m├[0m[90m─[0m [90mprivate ip[0m: 10.0.4.1
[90m└[0m[90m───────[0m [90margs[0m: /usr/bin/wrapper.sh /usr/bin/node /app/server.js
```
1. Compressed `EROFS`
```ansi title="Compressed EROFS"
[90m[[0m[92m●[0m[90m][0m Deployed successfully!
[90m│[0m
[90m├[0m[90m──────[0m [90mname[0m: node-playwright-chromium-9yb9y
[90m├[0m[90m──────[0m [90muuid[0m: d7255b2c-394c-4578-a21b-2f879f823cd9
[90m├[0m[90m─────[0m [90mmetro[0m: fra
[90m├[0m[90m─────[0m [90mstate[0m: [92mrunning[0m
[90m├[0m[90m────[0m [90mdomain[0m: https://damp-dream-rpmyh0h0.fra.unikraft.app
[90m├[0m[90m─────[0m [90mimage[0m: cezar.unikraft.io/node-playwright-chromium@sha256:7a93fbfa414eb958cd4f61b9cbe4874eb4fe6836e74fb292bca3e2927df71a88
[90m├[0m[90m─[0m [90mboot time[0m: 92.36 ms
[90m├[0m[90m────[0m [90mmemory[0m: 900 MiB
[90m├[0m[90m───[0m [90mservice[0m: damp-dream-rpmyh0h0
[90m├[0m [90mprivate ip[0m: 10.0.3.253
[90m└[0m[90m──────[0m [90margs[0m: /usr/bin/wrapper.sh /usr/bin/node /app/server.js
```
### Results
| rootfs Type | Compression | Image Size | Boot Time | Instance Memory Usage |
|---------------|-------------|------------|------------|-----------------------|
| CPIO | No | 824 MiB | 621.28 ms | 2400 MiB |
| CPIO | Yes (gzip) | 299 MiB | 2079.74 ms | 2400 MiB |
| EROFS | No | 830 MiB | 121.75 ms | 900 MiB |
| EROFS | Yes (lz4hc) | 410 MiB | 92.36 ms | 900 MiB |
As expected from the previous assumptions, the compressed CPIO rootfs boots slower than the uncompressed one.
This happens due to the need to decompress the entire archive before use.
EROFS performs better in both compressed and uncompressed forms, compared to CPIO.
The compressed EROFS rootfs boots even faster than the uncompressed one.
This is likely due to the reduced amount of data that the system reads from disk, resulting in faster access times despite the decompression overhead.
This only happens in the cold boot scenario.
In [warm boots](/features/snapshots), the uncompressed EROFS rootfs tops the charts as the system needs no decompression at all on demand.
## Conclusion
As a rule of thumb, use uncompressed rootfses whenever possible and try to always use `EROFS` over `CPIO` for better performance.
If limited by size constraints, try using compressed `EROFS` rootfses with a fast decompression algorithm like `lz4hc`.
## Learn more
* The [rootfs formats tutorial](/tutorials/rootfs-formats) for understanding the differences between `CPIO` and `EROFS`.
* The [`mkfs.erofs` documentation](https://man.archlinux.org/man/extra/erofs-utils/mkfs.erofs.1.en) for more information on creating compressed `EROFS` file systems.
* The [CLI reference](/docs/cli/unikraft) and the [legacy CLI reference](/docs/cli/kraft/overview).
* The `kraft pkg` [command reference](https://unikraft.org/docs/cli/reference/kraft/pkg) for packaging images.