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library: Enable transparent compression of btrfs file systems

Sunil Mohan Adapa requested to merge sunilmohan/freedom-maker:btrfs-compression into master

  • Reduces write amplification on flash based storage.

  • Improve read/write performance on slow disks.

  • Reduce the disk usage of files.

  • Uses more CPU and reduces overall throughput but this is not noticeable in most common scenarios.

  • It is possible to compress the files while the image is being prepared. This leads to significantly less size occupied by files (~1.7GiB uncompressed vs. ~0.8GiB compressed) in the final system. However, the size of the final xz image has doubled to more than 1GiB. This happens even when highest compression level (zstd:15) is used in btrfs/zstd. Due to concern for increased download image size, this change is not pursued at the moment.

Tests:

  • Build amd64, arm64 and raspberry3b-plus image with the patch. Build is successful.

  • In the built images, /etc/fstab has compress=zstd:1 only for btrfs partitions.

  • Run amd64 image. It boots without issues.

  • Install btrfs-compsize package inside the running image. Reinstall a package like freedombox-doc-en. Notice that running compsize /usr/share/freedombox now shows that some of the files are now compressed.

Signed-off-by: Sunil Mohan Adapa sunil@medhas.org

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