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### Understand the alert
This alert is triggered when the percentage of used Btrfs data space exceeds the configured threshold. Btrfs (B-tree file system) is a modern copy-on-write (CoW) filesystem for Linux which focuses on fault tolerance, repair, and easy administration. This filesystem also provides advanced features like snapshots, checksums, and multi-device spanning.
### What does high Btrfs data usage mean?
High Btrfs data usage indicates that a significant amount of the allocated space for data blocks in the filesystem is being used. This could be a result of many factors, such as large files, numerous smaller files, or multiple snapshots.
### Troubleshoot the alert
Before you attempt any troubleshooting, make sure you have backed up your data to prevent potential data loss or corruption.
1. **Add more physical space**: You can add a new disk to the filesystem, depending on your infrastructure and disk RAID configuration. Remember to unmount the drive if it's already mounted, then use the `btrfs device add` command to add the new disk and balance the system.
2. **Delete snapshots**: Review the snapshots in your Btrfs filesystem and delete any unnecessary snapshots. Use the `btrfs subvolume list` command to find snapshots and `btrfs subvolume delete` to remove them.
3. **Enable compression**: By enabling compression, you can save disk space without deleting files or snapshots. Add the `compress=alg` mount option in your `fstab` configuration file or during the mount procedure, where `alg` is the compression algorithm you want to use (e.g., `zlib`, `lzo`, `zstd`). You can apply compression to existing files using the `btrfs filesystem defragment` command.
4. **Enable deduplication**: Implement deduplication to identify and merge blocks of data with common sequences using copy-on-write semantics. You can use third-party tools dedicated to Btrfs deduplication, such as duperemove, bees, and dduper. However, research their stability and reliability before employing them.
5. **Perform a balance**: If the data and metadata are unevenly allocated among disks, especially in Btrfs filesystems with multiple disks, you can perform a balance operation to reallocate space between data and metadata. Use the `btrfs balance` command with appropriate usage parameters to start the balance process.
### Useful resources
1. [Btrfs Wiki](https://btrfs.wiki.kernel.org)
2. [The Btrfs filesystem on the Arch Linux website](https://wiki.archlinux.org/title/btrfs)
3. [Ubuntu man pages for Btrfs commands](https://manpages.ubuntu.com/manpages/bionic/man8)
4. [duperemove](https://github.com/markfasheh/duperemove)
5. [bees](https://github.com/Zygo/bees)
6. [dduper](https://github.com/lakshmipathi/dduper)
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