Btrfs Mount Options

Btrfs has emerged as a powerful contender in the Linux filesystem landscape, offering advanced features like snapshots, checksumming, and dynamic volume management. When properly configured, it can provide an optimal balance of performance, reliability, and space efficiency. This guide will help you understand the most appropriate mount options for different scenarios, whether you're using SSDs, HDDs, or setting up specialized partitions.

The Evolution of Btrfs

Btrfs (pronounced as "Butter FS," "Better FS," or "B-Tree FS") began its journey at Oracle Corporation in 2007, under the leadership of Chris Mason, who had previously worked on ReiserFS. The core data structure—the copy-on-write B-tree—was initially proposed by IBM researcher Ohad Rodeh at a USENIX conference that same year.

By 2009, Btrfs was accepted into the Linux kernel mainline, and by November 2013, its on-disk format was declared stable. Theodore Ts'o, the principal developer of ext3 and ext4, has stated that Btrfs represents a better direction for Linux filesystems due to "improvements in scalability, reliability, and ease of management".

Over the years, Btrfs has gained significant adoption. SUSE Linux Enterprise Server chose it as the default filesystem in 2015, and Fedora 33 followed suit for desktop variants in 2020. However, Red Hat announced in 2017 that it wouldn't fully support Btrfs in RHEL and removed it from RHEL 8 in 2019.

Understanding Btrfs Mount Options

Btrfs offers numerous mount options that control filesystem behavior. A key concept to understand is that most options affect the entire filesystem, not just individual subvolume mounts (with exceptions like subvol and subvolid). When you configure Btrfs in your fstab with certain options (e.g., compress=zstd), all subsequent mounts, including other subvolumes, will inherit those options.

Let's explore how to optimize Btrfs for different scenarios.

Recommended Mount Options for SSDs

General SSD Options

• ssd: This option is typically auto-detected for SSDs. It enables several optimizations:
  • Larger metadata cluster allocation
  • More sequential data allocation where possible
  • Disabled btree leaf rewriting
  • Immediate log fragment commits without batching
• noatime: Disables access time updates for files, reducing unnecessary writes and extending SSD lifespan.
• space_cache=v2: Caches free space information, improving performance by avoiding full tree parsing when creating files.
• discard=async: For modern kernels, this provides efficient TRIM functionality without the performance penalties of synchronous discard.

Compression Options for SSDs

For standard SATA SSDs:

• compress=zstd:1 or compress=zstd:3: Provides good compression with reasonable CPU overhead.

For NVMe SSDs:

• compress=lzo: Some tests suggest that lzo might be more appropriate for ultra-fast NVMe drives. According to benchmarks: "zstd:1 still has huge penalty on I/O speed on modern PC system... On NVME SSD the penalty become far serious".

Example Fstab Entry for SSDs

UUID=xxxx-xxxx / btrfs noatime,compress=zstd:1,ssd,discard=async,space_cache=v2 0 0

For NVMe drives:

UUID=xxxx-xxxx / btrfs noatime,compress=lzo,ssd,discard=async,space_cache=v2 0 0

TRIM Considerations

While discard=async provides continuous TRIM, some users prefer scheduled TRIM via fstrim.timer. The scheduled approach runs TRIM periodically (usually weekly) rather than continuously. If you choose this method, omit the discard=async option and enable the systemd timer:

systemctl enable fstrim.timer

Recommended Mount Options for HDDs

General HDD Options

• autodefrag: Automatically defragments files, particularly helpful for HDDs where fragmentation significantly impacts performance.
• noatime: Just like with SSDs, this reduces unnecessary writes.

Compression Options for HDDs

• compress=zstd:3 or higher: HDDs can benefit from stronger compression as the CPU time spent compressing data is often less than the time saved in reduced I/O.

Example Fstab Entry for HDDs

UUID=xxxx-xxxx / btrfs noatime,compress=zstd:3,autodefrag,space_cache=v2 0 0

Mount Options for Different Partition Types

Root Partition (/)

The root partition contains system files and is typically read-heavy with occasional writes during updates:

UUID=xxxx-xxxx / btrfs subvol=@,noatime,compress=zstd:1,space_cache=v2 0 0

For SSDs, add ssd,discard=async to these options.

Home Partition (/home)

The home partition contains user data and may benefit from different options:

UUID=xxxx-xxxx /home btrfs subvol=@home,noatime,compress=zstd:3,autodefrag,space_cache=v2 0 0

Security Considerations

For improved security, consider these additional options:

• nosuid: Prevents executables from running with elevated privileges via the setuid bit.
• noexec: Prevents execution of any binary files on the mounted partition.

For a more security-conscious /home setup:

UUID=xxxx-xxxx /home btrfs subvol=@home,noatime,compress=zstd:3,nosuid,noexec,space_cache=v2 0 0

Database or VM Storage

For partitions hosting databases or virtual machines that perform many small random writes:

UUID=xxxx-xxxx /var/lib/mysql btrfs noatime,nodatacow,space_cache=v2 0 0

Note: nodatacow disables copy-on-write, which can improve performance for database workloads but removes some of Btrfs's data integrity features. It also implicitly enables nodatasum.

Subvolume Strategies and Best Practices

Btrfs subvolumes provide a powerful way to organize your filesystem and implement different backup and snapshot strategies.

Basic Subvolume Layout

A common practice is to create subvolumes for different types of data:

• @ or @root: For the root filesystem
• @home: For user home directories
• @var: For variable data
• @snapshots: For storing snapshots

This separation allows you to implement different backup and snapshot policies for different types of data.

Example /etc/fstab Entries

UUID=xxxx-xxxx / btrfs defaults,noatime,compress=zstd:1,space_cache=v2,subvol=@ 0 0
UUID=xxxx-xxxx /home btrfs defaults,noatime,compress=zstd:1,space_cache=v2,subvol=@home 0 0
UUID=xxxx-xxxx /.snapshots btrfs defaults,noatime,compress=zstd:1,space_cache=v2,subvol=@snapshots 0 0

Performance Tuning Tips

Commit Interval

The commit option (e.g., commit=120) controls how frequently filesystem changes are committed to disk in seconds. A longer interval can improve performance but may increase data loss risk in case of a crash.

Autodefrag

The autodefrag option detects small random writes to files and queues them for defragmentation. This is particularly useful for HDDs and for systems that frequently update many small files.

Space Cache

The space_cache=v2 option is a newer, more efficient implementation of the space cache that improves performance, especially on larger filesystems.

Failure Handling

Consider adding the nofail option to prevent boot failures if a device is not available:

UUID=xxxx-xxxx /mnt/data btrfs defaults,nofail,noatime,compress=zstd 0 0

Conclusion

Choosing the right mount options for your Btrfs filesystem can significantly impact performance, reliability, and space efficiency. To summarize:

• For SSDs: Use noatime, ssd, discard=async (or scheduled TRIM), and appropriate compression
• For NVMe: Consider using lzo compression to avoid CPU bottlenecks
• For HDDs: Higher compression levels and autodefrag can improve performance
• Use subvolumes to organize different types of data with appropriate backup strategies

Remember that most mount options apply to the entire filesystem, not just individual subvolumes. Plan your mount options carefully, especially for the first mount point.

As your needs change and as Btrfs continues to evolve, revisit your mount options to ensure they remain optimal for your use case. The flexibility of Btrfs allows you to adapt your filesystem to a wide range of scenarios, from high-performance workstations to reliable network storage systems.

References

Btrfs - ArchWiki
Btrfs SSD Optimization - Red Hat Enterprise Linux 7
Btrfs options in fstab - EndeavourOS Forum
How to mount a hard drive with btrfs filesystem with user permissions - Garuda Linux Forum
Btrfs adding new hard drive as home after installation - Unix & Linux Stack Exchange
What would be the correct mount options for a separated btrfs home partition in fstab - Super User
Upgrading btrfs home mount with dedicated drive - Fedora Discussion
Btrfs mount options - Arch Linux Forums
How to mount a btrfs file system with the mount command - openSUSE Forums
fstab btrfs mount options update - Manjaro Forum
fstab for SSD on F38/F39 - Fedora Discussion
Created btrfs on external SSD, what now? - openSUSE Forums