Btrfs facts for kids

Btrfs (say "better F S" or "b-tree F S") is a special way computers store information. It's a file system, which is like an organized library for your computer's files. Btrfs uses a clever method called copy-on-write (COW). This means when you change a file, Btrfs doesn't change the original data right away. Instead, it makes a copy of the changed part. This helps keep your data safe and allows for cool features like snapshots.

Btrfs was created by Chris Mason in 2007 for Linux computers. It helps Linux handle large amounts of storage better. It also adds features like snapshots (saving a file's state at a certain time) and checksums (checking for errors). Btrfs aims to make managing computer storage easier and more reliable.

History of Btrfs

Screenshot of usage information of a Btrfs filesystem

The main idea behind Btrfs came from an IBM researcher in 2007. Chris Mason, who worked on other file systems, started building Btrfs based on this idea. He wanted to create a new, more advanced file system.

In 2008, another important developer, Theodore Ts'o, said that older Linux file systems like ext4 were not enough. He believed Btrfs was the future. It offered better ways to handle large storage and was easier to manage.

Btrfs was added to the main Linux kernel in 2009. This meant many Linux systems could start using it. Some Linux versions even offered Btrfs as an experimental choice when installing the operating system.

Over the years, Btrfs gained more features. These included automatic defragmentation (organizing data) and scrubbing (checking for errors). Developers from companies like Oracle and Fujitsu helped improve it. In 2015, Btrfs became the default file system for SUSE Linux Enterprise Server. This showed it was becoming a trusted choice.

In 2020, Fedora 33 also chose Btrfs as its default file system for desktop computers.

Cool Features of Btrfs

Btrfs has many advanced features that make it powerful and reliable.

Key Features of Btrfs

What Btrfs Can Do

As of Linux kernel version 5.0, Btrfs offers these features:

• Self-healing: It can often fix itself if there are minor data problems.
• Online Defragmentation: It can organize your files to work faster, even while you are using the computer.
• Volume Growth and Shrinking: You can easily make your storage space bigger or smaller without restarting.
• Adding/Removing Drives: You can add or remove hard drives to your system while it's running.
• Balancing Data: It can move data around your drives to make sure they are used evenly.
• Offline Checks: You can check the file system for errors when the computer is not in use.
• Data Scrubbing: It actively scans for and fixes errors in your files, especially if you have copies.
• RAID Support: Btrfs can combine multiple hard drives using RAID 0, RAID 1, and RAID 10. This helps with speed or keeping your data safe.
• Subvolumes: These are like separate folders that act as their own file systems. You can use them to organize your data.
• Compression: It can automatically make your files smaller to save space.
• Snapshots: You can create quick copies of your subvolumes. These snapshots save the state of your files at a specific moment.
• File Cloning: You can make copies of files instantly. The copies don't take up extra space until you change them.
• Checksums: Btrfs checks your data and file information for errors using special codes.
• Easy Upgrades: You can change older file systems like ext3/4 to Btrfs without losing your data.
• Union Mounting: You can combine a read-only storage with a writable Btrfs. This is useful for things like Live CDs.
• TRIM Support: It helps SSDs (fast storage drives) work better and last longer.
• Send/Receive: You can send changes between snapshots to another Btrfs system. This is great for backups.
• Incremental Backups: You can easily back up only the changes made since your last backup.
• Deduplication: It can find and remove duplicate copies of data to save space.
• Swap Files: Btrfs can handle swap files, which computers use as extra memory.

Features Not Yet Fully Ready

Some features, like RAID 5 and RAID 6, are available but not fully recommended for important systems. This is because they might not protect against all types of data loss.

Cloning Files with Btrfs

Btrfs has a special "clone" feature. This lets you make an instant copy of a file. It's often called a reflink. When you clone a file, Btrfs doesn't create a whole new copy of the data. Instead, it creates a new entry that points to the same data blocks as the original file.

This means cloning is very fast and uses almost no extra disk space at first. If you change the cloned file, Btrfs only copies the parts that are different. The original file remains untouched. This is different from a hard links, which are just different names for the exact same file. Cloning gives you independent files that start out sharing data.

Subvolumes and Snapshots

Example of snapshots of a Btrfs filesystem, managed with snapper

A Btrfs subvolume is like a special folder that can act as its own separate file system. You can mount it (make it accessible) just like a regular drive. Subvolumes can be created anywhere within your Btrfs system. You can even have subvolumes inside other subvolumes.

A Btrfs snapshot is a copy of a subvolume at a specific moment in time. Because Btrfs uses copy-on-write, snapshots are created very quickly. They don't take up much space at first. If you make changes to the original subvolume, the snapshot stays the same. If you change the snapshot, the original subvolume is not affected.

Snapshots are great for backups or for testing changes. If something goes wrong, you can easily go back to a previous snapshot.

Send and Receive Feature

Btrfs has a "send and receive" feature. This allows you to create a small file that contains only the differences between two snapshots. You can then "send" this file to another Btrfs system. The other system can "receive" it and apply those changes.

This feature is very useful for making incremental backups. You only back up the new changes, not the whole system every time. It's also good for copying data between computers.

Quota Groups

Example of Btrfs quota groups

Quota groups (or qgroups) help you limit how much space a subvolume or snapshot can use. When you create a new snapshot, it doesn't use any quota space at first. This is because it shares data with its parent. But if you add new files or change existing ones, it will start using up its quota.

Quota groups work for whole subvolumes or snapshots. They don't work for individual folders or users directly. However, you can create separate subvolumes for different users if you need to set space limits for them.

Converting Older File Systems

Btrfs has a tool that lets you convert older file systems like ext2, ext3, ext4, or ReiserFS to Btrfs. This conversion happens "in-place," meaning it uses the existing space on your drive. It even keeps a copy of your original file system.

This means you can try Btrfs without completely wiping your drive. If you don't like it, you can go back to your old file system. All your files are available and writable in the new Btrfs system. The conversion tool was greatly improved in 2016 and is now considered stable.

Union Mounting / Seed Devices

When you create a new Btrfs system, you can use an existing Btrfs as a "seed." This seed acts as a read-only base. Your new file system then works on top of it, using copy-on-write. It's like having a transparent layer over the original.

This can be useful for things like a Live CD. A Live CD lets you run an operating system from a disc without installing it. With Btrfs, the system could run from the disc and slowly copy itself to your hard drive in the background.

Encryption Plans

The developers of Btrfs have plans to add built-in encryption. This would help keep your data private and secure. For now, people often use other tools like dm-crypt or LUKS to encrypt their entire hard drive before putting Btrfs on it.

Checking and Recovering Data

Like other file systems, Btrfs has tools to check for and fix errors. The `btrfs check` program can help. However, it's usually recommended to use this tool carefully and only if you know what you're doing.

There's also a tool called `btrfs-restore`. This tool can help you get your files back from a damaged file system without changing the damaged one.

Normally, Btrfs is very good at fixing itself. It saves data regularly, so if your computer crashes, you usually only lose a few seconds of changes.

How Btrfs Works Inside

Btrfs uses a special structure called a B-tree. Imagine a B-tree as a very organized way to store and find information quickly. Everything in Btrfs, from your files to how space is used, is managed by these B-trees.

This design allows Btrfs to do many things efficiently. For example, when you make a snapshot, it doesn't copy all the data. It just creates new pointers in the B-tree. This is why snapshots are so fast and save space.

File System Tree

Inside each folder, Btrfs keeps track of your files. It uses a special way to sort them so it can find them quickly. Even if you have many files in one folder, Btrfs can still find the one you need fast.

Extents

Your actual file data is stored in "extents." These are like continuous blocks of data on your hard drive. When you have a large file, it might be made up of several extents. If you have cloned files or snapshots, they can share these extents. If you change a small part of a shared file, Btrfs only creates a new small extent for that change.

Extent Allocation Tree

This tree keeps track of all the space on your hard drive. It knows which parts are used and which are free. It divides your storage into "block groups" to help organize data and prevent fragmentation. This means your files stay neat and easy to access.

Checksum Tree and Scrubbing

Btrfs calculates special codes called CRC-32C checksums for all your data and file information. These checksums are like fingerprints for your data. They are stored in a "checksum tree." If Btrfs reads a block of data and its checksum doesn't match, it knows there's an error. If you have copies of your data (like with RAID), Btrfs will try to get a good copy from another drive.

Btrfs can also perform a "scrub" job. This is like a deep clean for your file system. It scans all your data, checks the checksums, and tries to fix any errors it finds.

Log Tree

Sometimes, programs need to save changes to your disk immediately. This is called an `fsync` request. To make this faster, Btrfs uses a temporary "log tree." It records these quick changes here. This avoids slowing down the main file system. If your computer crashes, these changes are saved and applied when you restart.

Chunk and Device Trees

Your hard drives are divided into "physical chunks." These chunks can be combined in different ways (like mirroring or striping) to create "logical chunks." Btrfs keeps track of all this in a "chunk tree." This allows Btrfs to manage multiple drives as one big storage pool.

Superblock

The "superblock" is a very important part of Btrfs. It's like the map that tells Btrfs where all its main trees are located. Copies of the superblock are stored in different places on your hard drive. This way, if one copy gets damaged, Btrfs can use another to start up.

Commercial Support

Currently Supported

• Oracle Linux (from version 7)
• SUSE Linux Enterprise Server (from version 12)
• Synology DiskStation Manager (from version 6.0)

No Longer Supported

• Red Hat Enterprise Linux (RHEL) included Btrfs as a "technology preview" in versions 6 and 7. However, it was removed from RHEL 8 in 2018.

See also

In Spanish: Btrfs para niños

• APFS – a similar file system for Apple devices
• Comparison of file systems
• HAMMER – a file system for DragonFly BSD that also uses B-trees
• List of file systems
• ReFS – a copy-on-write file system for Windows Server
• ZFS