Btrfs

From Btrfs Wiki:Btrfs is a modern copy on write (CoW) filesystem for Linux aimed at implementing advanced features while also focusing on fault tolerance, repair and easy administration. Jointly developed at multiple companies, Btrfs is licensed under the GPL and open for contribution from anyone.Warning: Btrfs has some features that are unstable. See the Btrfs Wiki’s Status, Is Btrfs stable? and Getting started for more detailed information. See the #Known issues section.

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Preparation

The official kernels linux and linux-lts include support for Btrfs.

For user space utilities install the btrfs-progs package, which is not part of the base group, and is required for basic operations.

If you need to boot from a Btrfs file system (i.e., your kernel and initramfs reside on a Btrfs partition), check if your boot loader supports Btrfs.

File system creation

The following shows how to create a new Btrfs file system. To convert an ext3/4 partition to Btrfs, see #Ext3/4 to Btrfs conversion. To use a partitionless setup, see #Partitionless Btrfs disk.

See mkfs.btrfs(8) for more information.

File system on a single device

To create a Btrfs filesystem on partition /dev/partition:

# mkfs.btrfs -L mylabel /dev/partition

The Btrfs default blocksize is 16KB. To use a larger blocksize for data/metadata, specify a value for the nodesize via the -n switch as shown in this example using 16KB blocks:

# mkfs.btrfs -L mylabel -n 16k /dev/partition

Multi-device file system

Warning: The RAID 5 and RAID 6 modes of Btrfs are fatally flawed, and should not be used for “anything but testing with throw-away data.” See the Btrfs page on RAID5 and RAID6 for status updates.

Multiple devices can be used to create a RAID. Supported RAID levels include RAID 0, RAID 1, RAID 10, RAID 5 and RAID 6. The RAID levels can be configured separately for data and metadata using the -d and -moptions respectively. By default the data is striped (raid0) and the metadata is mirrored (raid1). See Using Btrfs with Multiple Devices for more information about how to create a Btrfs RAID volume.

# mkfs.btrfs -d raid0 -m raid1 /dev/part1 /dev/part2 ...

You must include either the udev hook or the btrfs hook in /etc/mkinitcpio.conf in order to use multiple Btrfs devices in a pool. See the Mkinitcpio#Common hooks article for more information.Note:

If the disks in your multi-disk array have different sizes, this may not use the full capacity of all drives. In order to utilize the full capacity of all disks, use -d single instead of -d raid0 -m raid1 (metadata mirrored, data not mirrored and not striped)
Mounting such a filesystem may result in all but one of the according .device-jobs getting stuck and systemd never finishing startup due to a bug in handling this type of filesystem.
Some boot loaders such as Syslinux do not support multi-device file systems.

See #RAID for advice on maintenance specific to multi-device Btrfs file systems.

Configuring the file system

Copy-on-Write (CoW)

By default, Btrfs uses copy-on-write for all files all the time. See the Btrfs Sysadmin Guide section for implementation details, as well as advantages and disadvantages.

Disabling CoW

To disable copy-on-write for newly created files in a mounted subvolume, use the nodatacow mount option. This will only affect newly created files. Copy-on-write will still happen for existing files. The nodatacowoption also disables compression. See btrfs(5) for details.Note: From btrfs(5): “within a single file system, it is not possible to mount some subvolumes with nodatacow and others with datacow. The mount option of the first mounted subvolume applies to any other subvolumes.”

To disable copy-on-write for single files/directories do:

$ chattr +C /dir/file

This will disable copy-on-write for those operation in which there is only one reference to the file. If there is more than one reference (e.g. through cp --reflink=always or because of a filesystem snapshot), copy-on-write still occurs.Note: From chattr man page: “For btrfs, the ‘C’ flag should be set on new or empty files. If it is set on a file which already has data blocks, it is undefined when the blocks assigned to the file will be fully stable. If the ‘C’ flag is set on a directory, it will have no effect on the directory, but new files created in that directory will have the No_COW attribute.”Tip: In accordance with the note above, you can use the following trick to disable copy-on-write on existing files in a directory:

$ mv /path/to/dir /path/to/dir_old
$ mkdir /path/to/dir
$ chattr +C /path/to/dir
$ cp -a /path/to/dir_old/* /path/to/dir
$ rm -rf /path/to/dir_old

Make sure that the data are not used during this process. Also note that mv or cp --reflink as described below will not work.

Creating lightweight copies

By default, when copying files on a Btrfs filesystem with cp, actual copies are created. To create a lightweight copy referencing to the original data, use the reflink option:

$ cp --reflink source dest

See the man page on cp(1) for more details on the --reflink flag.

Compression

Btrfs supports transparent compression, meaning every file on the partition is automatically compressed. This not only reduces the size of files, but can also improve performance, in some specific use cases (e.g. single thread with heavy file I/O), while obviously harming performance in other cases (e.g. multithreaded and/or cpu intensive tasks with large file I/O). Better performance is generally achieved with the fastest compress algorithms, zstd and lzo, and some benchmarks provide detailed comparisons.

Compression is enabled using the compress mount option, which can be set to zlib, lzo, zstd, or no (for no compression). Only files created or modified after the mount option is added will be compressed.

To apply compression to existing files, use the btrfs filesystem defragment -calg command, where alg is either zlib, lzo or zstd. For example, in order to re-compress the whole file system with zstd, run the following command:

# btrfs filesystem defragment -r -v -czstd /

To enable compression when installing Arch to an empty Btrfs partition, use the compress option when mounting the file system: mount -o compress=zstd /dev/sdxY /mnt/. During configuration, add compress=zstd to the mount options of the root file system in fstab.Tip: Compression can also be enabled per-file without using the compress mount option; to do so apply chattr +c to the file. When applied to directories, it will cause new files to be automatically compressed as they come.Warning:

Systems using older kernels or btrfs-progs without zstd support may be unable to read or repair your filesystem if you use this option.
Stable GRUB and rEFInd currently lack support for zstd, either switch to grub-git^AUR, use a separate boot partition without zstd or reset compression of boot files to something supported using for example the command:$ btrfs filesystem defragment -v -clzo /boot/*

Subvolumes

“A btrfs subvolume is not a block device (and cannot be treated as one) instead, a btrfs subvolume can be thought of as a POSIX file namespace. This namespace can be accessed via the top-level subvolume of the filesystem, or it can be mounted in its own right.” [1]

Each Btrfs file system has a top-level subvolume with ID 5. It can be mounted as / (by default), or another subvolume can be mounted instead. Subvolumes can be moved around in the filesystem and are rather identified by their id than their path.

See the following links for more details:

Creating a subvolume

To create a subvolume:

# btrfs subvolume create /path/to/subvolume

Listing subvolumes

To see a list of current subvolumes under path:

# btrfs subvolume list -p path

Deleting a subvolume

To delete a subvolume:

# btrfs subvolume delete /path/to/subvolume

Attempting to remove the directory /path/to/subvolume without using the above command will not delete the subvolume.

Mounting subvolumes

Subvolumes can be mounted like file system partitions using the subvol=/path/to/subvolume or subvolid=objectid mount flags. For example, you could have a subvolume named subvol_root and mount it as /. One can mimic traditional file system partitions by creating various subvolumes under the top level of the file system and then mounting them at the appropriate mount points. Thus one can easily restore a file system (or part of it) to a previous state using #Snapshots.Tip: Changing subvolume layouts is made simpler by not using the toplevel subvolume (ID=5) as / (which is done by default). Instead, consider creating a subvolume to house your actual data and mounting it as /.Note: From btrfs(5): “Most mount options apply to the whole filesystem, and only the options for the first subvolume to be mounted will take effect. This is due to lack of implementation and may change in the future.”. See the Btrfs Wiki FAQ for which mount options can be used per subvolume.

See Snapper#Suggested filesystem layout, Btrfs SysadminGuide#Managing Snapshots, and Btrfs SysadminGuide#Layout for example file system layouts using subvolumes.

See btrfs(5) for a full list of btrfs-specific mount options.

Changing the default sub-volume

The default sub-volume is mounted if no subvol= mount option is provided. To change the default subvolume, do:

# btrfs subvolume set-default subvolume-id /

where subvolume-id can be found by listing.Note: After changing the default subvolume on a system with GRUB, you should run grub-install again to notify the bootloader of the changes. See this forum thread.

Changing the default subvolume with btrfs subvolume set-default will make the top level of the filesystem inaccessible, except by use of the subvol=/ or subvolid=5 mount options [2].

Quota

Warning: Qgroup is not stable yet and combining quota with (too many) snapshots of subvolumes can cause performance problems, for example when deleting snapshots. Plus there are several more known issues.

Quota support in Btrfs is implemented at a subvolume level by the use of quota groups or qgroup: Each subvolume is assigned a quota groups in the form of 0/<subvolume id> by default. However it is possible to create a quota group using any number if desired.

To use qgroups you need to enable quota first using

# btrfs quota enable <path>

From this point onwards newly created subvolumes will be controlled by those groups. In order to retrospectively enable them for already existing subvolumes, enable quota normally, then create a qgroup (quota group) for each of those subvolume using their <subvolume id> and rescan them:

# btrfs subvolume list <path> | cut -d' ' -f2 | xargs -I{} -n1 btrfs qgroup create 0/{} <path>
# btrfs quota rescan <path>

Quota groups in Btrfs form a tree hierarchy, whereby qgroups are attached to subvolumes. The size limits are set per qgroup and apply when any limit is reached in tree that contains a given subvolume.

Limits on quota groups can be applied either to the total data usage, un-shared data usage, compressed data usage or both. File copy and file deletion may both affect limits since the unshared limit of another qgroup can change if the original volume’s files are deleted and only one copy is remaining. For example a fresh snapshot shares almost all the blocks with the original subvolume, new writes to either subvolume will raise towards the exclusive limit, deletions of common data in one volume raises towards the exclusive limit in the other one.

To apply a limit to a qgroup, use the command btrfs qgroup limit. Depending on your usage either use a total limit, unshared limit (-e) or compressed limit (-c). To show usage and limits for a given path within a filesystem use

# btrfs qgroup show -reF <path>

Commit Interval

The resolution at which data are written to the filesystem is dictated by Btrfs itself and by system-wide settings. Btrfs defaults to a 30 seconds checkpoint interval in which new data are committed to the filesystem. This can be changed by appending the commit mount option in /etc/fstab for the btrfs partition.

LABEL=arch64 / btrfs defaults,noatime,compress=lzo,commit=120 0 0

System-wide settings also affect commit intervals. They include the files under /proc/sys/vm/* and are out-of-scope of this wiki article. The kernel documentation for them resides in Documentation/sysctl/vm.txt.

SSD TRIM

A Btrfs filesystem is able to free unused blocks from an SSD drive supporting the TRIM command.

More information about enabling and using TRIM can be found in Solid State Drives#TRIM.

Usage

Displaying used/free space

General linux userspace tools such as df will inaccurately report free space on a Btrfs partition. It is recommended to use btrfs filesystem usage to query Btrfs partitions. For example:

# btrfs filesystem usage /

Note: The btrfs filesystem usage command does not currently work correctly with RAID5/RAID6 RAID levels.

See [3] for more information.

Defragmentation

Btrfs supports online defragmentation through the mount option autodefrag, see btrfs(5). To manually defragment your root, use:

# btrfs filesystem defragment -r /

Using the above command without the -r switch will result in only the metadata held by the subvolume containing the directory being defragmented. This allows for single file defragmentation by simply specifying the path.

Defragmenting a file which has a COW copy (either a snapshot copy or one made with cp --reflink or bcp) plus using the -c switch with a compression algorithm may result in two unrelated files effectively increasing the disk usage.

RAID

Btrfs offers native “RAID” for #Multi-device file systems. Notable features which set btrfs RAID apart from mdadm are self-healing redundant arrays and online balancing. See the Btrfs wiki page for more information. The Btrfs sysadmin page also has a section with some more technical background.Warning: Parity RAID (RAID 5/6) code has multiple serious data-loss bugs in it. See the Btrfs Wiki’s RAID5/6 page and a bug report on linux-btrfs mailing list for more detailed information.

Scrub

The Btrfs Wiki Glossary says that Btrfs scrub is “[a]n online filesystem checking tool. Reads all the data and metadata on the filesystem, and uses checksums and the duplicate copies from RAID storage to identify and repair any corrupt data.”Warning: A running scrub process will prevent the system from suspending, see this thread for details.

Start manually

To start a (background) scrub on the filesystem which contains /:

# btrfs scrub start /

To check the status of a running scrub:

# btrfs scrub status /

Start with a service or timer

The btrfs-progs package brings the [email protected] unit for monthly scrubbing the specified mountpoint. Enable the timer with an escaped path, e.g. [email protected] for / and [email protected] for /home. You can use systemd-escape -p /path/to/mountpoint to escape the path, see systemd-escape(1) for details.

You can also run the scrub by starting [email protected] (with the same encoded path). The advantage of this over # btrfs scrub is that the results of the scrub will be logged in the systemd journal.

Balance

“A balance passes all data in the filesystem through the allocator again. It is primarily intended to rebalance the data in the filesystem across the devices when a device is added or removed. A balance will regenerate missing copies for the redundant RAID levels, if a device has failed.” [4] See Upstream FAQ page.

On a single-device filesystem a balance may be also useful for (temporarily) reducing the amount of allocated but unused (meta)data chunks. Sometimes this is needed for fixing “filesystem full” issues.

# btrfs balance start /
# btrfs balance status /

Snapshots

“A snapshot is simply a subvolume that shares its data (and metadata) with some other subvolume, using btrfs’s COW capabilities.” See Btrfs Wiki SysadminGuide#Snapshots for details.

To create a snapshot:

# btrfs subvolume snapshot source [dest/]name

To create a readonly snapshot add the -r flag. To create writable version of a readonly snapshot, simply create a snapshot of it.Note: Snapshots are not recursive. Every nested subvolume will be an empty directory inside the snapshot.

Send/receive

A subvolume can be sent to stdout or a file using the send command. This is usually most useful when piped to a Btrfs receive command. For example, to send a snapshot named /root_backup (perhaps of a snapshot you made of / earlier) to /backup you would do the following:

 # btrfs send /root_backup | btrfs receive /backup

The snapshot that is sent must be readonly. The above command is useful for copying a subvolume to an external device (e.g. a USB disk mounted at /backup above).

You can also send only the difference between two snapshots. For example, if you have already sent a copy of root_backup above and have made a new readonly snapshot on your system named root_backup_new, then to send only the incremental difference to /backup do:

 # btrfs send -p /root_backup /root_backup_new | btrfs receive /backup

Now a new subvolume named root_backup_new will be present in /backup.

See Btrfs Wiki’s Incremental Backup page on how to use this for incremental backups and for tools that automate the process.

Deduplication

Using copy-on-write, Btrfs is able to copy files or whole subvolumes without actually copying the data. However whenever a file is altered a new proper copy is created. Deduplication takes this a step further, by actively identifying blocks of data which share common sequences and combining them into an extent with the same copy-on-write semantics.

Tools dedicated to deduplicate a Btrfs formatted partition include duperemove^AUR, bedup^AUR and btrfs-dedup. One may also want to merely deduplicate data on a file based level instead using e.g. rmlint or jdupes^AUR. For an overview of available features of those programs and additional information have a look at the upstream Wiki entry.

Furthermore Btrfs developers are working on inband (also known as synchronous or inline) deduplication, meaning deduplication done when writing new data to the filesystem. Currently it is still an experiment which is developed out-of-tree. Users willing to test the new feature should read the appropriate kernel wiki page.

Known issues

A few limitations should be known before trying.

Encryption

Btrfs has no built-in encryption support, but this may come in the future. Users can encrypt the partition before running mkfs.btrfs. See dm-crypt/Encrypting an entire system#Btrfs subvolumes with swap.

Existing Btrfs file systems can use something like EncFS or TrueCrypt, though perhaps without some of Btrfs’ features.

Swap file

Btrfs does not yet support swap files. This is due to swap files requiring a function that Btrfs intentionally does not have for possibility of file system corruption [5]. Patches for swapfile support are already available [6] and may be included in an upcoming kernel release. As an alternative a swap file can be mounted on a loop device with poorer performance but will not be able to hibernate. Install the package systemd-swap and enable swapfc in its configuration to automate this.

TLP

Using TLP requires special precautions in order to avoid filesystem corruption. Refer to the according TLP section for more information.

Tips and tricks

Partitionless Btrfs disk

Warning: Most users do not want this type of setup and instead should install Btrfs on a regular partition. Furthermore GRUB strongly discourages installation to a partitionless disk.

Btrfs can occupy an entire data storage device, replacing the MBR or GPT partitioning schemes, using subvolumes to simulate partitions. However, using a partitionless setup is not required to simply create a Btrfs filesystem on an existing partition that was created using another method. There are some limitations to partitionless single disk setups:

Cannot use different file systems for different mount points.
Cannot use swap area as Btrfs does not support swap files and there is no place to create swap partition. This also limits the use of hibernation/resume, which needs a swap area to store the hibernation image.
Cannot use UEFI to boot.

To overwrite the existing partition table with Btrfs, run the following command:

# mkfs.btrfs /dev/sdX

For example, use /dev/sda rather than /dev/sda1. The latter would format an existing partition instead of replacing the entire partitioning scheme.

Install the boot loader like you would for a data storage device with a Master Boot Record. See Syslinux#Manual install or GRUB/Tips and tricks#Install to partition or partitionless disk.

Ext3/4 to Btrfs conversion

Warning: There are many reports on the btrfs mailing list about incomplete/corrupt/broken conversions. Make sure you have working backups of any data you cannot afford to lose. See Conversion from Ext3on the btrfs wiki for more information.

Boot from an install CD, then convert by doing:

# btrfs-convert /dev/partition

Mount the partion and test the conversion by checking the files. Be sure to change the /etc/fstab to reflect the change (type to btrfs and fs_passno [the last field] to 0 as Btrfs does not do a file system check on boot). Also note that the UUID of the partition will have changed, so update fstab accordingly when using UUIDs. chroot into the system and rebuild the GRUB menu list (see Install from existing Linux and GRUBarticles). If converting a root filesystem, while still chrooted run mkinitcpio -p linux to regenerate the initramfs or the system will not successfully boot. If you get stuck in grub with ‘unknown filesystem’ try reinstalling grub with grub-install /dev/partition and regenerate the config as well grub-mkconfig -o /boot/grub/grub.cfg.

After confirming that there are no problems, complete the conversion by deleting the backup ext2_saved sub-volume. Note that you cannot revert back to ext3/4 without it.

# btrfs subvolume delete /ext2_saved

Finally balance the file system to reclaim the space.

Remember that some applications which were installed prior have to be adapted to Btrfs. Notably TLP#Btrfs needs special care to avoid filesystem corruption but other applications may profit from certain features as well.

Checksum hardware acceleration

To verify if Btrfs checksum is hardware accelerated:

$ dmesg | grep crc32c

Btrfs loaded, crc32c=crc32c-intel

If you see crc32c=crc32c-generic, it is probably because your root partition is Btrfs, and you will have to compile crc32c-intel into the kernel to make it work. Putting crc32c-intel into mkinitcpio.conf does not work.

Corruption recovery

btrfs-check cannot be used on a mounted file system. To be able to use btrfs-check without booting from a live USB, add it to the initial ramdisk:

/etc/mkinitcpio.conf

BINARIES=("/usr/bin/btrfs")

Regenerate the initramfs.

Then if there is a problem booting, the utility is available for repair.Note: If the fsck process has to invalidate the space cache (and/or other caches?) then it is normal for a subsequent boot to hang up for a while (it may give console messages about btrfs-transaction being hung). The system should recover from this after a while.

See the Btrfs Wiki page for more information.

Booting into snapshots

In order to boot into a snapshot you must specify the subvolume via a kernel parameter using rootflags=subvol=/path/to/subvolume and alter your /etc/fstab to point to the same subvolume using subvol=. Alternatively the subvolume can be specified with its id – retrievable with e.g. btrfs subvolume list /root/path – and rootflags=subvolid=objectid as kernel parameter respectively subvolid=objectid as mount option in /etc/fstab.

If using GRUB you can automatically populate your boot menu with btrfs snapshots when regenerating the configuration file with the help of grub-btrfs or grub-btrfs-git^AUR.

Use Btrfs subvolumes with systemd-nspawn

See the Systemd-nspawn#Use Btrfs subvolume as container root and Systemd-nspawn#Use temporary Btrfs snapshot of container articles.

Troubleshooting

See the Btrfs Problem FAQ for general troubleshooting.

GRUB

Partition offset

The offset problem may happen when you try to embed core.img into a partitioned disk. It means that it is OK to embed grub’s core.img into a Btrfs pool on a partitionless disk (e.g. /dev/sdX) directly.

GRUB can boot Btrfs partitions, however the module may be larger than other file systems. And the core.img file made by grub-install may not fit in the first 63 sectors (31.5KiB) of the drive between the MBR and the first partition. Up-to-date partitioning tools such as fdisk and gdisk avoid this issue by offsetting the first partition by roughly 1MiB or 2MiB.

Missing root

The factual accuracy of this article or section is disputed.Reason: Suggests editing a non-configuration file manually. (Discuss in Talk:Btrfs#Should not suggest to edit files in /usr/share)

Users experiencing the following: error no such device: root when booting from a RAID style setup then edit /usr/share/grub/grub-mkconfig_lib and remove both quotes from the line echo " search --no-floppy --fs-uuid --set=root ${hints} ${fs_uuid}". Regenerate the config for grub and the system should boot without an error.

BTRFS: open_ctree failed

As of November 2014 there seems to be a bug in systemd or mkinitcpio causing the following error on systems with multi-device Btrfs filesystem using the btrfs hook in mkinitcpio.conf:

BTRFS: open_ctree failed
mount: wrong fs type, bad option, bad superblock on /dev/sdb2, missing codepage or helper program, or other error

In some cases useful info is found in syslog - try dmesg|tail or so.

You are now being dropped into an emergency shell.

A workaround is to remove btrfs from the HOOKS array in /etc/mkinitcpio.conf and instead add btrfs to the MODULES array. Then regenerate the initramfs with mkinitcpio -p linux (adjust the preset if needed) and reboot.

You will get the same error if you try to mount a raid array without one of the devices. In that case you must add the degraded mount option to /etc/fstab. If your root resides on the array, you must also add rootflags=degraded to your kernel parameters.

As of August 2016, a potential workaround for this bug is to mount the array by a single drive only in /etc/fstab, and allow btrfs to discover and append the other drives automatically. Group-based identifiers such as UUID and LABEL appear to contribute to the failure. For example, a two-device RAID1 array consisting of ‘disk1′ and disk2’ will have a UUID allocated to it, but instead of using the UUID, use only /dev/mapper/disk1in /etc/fstab. For a more detailed explanation, see the following blog post.

Another possible workaround is to remove the udev hook in mkinitcpio.conf and replace it with the systemd hook. In this case, btrfs should not be in the HOOKS or MODULES arrays.

See the original forums thread and FS#42884 for further information and discussion.

btrfs check

Warning: Since Btrfs is under heavy development, especially the btrfs check command, it is highly recommended to create a backup and consult the following Btfrs documentation before executing btrfs checkwith the --repair switch.

The btrfs check command can be used to check or repair an unmounted Btrfs filesystem. However, this repair tool is still immature and not able to repair certain filesystem errors even those that do not render the filesystem unmountable.

See Btrfsck for more information.

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BTRFS——新型RAID文件系统介绍

于2018年12月27日2018年12月27日由Starx发布