Btrfs/fr

Btrfs is Article description::a copy-on-write (CoW) [[filesystem for Linux aimed at implementing advanced features while focusing on fault tolerance, repair, and easy administration.]] Jointly developed at Oracle, Red Hat, Fujitsu, Intel, SUSE, STRATO, and many others, btrfs is licensed under the GPL and open for contribution from anyone.

Fonctionnalités
Ext4 is safe and stable and can handle large filesystems with extents, but why switch? While it is true that Btrfs is still considered experimental and is growing in stability, the time when Btrfs will become the default filesystem for Linux systems is getting closer. Some Linux distributions have already begun to switch to it with their current releases. Btrfs has a number of advanced features in common with ZFS, which is what made the ZFS filesystem popular with BSD distributions and NAS devices.


 * Copy on Write (CoW) and snapshotting - Make incremental backups painless even from a "hot" filesystem or virtual machine (VM).
 * File level checksums - Metadata for each file includes a checksum that is used to detect and repair errors.
 * Compression - Files may be compressed and decompressed on the fly, which speeds up read performance.
 * Auto defragmentation - The filesystems are tuned by a background thread while they are in use.
 * Subvolumes - Filesystems can share a single pool of space instead of being put into their own partitions.
 * RAID - Btrfs does its own RAID implementations so LVM or are not required in to have RAID. Currently RAID 0 and 1 are supported; RAID 5 and 6 are upcoming.
 * Partitions are optional - While Btrfs can work with partitions, it has the potential to use raw devices directly.
 * Data deduplication - There is limited data deduplication support; however, deduplication will eventually become a standard feature in Btrfs. This enables Btrfs to save space by comparing files via binary diffs.

Down the road, new clustered filesystems will readily take advantage of Btrfs with its copy on write and other advanced features for their object stores. Ceph is one example of a clustered filesystem that looks very promising, and can take advantage of Btrfs.

Noyau
Activez les options suivantes du noyau pour activer la prise en charge de Btrfs :

Emerge
Le paquet contient les utilitaires nécessaire au fonctionnent de Btrfs.

Utilisation
Typing long Btrfs commands can quickly become a hassle. Each command (besides the initial command) can be reduced to a very short set of instructions. This method is helpful when working from the command line to reduce the amount of characters typed.

Par exemple, pour défragmenter un système de fichiers situé à (ci-dessous la commande longue) :

Shorten each of the longer commands after the command by reducing them to their unique, shortest prefix. In this context, unique means that no other commands will match the command at the command's shortest length. The shortened version of the above command is:

No other commands start with  ;   is the only one. The same goes for the  sub-command under the   command.

Création
Pour créer un système de fichiers sur la partition  :

Dans l'exemple suivant, remplacez  par le numéro de partition et   par la lettre correspondant au disque qui va être formaté. Par exemple, pour formater la troisième partition du premier disque en Btrfs, lancez :

Montage
Après création, les systèmes de fichiers peuvent être montés de différentes manières :


 * - Montage manuel.
 * - Définir des points de montage dans activant un montage automatique à chaque démarrage.
 * Removable media - Montages automatiques à la demande (utile pour les périphériques USB).
 * AutoFS - Montage automatique à l'accès au système de fichiers.

Conversion de systèmes de fichiers basés sur ext*
Il est possible de convertir des systèmes de fichiers ext2, ext3, et ext4 en Btrfs en utilisant l'utilitaire.

The following instructions only supports the conversion of filesystems that are unmounted. To convert the root partition, boot to a system rescue disk (SystemRescueCD works nicely) and run on the conversion commands on the root partition.

Premièrement, soyez sûr que le point de montage est bien démonté :

Vérifiez l'intégrité du système de fichier en utilisant l'outil adéquat. Dans l'exemple suivant, le système de fichier est du ext4 :

Utilisez pour convertir un périphérique formaté en ext*, en Btrfs :

Assurez-vous d'éditer après que le périphérique est été formaté, en changeant la colonne correspondant au système de fichiers utilisé, d'ext4 en Btrfs :

Défragmentation
Une autre fonctionnalité du Btrfs est la défragmentation en ligne. Pour défragmenter un système de fichiers racine en Btrfs, lancez :

Compression
Btrfs supporte la compression transparente : trois alghorithmes de compression sont disponibles zlib, lzo, et zstd.

Il est possible de compresser des fichiers spécifiques, en utilisant l'attribut suivant :

L'option de montage  spécifie le comportement à suivre lors de la création de tous nouveaux fichiers. Pour recompresser tous le système de fichiers, lancer la commande suivante :

Depending on the CPU and disk performance, using lzo compression could improve the overall throughput.

Il est possible d'utiliser la compression avec l'algorithme zlib plutôt que lzo. zlib est plus lent mais a un ratio de compression supérieur :

Niveau de compression
Depuis la version 4.15.0 du noyau, la compression utilisant zlib peut maintenant utiliser différents niveaux de compression, allant de 1 à 9. Par exemple, pour utiliser le niveau de compression maximum au montage :

Ou pour utiliser le niveau de compression minimal :

Ou ajuster la compression au remontage :

The compression level should be visible in or by checking the most recent output of.

Ratio de compression et usage du disque
The usual userspace tools for determining used and free space like  and   may provide inaccurate results on a Btrfs partition due to inherent design differences in the way files are written compared to, for example, ext2/3/4.

It is therefore advised to use the du/df alternatives provided by the btrfs userspace tool. In Addition to that, The  tool found from the sys-fs/compsize package can be helpful in providing additional information regarding compression ratios and the disk usage of compressed files. The following are example uses of these tools for a btrfs partition mounted under.

Périphériques multiples (RAID)
Btrfs can be used with multiple devices in order to create RAIDs. Using Btrfs to create filesystems that span multiple devices is much easier than creating using ; there is no initialization time needed for creation.

The simplest method is to use the entirety of the unpartitioned block device to create a filesystem that spans multiple devices. For example, to create a filesystem in RAID1 mode across two devices:

Converting between RAID modes is possible with the balance sub-command. For example, say a multiple device RAID 1 is mounted at. It is possible to convert this RAID1 to RAID0 with using the following command:

RAID mode conversion can be performed while the filesystem is online and in use. Possible RAID modes in btrfs include RAID0, RAID1, RAID5, RAID6, and RAID10. See the upstream BTRFS wiki for more information.

Sous-volumes
As mentioned above in the features list, Btrfs can create subvolumes. Subvolumes can be used to better organize and manage data. They become especially powerful when combined with snapshots. Important distinctions must be made between Btrfs subvolumes and subvolumes created by Logical Volume Management (LVM). Btrfs subvolumes are not block level devices, they are POSIX file namespaces. They can be created at any location in the filesystem and will act like any other directory on the system with one caveat: subvolumes can be mounted and unmounted. Subvolumes are nestable (subvolumes can be created inside other subvolumes), and easily created or removed.

Création
To create a subvolume, issue the following command inside a Btrfs filesystem's name space:

Replace  with the desired destination and subvolume name. For example, if a Btrfs filesystem exists at, a subvolume could be created inside it using the following command:

Listage
To see the subvolume(s) that have been created, use the  command followed by a Btrfs filesystem location. If the current directory is somewhere inside a Btrfs filesystem, the following command will display the subvolume(s) that exist on the filesystem:

If a Btrfs filesystem with subvolumes exists at the mount point created in the example command above, the output from the list command will look similar to the following:

Suppression
Subvolumes can be properly removed by using the  command followed by the path to the subvolume. All available subvolume paths in a Btrfs filesystem can be seen using the list command above.

As above, replace  with the actual path to the subvolume to be removed. To delete the subvolume used in the examples above, the following command would be issued:

Instantanés
Snapshots are subvolumes that share data and metadata with other subvolumes. This is made possible by Btrfs' Copy on Write (CoW) ability. Snapshots can be used for several purposes, one of which is to create backups of file system structures at specific points in time.

Si la racine est en Btrfs, il est possible de créer un instantané en utilisant les commandes  :

The following small shell script can be added to a timed cron job to create a timestamped snapshot backup of a Btrfs formatted root filesystem. The timestamps can be adjusted to whatever is preferred by the user.

Montage
A subvolume can be mounted in a location different from where it was created, or users can choose to not mount them at all. For example, a user could create a Btrfs filesystem in and create  and  subvolumes. The subvolumes could then be mounted at and, with the original top level subvolume left unmounted. This results in a configuration where the subvolumes' relative path from the top level subvolume is different from their actual path.

To mount a subvolume, perform the following command, where  is the relative path of the subvolume from the top level subvolume, obtainable through the   command:

Similarly, one can update the filesystem tab to mount their Btrfs subvolumes like so:

Using with VM disk images
When using Btrfs with virtual machine disk images, it is best to disable copy-on-write on the disk images in order to speed up IO performance. This can only be performed on files that are newly created. It also possible to disable CoW on all files created within a certain directory. For example, using the command:

Clear the free space cache
It is possible to clear Btrfs' free space cache by mounting the filesystem with the  mount option. For example:

Btrfs hogging memory (disk cache)
When utilizing some of Btrfs' special abilities (like making many  copies or creating a crazy amount of snapshots), lot of memory can be eaten and not freed fast enough by the kernel's inode cache. This issue can go undiscovered since memory dedicated to the disk cache might not be clearly visible in traditional system monitoring utilities. The utility (available as part of the  package) was specifically created to determine how much memory kernel objects are consuming:

If the inode cache is consuming too much memory, the kernel can be manually instructed to drop the cache by echoing an integer value to the file.

To be safe, and to help the kernel determine the maximum amount of freeable memory, be sure to run a before running the  commands below:

Most of the time Btrfs users will probably want to to reclaim just the slab objects (dentries and btrfs_inodes):

To clear the entire disk cache (slab objects and the page cache) use instead:

More information on kernel slabs can be found in this dedoimedo blog entry.

Mounting Btrfs fails, returning mount: unknown filesystem type 'btrfs'
The original solution by Tim on Stack Exchange inspired the following solution: build the kernel manually instead of using genkernel:

La racine Btrfs ne démarre pas
Genkernel's initramfs as created with the command below doesn't load btrfs:

Compile support for btrfs in the kernel rather than as a module, or use genkernel-next or Dracut to generate the initramfs.

Voir aussi

 * Btrfs snapshots - Script that creates snapshots when files have changed
 * Btrfs/System Root Guide - Use the Btrfs filesystem as a collection of subvolumes including one as a system root.
 * Btrfs native system root guide - An alternative guide on using a subvolume in a Btrfs filesystem as the system's root.
 * ext4 - The default filesystem for most Linux distributions.
 * Samba shadow copies - Using Samba to expose Shadow Copies as 'Previous Versions' to Windows clients.
 * Snapper - A command-line program capable of managing Btrfs filesystem snapshots.
 * ZFS - A filesystem that shares much in common with Btrfs, but has licensing issues.

Ressources externes

 * https://wiki.debian.org/Btrfs - Comme décrit par le wiki de Debian.
 * https://wiki.archlinux.org/index.php/Btrfs article sur le Btrfs - Comme décrit par le wiki de Arch Linux.
 * http://www.funtoo.org/BTRFS_Fun - BTRFS Fun sur le wiki de Funtoo.
 * http://marc.merlins.org/perso/btrfs/post_2014-05-04_Fixing-Btrfs-Filesystem-Full-Problems.html - Trucs et astuces pour résoudre les problèmes de Btrfs de niche dans certaines situations.