Btrfs (Русский)

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Состояние перевода: На этой странице представлен перевод статьи Btrfs. Дата последней синхронизации: 7 Декабря 2016. Вы можете помочь синхронизировать перевод, если в английской версии произошли изменения.

Из Wikipedia:Btrfs:

Btrfs (файловая система основанная на структурах B-деревьев, произносится как "butter F S", "better F S", "b-tree F S", или просто как аббревиатура) эта файловая система работает по принципу «копирование при записи» (COW), первоначально разработанная Oracle Corporation для использования в Linux. Разработка Btrfs началась в 2007, а в августе 2014 файловая система была помечена как стабильная.

Из Btrfs Wiki:

Btrfs это новая файловая система для Linux с принципом копирования при записи (CoW), направленная на реализацию дополнительных функций с особым упором на отказоустойчивость, восстановление и простоту администрирования. Совместная разработка Oracle, Red Hat, Fujitsu, Intel, SUSE, STRATO и многих других, Btrfs распространяется под лицензией GPL, что позволяет внести свой вклад любому желающему.
Важно: Btrfs содержит некоторые возможности, считающиеся экспериментальными. За подробностями, пройдите по ссылкам Btrfs Wiki's Статус, Btrfs стабильна? и Начинаем. Смотрите раздел #Известные проблемы.

Contents

Подготовка

Официальные ядра linux и linux-lts содержат поддержку Btrfs. Если вы хотите разместить загрузчик на разделе с файловой системой Btrfs, проверьте, поддерживает ли ваш boot loader Btrfs.

Установите пакет пользовательских утилит btrfs-progs.

Создание раздела диска Btrfs

Btrfs может обладать всем устройством хранения данных, заменяя схемы разбиения MBR или GPT, используя подтома для имитации разделов. Не нужно разбивать разделы, чтобы просто создать файловую систему Btrfs[broken link: invalid section] на существующем разделе который был создан с использованием другого метода. There are some limitations to partitionless single disk setups:

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#Install to partition or partitionless disk[broken link: invalid section].

Важно: GRUB strongly discourages installation to a partitionless disk.

Создание файловой системы

Файловая система Btrfs может быть создана с нуля или конвертирована из имеющейся ext3/ext4.

Создание новой файловой системы

Файловая система на одном устройстве

Для форматирования раздела:

# mkfs.btrfs -L mylabel /dev/partition

В Btrfs размер блока по умолчанию 16KB. Для того чтобы использовать больший размер блока для данных/метаданных, укажите значение nodesize с помощью -n как показано в примере с 16KB блоками:

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

Файловая система на нескольких устройствах

Важно:
  • As of August 2016, the RAID 5, RAID 6 mode of Btrfs is considered fatally flawed, and shouldn't be used for "anything but testing with throw-away data." [1]
  • Some boot loaders such as Syslinux do not support multi-device file systems.

Multiple devices can be entered 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 -m options 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 as well as the manpage for mkfs.btrfs.

# 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.

Примечание: 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)
Примечание: 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.

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

Конвертация Ext3/4 в Btrfs

Важно: As of mid-to-late 2015, there are many reports on the btrfs mailing list about incomplete/corrupt/broken conversions. The situation is improving as patches are being submitted, but proceed very carefully. Make sure you have working backups of any data you cannot afford to lose. See Conversion from Ext3 on the btrfs wiki.

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 GRUB articles). 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.

Настройка файловой системы

Копирование при записи (CoW)

По умолчанию, Btrfs использует Копирование при записи для всех файлов постоянно. Чтобы узнать как это реализовано и какие есть преимущества и недостатки, смотрите the Btrfs Sysadmin Guide section.

Отключение 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.

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.

Примечание: 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."
Совет: 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.

Принудительное CoW

To force copy-on-write when copying files use:

$ cp --reflink source dest 

This would only be required if CoW was disabled for the file to be copied (as implemented above). See the man page on cp for more details on the --reflink flag.

Сжатие

Btrfs supports transparent compression, meaning every file on the partition is automatically compressed. This not only reduces the size of files, but also improves performance, in particular if using the lzo algorithm, in some specific use cases (e.g. single thread with heavy file IO), while obviously harming performance on other cases (e.g. multithreaded and/or cpu intensive tasks with large file IO).

Compression is enabled using the compress=zlib or compress=lzo mount options. Only files created or modified after the mount option is added will be compressed. However, it can be applied quite easily to existing files (e.g. after a conversion from ext3/4) using the btrfs filesystem defragment -calg command, where alg is either zlib or lzo. In order to re-compress the whole file system with lzo, run the following command:

# btrfs filesystem defragment -r -v -clzo /
Совет: Compression can also be enabled per-file without using the compress mount option; simply apply chattr +c to the file. When applied to directories, it will cause new files to be automatically compressed as they come.

When installing Arch to an empty Btrfs partition, use the compress option when mounting the file system: mount -o compress=lzo /dev/sdxY /mnt/. During configuration, add compress=lzo to the mount options of the root file system in fstab.

Подтома

"A btrfs подтома (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." [2]

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[broken link: invalid section] instead.

See the following links for more details:

Создание подтома

Чтобы создать подтом:

# btrfs subvolume create /path/to/subvolume

Просмотр подтомов

Чтобы просмотреть список текущих подтомов по пути:

# btrfs subvolume list -p путь

Удаление подтома

Для удаления подтома:

# btrfs subvolume delete /path/to/subvolume

Попытка удалить каталог /путь/к/подтому без использования указанной выше команды не удалит подтом.

Монтирование подтомов

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 restore a file system (or part of it) to a previous state easily using #Snapshots[broken link: invalid section].

Совет: 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 /.
Примечание: "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." [3] 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.

Изменение подтома по умолчанию

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[broken link: invalid section].

Примечание: 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.
Важно: Changing the default subvolume with btrfs subvolume set-default will make the top level of the filesystem inaccessible when the default subvolume is mounted . Reference: Btrfs Wiki Sysadmin Guide.

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,ssd,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

Файловая система Btrfs способна освобождать неиспользуемые блоки из SSD-диска, поддерживающего команду TRIM. Больше информации о задействовании и использовании TRIM можно найти в разделе Solid State Drives#TRIM.

Использование

Показать использованное/свободное место

General linux userspace tools such as /usr/bin/df will inaccurately report free space on a Btrfs partition. It is recommended to use /usr/bin/btrfs to query a Btrfs partition. Below is an illustration of this effect, first querying using df -h, and then using btrfs filesystem df:

$ df -h /
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda3       119G  3.0G  116G   3% /
$ btrfs filesystem df /
Data: total=3.01GB, used=2.73GB
System: total=4.00MB, used=16.00KB
Metadata: total=1.01GB, used=181.83MB

Notice that df -h reports 3.0GB used but btrfs filesystem df reports 2.73GB for the data. This is due to the way Btrfs allocates space into the pool. The true disk usage is the sum of all three 'used' values which is inferior to 3.0GB as reported by df -h.

Примечание: If you see an entry of type unknown in the output of btrfs filesystem df at kernel >= 3.15, this is a display bug. As of this patch, the entry means GlobalReserve, which is kind of a buffer for changes not yet flushed. This entry is displayed as unknown, single in RAID setups and is not possible to re-balance.

Another useful command to show a less verbose readout of used space is btrfs filesystem show:

# btrfs filesystem show /dev/sda3

The newest command to get information on free/used space of a is btrfs filesystem usage:

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

Дефрагментация

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: Caveat from [4] plus the solution. (Discuss in Talk:Btrfs (Русский)#)

Btrfs supports online defragmentation. To defragment the metadata of the root folder:

# btrfs filesystem defragment /

This will not defragment the entire file system. For more information read this page on the Btrfs wiki.

To defragment the entire file system verbosely:

# btrfs filesystem defragment -r -v /

RAID

Btrfs offers native "RAID" for #Multi-device file system[broken link: invalid section]s. 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.

Важно: 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."

Важно: 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 /
Запуск службы или таймера

The btrfs-progs package brings the btrfs-scrub@.timer unit for monthly scrubbing the specified mountpoint. Enable the timer with an escaped path, e.g. btrfs-scrub@-.timer for / and btrfs-scrub@home.timer for /home. You can use the systemd-escape tool to escape a given string, see systemd-escape(1) for examples.

You can also run the scrub by starting btrfs-scrub@.service (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." [5] 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 /

Снимки

"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.

Примечание: Snapshots are not recursive. Every nested subvolume will be an empty directory inside the snapshot.

Отправить / получить

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 an incremental backups and for tools that automate the process.

Известные проблемы

Несколько ограничений должны быть известны перед использованием.

Шифрование

Btrfs has no built-in encryption support, but this may come in 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.

Файл подкачки (Swap)

Btrfs does not yet support swap files. This is due to swap files requiring a function that Btrfs does not have for possibility of file system corruption [6]. Patches for swapfile support are already available [7] 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 to automate this.

Ядро Linux-rt

Tango-view-refresh-red.pngThis article or section is out of date.Tango-view-refresh-red.png

Reason: We're on 4.6.4. Is this still an issue? (Discuss in Talk:Btrfs (Русский)#)

As of version 3.14.12_rt9, the linux-rt kernel does not boot with the Btrfs file system. This is due to the slow development of the rt patchset.

Tips and tricks

Checksum hardware acceleration

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 kernel to make it work. Note: put 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 initial ramdisk using mkinitcpio.

Then if there is a problem booting, the utility is available for repair.

Примечание: 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.

Загрузка в снимки с помощью GRUB

You can manually create a GRUB#GNU/Linux menu entry with the rootflags=subvol= argument. The subvol= mount options in /etc/fstab of the snapshot to boot into also have to be specified correctly.

Alternatively, you can automatically populate your GRUB menu with btrfs snapshots when regenerating the GRUB configuration file by using grub-btrfsAUR or grub-btrfs-gitAUR.

Использование подтомов Btrfs с systemd-nspawn

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

Решение проблем

See the Btrfs Problem FAQ for general troubleshooting.

GRUB

Смещение разделов

Примечание: 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.

Отсутствует root

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.

Ошибка 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.

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

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.

Примечание: 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/disk1 in /etc/fstab.

For a more detailed explanation, see the following blog post.

Проверка btrfs

Важно: 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 check with 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.

Смотрите также