1 Overview of File Systems in Linux #

Btrfs provides fault tolerance, repair, and easy management features, such as the following:

By default, SUSE Linux Enterprise Server is set up using Btrfs and snapshots for the root partition. Snapshots allow you to easily roll back your system if needed after applying updates, or to back up files. Snapshots can easily be managed with the SUSE Snapper infrastructure as explained in Chapter 7, System Recovery and Snapshot Management with Snapper. For general information about the SUSE Snapper project, see the Snapper Portal wiki at OpenSUSE.org (http://snapper.io).

When using a snapshot to roll back the system, it must be ensured that data such as user's home directories, Web and FTP server contents or log files do not get lost or overwritten during a roll back. This is achieved by using Btrfs subvolumes on the root file system. Subvolumes can be excluded from snapshots. The default root file system setup on SUSE Linux Enterprise Server as proposed by YaST during the installation contains the following subvolumes. They are excluded from snapshots for the reasons given below.

Warning: Support for Rollbacks

Rollbacks are only supported by SUSE if you do not remove any of the preconfigured subvolumes. You may, however, add subvolumes using the YaST Partitioner.

1.2.2.1 Mounting Compressed Btrfs File Systems #

 

Note: GRUB 2 and Compressed Root

GRUB 2 cannot boot from lzo or zstd compressed root file systems. Use zlib compression, or create a separate /boot partition if you wish to use lzo or zstd compression for root.

The Btrfs file system supports transparent compression. While enabled, Btrfs compresses file data when written and uncompresses file data when read.

Use the compress or compress-force mount option and select the compression algorithm, zstd, lzo or zlib (the default). zlib compression has a higher compression ratio while lzo is faster and takes less CPU load. The zstd algorithm offers a modern compromise, with performance close to lzo and compression ratios similar to zlib.

For example:

# mount -o compress=zstd /dev/sdx /mnt

In case you create a file, write to it, and the compressed result is greater or equal to the uncompressed size, Btrfs will skip compression for future write operations forever for this file. If you do not like this behavior, use the compress-force option. This can be useful for files that have some initial non-compressible data.

Note, compression takes effect for new files only. Files that were written without compression are not compressed when the file system is mounted with the compress or compress-force option. Furthermore, files with the nodatacow attribute never get their extents compressed:

# chattr +C FILE
# mount -o nodatacow  /dev/sdx /mnt

In regard to encryption, this is independent from any compression. After you have written some data to this partition, print the details:

# btrfs filesystem show /mnt
btrfs filesystem show /mnt
Label: 'Test-Btrfs'  uuid: 62f0c378-e93e-4aa1-9532-93c6b780749d
        Total devices 1 FS bytes used 3.22MiB
      devid    1 size 2.00GiB used 240.62MiB path /dev/sdb1

If you want this to be permanent, add the compress or compress-force option into the /etc/fstab configuration file. For example:

UUID=1a2b3c4d /home btrfs subvol=@/home,compress 0 0

You can migrate data volumes from existing ReiserFS or Ext (Ext2, Ext3, or Ext4) to the Btrfs file system using the btrfs-convert tool. This allows you to do an in-place conversion of unmounted (offline) file systems, which may require a bootable install media with the btrfs-convert tool. The tool constructs a Btrfs file system within the free space of the original file system, directly linking to the data contained in it. There must be enough free space on the device to create the metadata or the conversion will fail. The original file system will be intact and no free space will be occupied by the Btrfs file system. The amount of space required is dependent on the content of the file system and can vary based on the number of file system objects (such as files, directories, extended attributes) contained in it. Since the data is directly referenced, the amount of data on the file system does not impact the space required for conversion, except for files that use tail packing and are larger than about 2 KiB in size.

Warning: Root file system conversion not supported

Converting the root file system to Btrfs is not supported and not recommended. Automating such a conversion is not possible due to various steps that need to be tailored to your specific setup—the process requires a complex configuration to provide a correct rollback, /boot must be on the root file system, and the system must have specific subvolumes, etc. Either keep the existing file system or re-install the whole system from scratch.

To convert the original file system to the Btrfs file system, run:

# btrfs-convert /path/to/device

Important: Check /etc/fstab

After the conversion, you need to ensure that any references to the original file system in /etc/fstab have been adjusted to indicate that the device contains a Btrfs file system.

When converted, the contents of the Btrfs file system will reflect the contents of the source file system. The source file system will be preserved until you remove the related read-only image created at fs_root/reiserfs_saved/image. The image file is effectively a 'snapshot' of the ReiserFS file system prior to conversion and will not be modified as the Btrfs file system is modified. To remove the image file, remove the reiserfs_saved subvolume:

# btrfs subvolume delete fs_root/reiserfs_saved

To revert the file system back to the original one, use the following command:

# btrfs-convert -r /path/to/device

Warning: Lost Changes

Any changes you made to the file system while it was mounted as a Btrfs file system will be lost. A balance operation must not have been performed in the interim, or the file system will not be restored correctly.

Btrfs is integrated in the YaST Partitioner and AutoYaST. It is available during the installation to allow you to set up a solution for the root file system. You can use the YaST Partitioner after the installation to view and manage Btrfs volumes.

Btrfs administration tools are provided in the btrfsprogs package. For information about using Btrfs commands, see the man 8 btrfs, man 8 btrfsck, and man 8 mkfs.btrfs commands. For information about Btrfs features, see the Btrfs wiki at http://btrfs.wiki.kernel.org.

The Btrfs root file system subvolumes (for example, /var/log, /var/crash, or /var/cache) can use all the available disk space during normal operation, and cause a system malfunction. To help avoid this situation, SUSE Linux Enterprise Server offers quota support for Btrfs subvolumes. If you set up the root file system from a YaST proposal, you are ready to enable and set subvolume quotas.

Tip: Nullifying a Quota

In case you want to nullify an existing quota, set a quota size of none:

> sudo btrfs qgroup limit none /var/tmp

To disable quota support for a partition and all its subvolumes, use btrfs quota disable:

> sudo btrfs quota disable /

See the man 8 btrfs-qgroup and man 8 btrfs-quota for more details. The UseCases page on the Btrfs wiki (https://btrfs.wiki.kernel.org/index.php/UseCases) also provides more information.

Important: Snapshots with swapping

You will not be able to create a snapshot if there is any enabled swap file in the source subvolume.

SLES supports swapping to a file on the btrfs filesystem if the following criteria relating to the resulting swap file are fulfilled:

Btrfs allows to make snapshots to capture the state of the file system. Snapper, for example, uses this feature to create snapshots before and after system changes, allowing a rollback. However, together with the send/receive feature, snapshots can also be used to create and maintain copies of a file system in a remote location. This feature can, for example, be used to do incremental backups.

A btrfs send operation calculates the difference between two read-only snapshots from the same subvolume and sends it to a file or to STDOUT. A btrfs receive operation takes the result of the send command and applies it to a snapshot.

1.2.7.2 Incremental Backups #

 

The following procedure shows the basic usage of Btrfs send/receive using the example of creating incremental backups of /data (source side) in /backup/data (target side). /data needs to be a subvolume.

Procedure 1.1: Initial Setup #

 

1. Create the initial snapshot (called snapshot_0 in this example) on the source side and make sure it is written to the disk:

   > sudo btrfs subvolume snapshot -r /data /data/bkp_data
   sync

   A new subvolume /data/bkp_data is created. It will be used as the basis for the next incremental backup and should be kept as a reference.

2. Send the initial snapshot to the target side. Since this is the initial send/receive operation, the complete snapshot needs to be sent:

   > sudo bash -c 'btrfs send /data/bkp_data | btrfs receive /backup'

   A new subvolume /backup/bkp_data is created on the target side.

When the initial setup has been finished, you can create incremental backups and send the differences between the current and previous snapshots to the target side. The procedure is always the same:

1. Create a new snapshot on the source side.

2. Send the differences to the target side.

3. Optional: Rename and/or clean up snapshots on both sides.

Procedure 1.2: Performing an Incremental Backup #

 

1. Create a new snapshot on the source side and make sure it is written to the disk. In the following example the snapshot is named bkp_data_CURRENT_DATE:

   > sudo btrfs subvolume snapshot -r /data /data/bkp_data_$(date +%F)
   sync

   A new subvolume, for example /data/bkp_data_2016-07-07, is created.

2. Send the difference between the previous snapshot and the one you have created to the target side. This is achieved by specifying the previous snapshot with the option -p SNAPSHOT.

   > sudo bash -c 'btrfs send -p /data/bkp_data /data/bkp_data_2016-07-07 \
   | btrfs receive /backup'

   A new subvolume /backup/bkp_data_2016-07-07 is created.

3. As a result four snapshots, two on each side, exist:

   /data/bkp_data

   /data/bkp_data_2016-07-07

   /backup/bkp_data

   /backup/bkp_data_2016-07-07

   Now you have three options for how to proceed:

   • Keep all snapshots on both sides. With this option you can roll back to any snapshot on both sides while having all data duplicated at the same time. No further action is required. When doing the next incremental backup, keep in mind to use the next-to-last snapshot as parent for the send operation.

   • Only keep the last snapshot on the source side and all snapshots on the target side. Also allows to roll back to any snapshot on both sides—to do a rollback to a specific snapshot on the source side, perform a send/receive operation of a complete snapshot from the target side to the source side. Do a delete/move operation on the source side.

   • Only keep the last snapshot on both sides. This way you have a backup on the target side that represents the state of the last snapshot made on the source side. It is not possible to roll back to other snapshots. Do a delete/move operation on the source and the target side.

   1. To only keep the last snapshot on the source side, perform the following commands:

      > sudo btrfs subvolume delete /data/bkp_data
      > sudo mv /data/bkp_data_2016-07-07 /data/bkp_data

      The first command will delete the previous snapshot, the second command renames the current snapshot to /data/bkp_data. This ensures that the last snapshot that was backed up is always named /data/bkp_data. As a consequence, you can also always use this subvolume name as a parent for the incremental send operation.

   2. To only keep the last snapshot on the target side, perform the following commands:

      > sudo btrfs subvolume delete /backup/bkp_data
      > sudo mv /backup/bkp_data_2016-07-07 /backup/bkp_data

      The first command will delete the previous backup snapshot, the second command renames the current backup snapshot to /backup/bkp_data. This ensures that the latest backup snapshot is always named /backup/bkp_data.

Tip: Sending to a Remote Target Side

To send the snapshots to a remote machine, use SSH:

> btrfs send /data/bkp_data | ssh root@jupiter.example.com 'btrfs receive /backup'

Btrfs supports data deduplication by replacing identical blocks in the file system with logical links to a single copy of the block in a common storage location. SUSE Linux Enterprise Server provides the tool duperemove for scanning the file system for identical blocks. When used on a Btrfs file system, it can also be used to deduplicate these blocks and thus save space on the file system. duperemove is not installed by default. To make it available, install the package duperemove .

Note: Deduplicating Large Datasets

If you intend to deduplicate a large amount of files, use the --hashfile option:

> sudo duperemove --hashfile HASH_FILE file1 file2 file3

The --hashfile option stores hashes of all specified files into the HASH_FILE instead of RAM and prevents it from being exhausted. HASH_FILE is reusable—you can deduplicate changes to large datasets very quickly after an initial run that generated a baseline hash file.

duperemove can either operate on a list of files or recursively scan a directory:

> sudo duperemove OPTIONS file1 file2 file3
> sudo duperemove -r OPTIONS directory

It operates in two modes: read-only and de-duping. When run in read-only mode (that is without the -d switch), it scans the given files or directories for duplicated blocks and prints them. This works on any file system.

Running duperemove in de-duping mode is only supported on Btrfs file systems. After having scanned the given files or directories, the duplicated blocks will be submitted for deduplication.

For more information see man 8 duperemove.

You may need to delete one of the default Btrfs subvolumes from the root file system for specific purposes. One of them is transforming a subvolume—for example @/home or @/srv—into a file system on a separate device. The following procedure illustrates how to delete a Btrfs subvolume:

SUSE Linux Enterprise Server supports the “on-disk format” (v5) of the XFS file system. The main advantages of this format are automatic checksums of all XFS metadata, file type support, and support for a larger number of access control lists for a file.

Note that this format is not supported by SUSE Linux Enterprise kernels older than version 3.12, by xfsprogs older than version 3.2.0, and GRUB 2 versions released before SUSE Linux Enterprise 12.

Important: The V4 is deprecated

XFS is deprecating file systems with the V4 format. This file system format was created by the command:

mkfs.xfs -m crc=0 DEVICE

The format was used in SLE 11 and older releases and currently it creates a warning message by dmesg:

Deprecated V4 format (crc=0) will not be supported after September 2030

If you see the message above in the output of the dmesg command, it is recommended that you update your file system to the V5 format:

1. Back up your data to another device.

2. Create the file system on the device.

   mkfs.xfs -m crc=1 DEVICE

3. Restore the data from the backup on the updated device.

The code for Ext2 is the strong foundation on which Ext3 could become a highly acclaimed next-generation file system. Its reliability and solidity are elegantly combined in Ext3 with the advantages of a journaling file system. Unlike transitions to other journaling file systems, such as XFS, which can be quite tedious (making backups of the entire file system and re-creating it from scratch), a transition to Ext3 is a matter of minutes. It is also very safe, because re-creating an entire file system from scratch might not work flawlessly. Considering the number of existing Ext2 systems that await an upgrade to a journaling file system, you can easily see why Ext3 might be of some importance to many system administrators. Downgrading from Ext3 to Ext2 is as easy as the upgrade. Perform a clean unmount of the Ext3 file system and remount it as an Ext2 file system.

To convert an Ext2 file system to Ext3:

Some other journaling file systems follow the “metadata-only” journaling approach. This means your metadata is always kept in a consistent state, but this cannot be automatically guaranteed for the file system data itself. Ext4 is designed to take care of both metadata and data. The degree of “care” can be customized. Mounting Ext4 in the data=journal mode offers maximum security (data integrity), but can slow down the system because both metadata and data are journaled. Another approach is to use the data=ordered mode, which ensures both data and metadata integrity, but uses journaling only for metadata. The file system driver collects all data blocks that correspond to one metadata update. These data blocks are written to disk before the metadata is updated. As a result, consistency is achieved for metadata and data without sacrificing performance. A third mount option to use is data=writeback, which allows data to be written to the main file system after its metadata has been committed to the journal. This option is often considered the best in performance. It can, however, allow old data to reappear in files after crash and recovery while internal file system integrity is maintained. Ext4 uses the data=ordered option as the default.

An inode stores information about the file and its block location in the file system. To allow space in the inode for extended attributes and ACLs, the default inode size was increased to 256 bytes.

When you create a new Ext4 file system, the space in the inode table is preallocated for the total number of inodes that can be created. The bytes-per-inode ratio and the size of the file system determine how many inodes are possible. When the file system is made, an inode is created for every bytes-per-inode bytes of space:

number of inodes = total size of the file system divided by the number of bytes per inode

The number of inodes controls the number of files you can have in the file system: one inode for each file.

Important: Changing the inode size of an existing ext4 file system not possible

After the inodes are allocated, you cannot change the settings for the inode size or bytes-per-inode ratio. No new inodes are possible without re-creating the file system with different settings, or unless the file system gets extended. When you exceed the maximum number of inodes, no new files can be created on the file system until some files are deleted.

When you make a new Ext4 file system, you can specify the inode size and bytes-per-inode ratio to control inode space usage and the number of files possible on the file system. If the blocks size, inode size, and bytes-per-inode ratio values are not specified, the default values in the /etc/mked2fs.conf file are applied. For information, see the mke2fs.conf(5) man page.

Use the following guidelines:

Use any of the following methods to set the inode size and bytes-per-inode ratio:

Periodic TRIM is handled by the fstrim command invoked by systemd on a regular basis. You can also run the command manually.

To schedule periodic TRIM, enable the fstrim.timer as follows:

> sudo systemctl enable fstrim.timer

systemd creates a unit file in /usr/lib/systemd/system. By default, the service runs once a week, which is usually sufficient. However, you can change the frequency by configuring the OnCalendar option to a required value.

The default behaviour of fstrim is to discard all blocks in the file system. You can use options when invoking the command to modify this behaviour. For example, you can pass the offset option to define the place where to start the trimming procedure. For details, see man fstrim.

The fstrim command can perform trimming on all devices stored in the /etc/fstab file, which support the TRIM operation—use the -A option when invoking the command for this purpose.

Online TRIM of a device is performed each time data is written to the device.

To enable online TRIM on a device, add the discard option to the /etc/fstab file as follows:

UID=83df497d-bd6d-48a3-9275-37c0e3c8dc74  /  btrfs  defaults,discard

Alternatively, on the Ext4 file system you can use the tune2fs command to set the discard option in /etc/fstab:

> sudo tune2fs -o discard DEVICE

The discard option is also added to /etc/fstab in case the device was mounted by mount with the discard option:

> sudo mount -o discard DEVICE

Note: Drawbacks of online TRIM

Using the discard option may decrease the lifetime of some lower-quality SSD devices. Online TRIM can also impact the performance of the device, for example, if a larger amount of data is deleted. In this situation, an erase block might be reallocated, and shortly afterwards, the same erase block might be marked as unused again.

The root (/) partition using the Btrfs file system stops accepting data. You receive the error “No space left on device”.

See the following sections for information about possible causes and prevention of this issue.

The btrfs balance command is part of the btrfs-progs package. It balances block groups on Btrfs file systems in the following example situations:

Tip

To fine-tune the default behavior of balancing data on Btrfs file systems—for example, how frequently or which mount points to balance— inspect and customize /etc/sysconfig/btrfsmaintenance. The relevant options start with BTRFS_BALANCE_.

For details about the btrfs balance command usage, see its manual pages (man 8 btrfs-balance).

Linux file systems contain mechanisms to avoid data fragmentation and usually it is not necessary to defragment. However, there are use cases, where data fragmentation cannot be avoided and where defragmenting the hard disk significantly improves the performance.

This only applies to conventional hard disks. On solid state disks (SSDs) which use flash memory to store data, the firmware provides an algorithm that determines to which chips the data is written. Data is usually spread all over the device. Therefore defragmenting an SSD does not have the desired effect and will reduce the lifespan of an SSD by writing unnecessary data.

For the reasons mentioned above, SUSE explicitly recommends not to defragment SSDs. Some vendors also warn about defragmenting solid state disks. This includes, but it is not limited to the following: