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 the SUSE support if you do not remove any of the preconfigured subvolumes. You may, however, add additional subvolumes using the YaST Partitioner.

1.2.2.1 Mounting Compressed Btrfs File Systems #

 

Note: GRUB 2 and LZO Compressed Root

GRUB 2 cannot read an lzo compressed root. You need a separate /boot partition to use compression.

Since SLE12 SP1, compression for Btrfs file systems is supported. Use the compress or compress-force option and select the compression algorithm, lzo or zlib (the default). The zlib compression has a higher compression ratio while lzo is faster and takes less CPU load.

For example:

root # mount -o compress /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-compressable 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:

root # chattr +C FILE
root # 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:

root # 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 Ext (Ext2, Ext3, or Ext4) or ReiserFS to the Btrfs file system. The conversion process occurs offline and in place on the device. The file system needs at least 15% of available free space on the device.

To convert the file system to Btrfs, take the file system offline, then enter:

sudo btrfs-convert DEVICE

To roll back the migration to the original file system, take the file system offline, then enter:

sudo btrfs-convert -r DEVICE

Warning: Root File System Conversion not Supported

Converting the root file system to Btrfs is not supported. Either keep the existing file system or re-install the whole system from scratch.

Important: Possible Loss of Data

When rolling back to the original file system, all data will be lost that you added after the conversion to Btrfs. That is, only the original data is converted back to the previous file system.

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 /var/log, /var/crash and /var/cache can use all of the available disk space during normal operation, and cause a system malfunction. To help avoid this situation, SUSE Linux Enterprise Server now offers Btrfs quota support for subvolumes. If you set up the root file system by using the respective YaST proposal, it is prepared accordingly: quota groups (qgroup) for all subvolumes are already set up. To set a quota for a subvolume in the root file system, proceed as follows:

Note: Btrfs Quota Groups Can Incur Degraded Performance

On SUSE Linux Enterprise Server 12 SP5, using Btrfs quota groups can degrade file system performance.

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.

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.

1.2.6.2 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. duperemove is not installed by default. To make it available, install the package duperemove .

Note: Use Cases

As of SUSE Linux Enterprise Server 12 SP5 duperemove is not suited to deduplicate the entire file system. It is intended to be used to deduplicate a set of 10 to 50 large files that possibly have lots of blocks in common, such as virtual machine images.

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 ReiserFS or 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.

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. Ext3 is designed to take care of both metadata and data. The degree of “care” can be customized. Enabling Ext3 in the data=journal mode offers maximum security (data integrity), but can slow down the system because both metadata and data are journaled. A relatively new 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 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. Ext3 uses the data=ordered option as the default.

To convert an Ext2 file system to Ext3:

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 for Ext3 was increased from 128 bytes on SLES 10 to 256 bytes on SLES 11. As compared to SLES 10, when you make a new Ext3 file system on SLES 11, the default amount of space preallocated for the same number of inodes is doubled, and the usable space for files in the file system is reduced by that amount. Thus, you must use larger partitions to accommodate the same number of inodes and files than were possible for an Ext3 file system on SLES 10.

When you create a new Ext3 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. To address the increased inode size and reduced usable space available, the default for the bytes-per-inode ratio was increased from 8192 bytes on SLES 10 to 16384 bytes on SLES 11. The doubled ratio means that the number of files that can be created is one-half of the number of files possible for an Ext3 file system on SLES 10.

Important: Changing the Inode Size of an Existing Ext3 File System

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 Ext3 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:

If you do not use extended attributes or ACLs on your Ext3 file systems, you can restore the SLES 10 behavior specifying 128 bytes as the inode size and 8192 bytes as the bytes-per-inode ratio when you make the file system. Use any of the following methods to set the inode size and bytes-per-inode ratio:

For information, see https://www.suse.com/support/kb/doc.php?id=7009075 (SLES11 ext3 partitions can only store 50% of the files that can be stored on SLES10 [Technical Information Document 7009075]).

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.

On solid-state drives (SSDs) and thinly provisioned volumes it is useful to trim blocks not in use by the file system. SUSE Linux Enterprise Server fully supports unmap or trim operations on all file systems supporting these methods.

The recommended way to trim a supported file system (except Btrfs) on SUSE Linux Enterprise Server is to run /sbin/wiper.sh. Make sure to read /usr/share/doc/packages/hdparm/README.wiper before running this script. For most desktop and server systems the sufficient trimming frequency is once a week. Mounting a file system with -o discard comes with a performance penalty and may negatively affect the lifetime of SSDs and is not recommended.

Warning: Do Not Use wiper.sh on Btrfs

The wiper.sh script trims read-write mounted ext4 or XFS file systems and read-only mounted/unmounted ext2, ext3, ext4, or XFS file systems. Do not use wiper.sh on the Btrfs file system as it may damage your data. Instead, use /usr/share/btrfsmaintenance/btrfs-trim.sh which is part of the btrfsmaintenance package.

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: