23. Jan 2012

Using BTRFS on openSUSE 12.1

Manu Gupta
This article is contributed by Kamila SouÍkova

Introduction

As the btrfs wiki says: “Btrfs is a new copy on write filesystem for Linux aimed at implementing advanced features while focusing on fault tolerance, repair and easy administration.” Although under heavy development, it has become stable enough for personal use, and there are plenty of reasons to try it. What distinguishes it from earlier filesystems is that it has been designed with scalability and robustness in mind: it can handle huge files (up to 16EiB — a lot!), it can pack lots of files and directories efficiently, has built-in error detection methods (checksums of data and metadata), support for transparent compression, integrated multiple devices support (RAID-0, RAID-1 and RAID-10 so far) and more — see here for a more complete list.

In this how-to I will focus on one particularly neat feature: snapshots. Btrfs allows you to make read-only or writable snapshots of the state of your filesystem without wasting space with redundant data. Together with YaST’s Snapper module, this makes tracking FS changes and undoing undesired modifications a breeze.

Some Background

So how does btrfs do all that snapshot awesomeness? The answer is, it uses subvolumes. Subvolumes are something like separate filesystems within one partition: you can have several filesystem roots in the same container. With copy-on-write (COW) this means that “almost-separate filesystems” can share duplicate data and therefore save space. (Note: Subvolumes do not have a fixed size, they grow and shrink with the data they contain.)

With snapshots, one creates a copy of the whole filesystem within a new subvolume on the partition. As data is actually duplicated only once it is changed, making snapshots is both time- and space-efficient.

Time to Test It

Preparations

You will need the btrfs tools (package btrfsprogs). The Snapper YaST module provides a nice interface for btrfs’s snapshot functionality. You may create a new btrfs partition just for experimenting, or convert an existing ext3/ext4 filesystem. As for conversion, this is done using btrfs-convert; the good thing is that it can preserve the original ext FS and one can easily go back to it (see the wiki page for more info). Alternatively, openSUSE offers btrfs as an option during installation since 11.3.

Listing Snapshots

Once you have a partition, you can look at its snapshots with snapper list or view plain btrfs submodules/snapshots with btrfs subvolume list

Note that in the snapper listing snapshot #0 is always the current system. Also note that for btrfs snapshots and submodules are the same thing.

Snapper Configs

Snapper keeps configs for your partitions; you can view them with snapper list-configs

Configs allow you to specify when to make snapshots, how many of each tipe should be kept etc. They are stored in /etc/snapper/configs; config templates are in /etc/snapper/config-templates.

In case your root filesystem is btrfs, Snapper will have created a config that makes a “pre” and “post” snapshot pair whenever you use YaST or zypper in addition to the “timeline” snapshots.

You can create your own configs (e.g. for /home or whatever) with snapper create-config: snapper -c create-config [-t template] You need a config whenever you want a path other than the default /. (Always specify it with -c.)

Making Snapshots

To make snapshots manually use either snapper [-c config-name] create --description "something that tells you what this is" (and see snapper –help for more options); or the vanilla

btrfs subvolume snapshot /[subvolume-name]

A snapshot made with the former command will show in snapper list; one made with the latter will appear as a directory named in. Side note: Snapper actually stores its snapshots in/.snapshots/.

Viewing Differences

To see the differences between snapshots you may either mount the snapshots somewhere (with snapper mount , or simply browse to the snapshot location) and see for yourself, or use the very convenient snapper diff: snapper diff will give you the list of files changed between the two given snapshots; snapper diff will show you the files’ diff.

Restoring Previous Snapshots

To restore a snapshot made with snapper use snapper undochange []... You can also specify ranges, as in snapper undochange 42..47.

To do the same with vanilla btrfs use btrfs subvolume set-default where can be found out with btrfs subvolume list and then unmount and remount.

Snapper GUI

Snapper also provides a YaST GUI for comparing and restoring snapshots or individual files.

** Final Words** Btrfs does not have an error-correcting fsck yet, so just now it is not ready for systems that require high reliability. But that doesn’t mean you shouldn’t use it ­— it merely means that you should keep backups, which you should do anyway. So go ahead and give it a try! Its snapshots feature (together with all the other awesomeness) makes it a really worthy filesystem for both personal use and servers, and with Snapper managing snapshots is really convenient. Note: Kernel 3.1 Fixes from kernel 3.2 have not been backported into 3.1 yet, so you may want to use the 3.2 kernel. See e.g. https://bugzilla.novell.com/show_bug.cgi?id=733843 in case you get crashes with 3.1.

References and Further Reading

BTRFS wiki

Linux.com Tuturial

openSUSE Snapper Portal

Categories: Distribution

Tags: btrfs

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