Btrfs Snapshot Integration in KDE

# Btrfs Snapshot Integration in KDE Source: [https://bharadwajraju.com/posts/btrfs-snapshots-in-kde/](https://bharadwajraju.com/posts/btrfs-snapshots-in-kde/) I have been working on integrating Btrfs snapshots intoKDEsoftware\. The central part of this work has been realized in the form of[KIOSnapshot](https://invent.kde.org/system/kio-snapshot), which has just been released\. Here I want to discuss what it is, how it works, how it was developed, and the surrounding work acrossKDE\. ## [Background](https://bharadwajraju.com/posts/btrfs-snapshots-in-kde/#background) Among the key features of Btrfs is the ability to take efficient*snapshots*ofsubvolumes\(Subvolumes are independently manageable directory trees within your filesystem\. You can snapshot or rollback a subvolume atomically and independently from the rest of your filesystem\. Snapshots are just special cases of subvolumes\.\)\. They are efficient because Btrfs makes snapshots share file extents with the originals, so snapshots only take up additional space where they differ from the original\. Thus it is cheap to take snapshots frequently without worrying about disk space\. This can be used to build very handy universal “undo” or “time travel” functionality for the user\. Yet, though there are graphical tools to work with Btrfs snapshots such as[Btrfs Assistant](https://gitlab.com/btrfs-assistant/btrfs-assistant), these are separate from normal file browsing, and are rather technical tools concerned with the orchestration of snapshots\. Direct integration of snapshots into the file browser itself for mundane end\-user purposes, like Windows has with Previous Versions or macOS with the famous Time Machine, has been lacking in the Linux world\. The aim ofKIOSnapshot is to build that kind of direct integration forKDE\. ## [What it is](https://bharadwajraju.com/posts/btrfs-snapshots-in-kde/#what-it-is) KIOSnapshot lets Dolphin \(indeed anyKDEsoftware\) list and access Btrfs snapshots of a file or subvolume\. You can right\-click on a file and go to a folder view showing you all the distinct past versions of it as saved in your snapshots\.​\(At first, I wrote the snapshots\-for\-file case as a dialog with buttons to open or restore \(like Windows’ Previous Versions feature\), but I changed it to be a full virtual folder, since that would be much more flexible — now a user could select multiple previous versions and open them in a comparison tool, or copy them somewhere, or check their metadata easily, or whatever else they wished\.\) You also have views into entire directory trees of subvolumes at their various snapshots\. Note that it does not*take*snapshots, it only allows access to them\. To take snapshots, you would have to do it manually, or through an orchestrator like[Snapper](https://snapper.io/)\(If you*are*using Snapper, you should add yourself to the`ALLOW\_USERS`setting for your Snapper config and turn on`SYNC\_ACL=yes`, to allow rootless access to your snapshots\. See[Snapper\-Configs\(5\)](http://snapper.io/manpages/snapper-configs.html)and[Snapper\(8\) § Permissions](http://snapper.io/manpages/snapper.html#permissions)for details\.\)\. ## [How it works](https://bharadwajraju.com/posts/btrfs-snapshots-in-kde/#how-it-works) Mainly it uses[libbtrfsutil](https://github.com/kdave/btrfs-progs/tree/master/libbtrfsutil)from btrfs\-progs to talk to the filesystem, andKDEFrameworks’[Solid](https://develop.kde.org/docs/features/solid/)to query filesystems and mounts on a more meta level\. Now, the BtrfsAPIis quite conservative in what it allows non\-superusers to do with it\. Even a question as seemingly innocuous as “what subvolume is this path in?” cannot be answered for a non\-superuser directly\. The consequence of this is that on my first attempt at building this integration, I had one component running as a system\-level DBus service which would let users query stuff like this for files they owned\. Luckily, a nudge from Méven Car made me realize that with some working\-around, I could build out all the features without anything running as root\. For example, while Btrfs is loath to let you get the subvolumeIDfor a path or vice versa, it will happily give you a listing of the subvolumes \(that you can access\) under a path\. From there you can derive all the information needed\. Using this,KIOSnapshot implements aKIOworker that provides the`snapshot://`protocol, which will be understood by all programs which useKDEFrameworks\. This protocol provides a virtual view into the snapshots in a filesystem along two dimensions: the snapshots for a given subvolume, each of which is a browsable directory tree in itself; and the snapshots for a given file across all snapshots of its containing subvolume\. ## [Allied work](https://bharadwajraju.com/posts/btrfs-snapshots-in-kde/#allied-work) Aside fromKIOSnapshot itself, I also worked on some small things in other parts ofKDEto support it\. - Fixed a bug inKIOwhich caused inconsistent behavior in Dolphin’s location bar:[KIOMR\#2305](https://invent.kde.org/frameworks/kio/-/merge_requests/2305) - Fixed how Solid handled Btrfs layouts like the one used inKDELinux:[SolidMR\#261](https://invent.kde.org/frameworks/solid/-/merge_requests/261) - Special default view settings forKIOSnapshot’s views in Dolphin:[DolphinMR\#1347](https://invent.kde.org/system/dolphin/-/merge_requests/1347) - Experimented with adding support for Btrfs subvolumes into Solid itself — this is just a rough proof\-of\-concept, and maybe this work is more appropriate further upstream in UDisks — but here it is anyway:[Solid branch btrfs\-subvolumes](https://invent.kde.org/frameworks/solid/-/tree/work/bharadwaj/btrfs-subvolumes?ref_type=heads) ## [KDELinux](https://bharadwajraju.com/posts/btrfs-snapshots-in-kde/#kde-linux) KDELinux will soon ship with Snapper andKIOSnapshot out\-of\-the\-box\. Hadi Chokr did a lot of excellent integration work here:[migrating the filesystem layout to make all user homes subvolumes](https://invent.kde.org/kde-linux/kde-linux/-/merge_requests/475), and[automatically integrating and configuring Snapper for all users](https://invent.kde.org/kde-linux/kde-linux/-/merge_requests/613)seamlessly\. This is part of a broader initiative inKDELinux to[improve data backup and restore systems](https://invent.kde.org/kde-linux/kde-linux/-/work_items/666), which has also overseen improvements elsewhere, such as in the Kup backup system\. ## [Release](https://bharadwajraju.com/posts/btrfs-snapshots-in-kde/#release) The[first stable release](https://download.kde.org/stable/kio-snapshot/)of it was made today \(thanks to Bhushan Shah for helping with the release process\)\. I imagine it should be getting packaged into distros fairly soon, thanks to the infrastructure that comes with being aKDEproject, but even then it should be easy enough to compile from source\. Please use it and[report bugs and requests](https://bugs.kde.org/enter_bug.cgi?product=kio-snapshot&component=general), thank you\!