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Fábio Rehm

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Getting to know BTRFS

I’ve been playing a lot with Docker recently and while learning more about it I came across “Another Union File System” (AUFS) + its Copy On Write (COW) capabilities and have been pretty impressed by it. In short, by using a COW + union file system Docker makes things really cheap when it comes to disk usage.

The idea of COW filesystems along with a couple GitHub issues, some tweets exchanged with @rcarmo and this post on improvements made on lxc-clone was enough to trigger my interest on BTRFS and I went out to learn more about it.

What’s with Copy on Write (aka COW)?

Some googling pointed me to this comprehensive thesis on Copy On Write Based File Systems by Sakis Kasampalis and while I havent read the whole thesis yet, I found this pretty nice definition about COW on page 19:

COW generally follows a simple principle. As long as multiple programs need read only access to a data structure, providing them only pointers which point to the same data structure, instead of copying the whole structure to a new one, is enough.

If you’ve never heard of COW take the time to grasp those words and think about applying that to a File System. If you can’t get the big picture don’t worry, just keep reading :)

Going further, Kasampalis writes:

If at least one of the programs needs at some point write access to the data structure, create a private copy for it. The same holds for each one of the programs which will need write access to the data structure. A new private copy will be created for them.

Let’s assume that we have an array data structure called “foo”, and two programs called “Bob” and “Alice” which need to access it in read/write mode. If during the access, none of the programs tries to change the contents of “foo”, both “Bob” and “Alice” will actually use exactly the same “foo” array. If at some point “Bob” changes the contents of “foo”, a private copy of the changed data of “foo” will automatically be created by the COW system and provided to “Bob”. Note that the unchanged data can still be shared between “Bob” and “Alice”. This is a powerful feature of COW.

He also highlights the benefits of COW file systems on page 20:

I believe that COW’s major contributions to file systems are the support for (1) taking cheap snapshots, (2) ensuring both data and metadata consistency and integrity with acceptable performance.

As another COW example, we have Ruby Enterprise Edition, that shipped with a “copy-on-write friendly garbage collector, capable of reducing Ruby on Rails applications’ memory usage by 33% on average” and had its EOL anounced once a copy-on-write patch was checked into Ruby 2.0. For other use cases please refer to Wikipedia.

B-tree data structure

If you forgot about your data structure lessons, here’s the definition from Wikipedia:

In computer science, a B-tree is a tree data structure that keeps data sorted and allows searches, sequential access, insertions, and deletions in logarithmic time. The B-tree is a generalization of a binary search tree in that a node can have more than two children. (Comer 1979, p. 123) Unlike self-balancing binary search trees, the B-tree is optimized for systems that read and write large blocks of data. It is commonly used in databases and filesystems.

B-tree file system (aka BTRFS)

From Wikipedia again:

Btrfs […] is a GPL-licensed experimental copy-on-write file system for Linux. […] Btrfs is intended to address the lack of pooling, snapshots, checksums and integral multi-device spanning in Linux file systems, these features being crucial as Linux use scales upward into the larger storage configurations common in enterprise. Chris Mason, the principal Btrfs author, has stated that its goal was “to let Linux scale for the storage that will be available. Scaling is not just about addressing the storage but also means being able to administer and to manage it with a clean interface that lets people see what’s being used and makes it more reliable.”

Although it looks pretty exciting from outside when you play with it, there are some things to keep in mind:

It is still in heavy development and marked as unstable. Especially when the filesystem becomes full, no-space conditions arise which might make it challenging to delete files.

Trying it out (aka “What it means for vagrant-lxc?”)

Enough theory and copy and pasting! Because an asciicast is worth more than a thousand words, check out the one below. I used the quantal machine from vagrant-lxc-vbox-hosts to fire up a VBox machine ready to rock and recorded the asciicast from there.

You can find out more about and play with BTRFS by looking at this entry from Funtoo Linux Wiki.

I hope I gave you enough reason to watch the asciicast with the information above but if you ended up not watching it, I’d just like to let you know that I was able to “break” one of the most basic Physics law that states that two things cannot occupy the same space at the same time by using lxc-clone and its snapshotting capabilities :P Thanks to BTRFS I was capable of fitting 22 GB of data into just 405MB!

So, can you guess what does that mean for vagrant-lxc? Basically even faster container creation times (under 0.1 millisecond for lxc-clone + BTRFS snapshotting) and lower disk usage. This also means that support for vagrant-lxc container snapshots should be easier to implement once the cloning functionality is in place.

That’s awesome! I want it now!

Well… Unfortunately I’m not sure when this is going to be easily supported from vagrant-lxc, so for now you’ll need to do things by hand. Please check the asciicast in order to find out how to set things up and to “trick” vagrant-lxc into using your cloned containers.

One thing that I know for sure is that we’ll have to wait for Vagrant 1.4 because of this pull request which enables us to hook into Vagrant’s process of removing base boxes. The reason behind that is that we’ll need to keep a “base container” around in order to create our lxc clones from and without that we are not able to automatically delete the base container rootfs in case the user removes a vagrant-lxc base box, leaving unused containers behind.

On a side note, I’ve had some trouble using lxc-clone with vagrant-lxc containers on both an Ubuntu 13.04 VBox VM and my own laptop so I had to switch back to a 12.10 VM in order to record the asciicast. I haven’t had the chance to find out what’s going on yet but it seems that I’m not the only one having trouble with it.

To finish this up, I want to make clear that I’ve only done some initial experiments with BTRFS and I have no idea how it behaves on the wild but I’ll definitely look into adding some experimental support for lxc-clone in a future vagrant-lxc version. I’m not really sure how that will look like but on the spirit of eating my own dog food I’ll try to create a BTRFS partition on my physical HD soon as a start see how it goes :)


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