The problem#

For my recent posts on ZFS, I wanted to quickly try out a bunch of variants of my proposed operations without worrying about accidentally modifying my real ZFS filesystems. Specifically, I wanted to know which ways of copying files would result in more efficiently reusing blocks from existing snapshots where possible.

The solution#

WARNING: The instructions below will modify the ZFS pool tank, which is the default name used in many ZFS examples, and therefore may be a real ZFS pool on your computer.

I strongly recommend doing all of this inside a VM to be sure you are not affecting any real filesystems. I used a VirtualBox VM that I installed Debian on and used the guest additions to share a directory between the VM and my actual machine.

First create a 1 GiB virtual (i.e. in a file instead of a physical device) ZFS pool to run tests on:

fallocate -l 1G /root/tank
zpool create tank /root/tank

Then perform various filesystem operations and inspect the result of zfs list -o space to determine if they were using more (or less) space than you expect. In order to make sure I was being consistent and make it easier to test out multiple variations, I wrote some scripts:

git clone https://git.aweirdimagination.net/perelman/zfs-test.git
cd zfs-test/bin
# dump logs from create-/copy-all- and-measure into ../logs/
./measure-all
# read ../logs/ and print space used as Markdown table
./logs-to-table --links

The details#

What is Madoko?#

Madoko is an extension of Markdown for scholarly papers. In essence, it is a competitor to LaTeX, which, along with Microsoft Word, is the way the vast majority of such papers are presently authored.

Madoko targets both HTML and LaTeX output in order to be compatible with existing workflows while encouraging the creation of HTML versions of papers which are presently rare as PDF is the default for publishing even though it is sub-optimal for reading on a screen.