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Day: February 3, 2021

Launching Linux VMs with Firecracker

Published 2021-02-03 by Kevin Feasel

Julia Evans gives us an introduction to Firecracker:

Firecracker says this about performance in their specification:

It takes <= 125 ms to go from receiving the Firecracker InstanceStart API call to the start of the Linux guest user-space /sbin/init process.

So far I’ve been using Firecracker to start relatively large VMs – Ubuntu VMs running systemd as an init system – and it takes maybe 2-3 seconds for them to boot. I haven’t been measuring that closely because honestly 5 seconds is fast enough and I don’t mind too much about an extra 200ms either way.

That’s pretty fast. Click through for more info on installation and configuration.

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Histograms versus Bar Charts

Published 2021-02-03 by Kevin Feasel

Alex Velez explains the difference between a histogram and a bar (or column) chart:

Consider the above illustration of two data visualizations. 

A histogram is on the left, and to the right is a bar chart (also known as a bar graph). Histograms and bar charts look almost identical, yet they are dramatically different. Understanding their differences is important, so you know when to use each one and accurately convey—or consume—the insights they contain. 

Let’s take a closer look. 

Click through for that closer look.

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DEFINE TABLE in DAX Queries

Published 2021-02-03 by Kevin Feasel

Marco Russo and Alberto Ferrari takes us through the DEFINE TABLE statement in DAX:

Introduced in December 2020, the DEFINE TABLE statement lets you define a calculated table local to a query. The table is not persisted in the model, it exists only for the lifetime of the query. Apart from that, it is a calculated table in every sense of the term albeit with some limitations.

The extension of DAX with the capability to define calculated tables local to a query is needed in order to support composite models (DirectQuery for Power BI datasets and Azure Analysis Services). There are no limitations on the use of the feature, so you can take advantage of local tables in any DAX query. We refer to calculated tables defined in a query as query calculated tables, or query tables for short.

Click through for an example of how it works.

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Powershell Editors and Environments

Published 2021-02-03 by Kevin Feasel

Greg Moore gives us an overview of the Powershell IDE landscape:

Before I go too deep into this article, I want to distinguish between editing a file and running it. I’m going to focus on editors here, but most development environments include a way to execute a PowerShell script or PowerShell commands. However, do not confuse the editor with the execution environment.

I used the Powershell ISE for a long while, but eventually stopped because its settings were just different enough from the shell’s settings that things which would work just fine in the ISE would fail when I set them up as automated tasks. I don’t remember what those things were, though, so further research may be required. Nowadays, I’ll use VS Code when I need a proper editor and just wing it on the shell for one-off stuff.

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Searching through Powershell History

Published 2021-02-03 by Kevin Feasel

Jess Pomfret describes a helpful module:

I was listening to a podcast last week about PowerShell, when one of the hosts mentioned having to ‘up arrow’ back through your history to find a command you wanted to rerun.  This made me realise that I should write this quick post on using PSReadLine’s interactive search function.  This tip is a serious time saver and I rely on it heavily.

The great news is that if you are using Windows PowerShell on Windows 10 or if you’re using PowerShell 6+, PSReadLine is already installed and you can immediately start using this tip.  If you don’t have the module though, it’s easy enough to install from the PowerShell Gallery:   

Read on to learn how to use it.

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The Limits of Filtered Indexes

Published 2021-02-03 by Kevin Feasel

Erik Darling lays out the pros and cons of filtered indexes:

Filtered indexes are really interesting things. Just slap a where clause on your index definition, and you can do all sorts of helpful stuff:

– Isolate hot data
– Make soft delete queries faster
– Get a histogram specific to the span of data you care about

Among other things, of course. There are some annoying things about them though.

– They only work with specific ANSI options
– If you don’t include the filter definition columns in the index, it might not get used
– They only work when queries use literals, not parameters or variables

Click through for examples of them in action. I would definitely like to see improvements to filtered indexes along the lines that Erik mentions. They have so much potential, but are really held back by those limitations.

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