Category: English

Multi-step Refactoring Pains in C++

Titus Winters talks about maintaining and refactoring large C++ code bases (i.e. code bodies that require multi-step refactoring). He describes how “higher-level” language features effectively make refactoring harder (e.g. functions, classes, templates, concepts).

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https://www.youtube.com/watch?v=v_yzLe-wnfk

Usefulness of Swap Explained

Chris Down explains how swap’s main role is being the missing backing store for anonymous (i.e. allocated by malloc) pages. While all other kinds of data (e.g. paged in files) can be reclaimed easily and later reloaded, because their “source of truth” is elsewhere. There’s no such source for anonymous pages hence these pages can “never” be reclaimed unless there’s swap space available (even if those pages aren’t “hot”).

Linux has historically had poor swap (and by extension OOM) handling with few and imprecise means for configuration. Chris describes the behavior of a machine with and without swap in different scenarios of memory contention. He thinks that poor swap performance is caused by having a poor measure “memory pressure.” He explains how work on cgroup v2 might give the kernel (and thus admins) better measures for memory pressure and knobs for dealing with it.

Moving LXD Containers From One Pool to Another

When I started playing with LXD I just accepted the default storage configuration which creates an image file and uses that to initialize a ZFS pool. Since I’m using ZFS as my main file system this seemed silly as LXD can use an existing dataset as a source for a storage pool. So I wanted to migrate my existing containers to the new storage pool.

Although others seemed to to have the same problem there was no ready answer. Digging through the documentation I finally found out that the lxc move  command had a  -s  option … I had an idea. ? Here’s what I came up with …

Preparation

First we create the dataset on the existing ZFS pool and add it to LXC.

lxc storage list should show something like this now:

pool1 is the old pool backed by the image file and is used by some containers at the moment as can be seen in the “Used By” column.  pool2 is added by not used by any contaiers yet.

Moving

We now try to move our containers to pool2.

We can check with  lxc storage list whether we succeeded.

Indeed  pool2 is beeing used now. ? Just to be sure we check that zfs list -r mypool/lxd  also reflects this.

Awesome!

⚠ Note that this only moves the container, but not the LXC image it was cloned off of.

We can repeat this until all containers we care about are moved over to pool2.

Cleanup

To prevent new containers to use pool1  we have to edit the default  profile.

Finally …. when we’re happy with the migration and we’ve verified that everything works as expected we can now remove pool1.