Re: /etc/sysctl.d/01_ec2.conf sets vm.swappiness = 0

["Phil Endecott" <debiancloud@chezphil.org>]

Hi Noah,

Noah Meyerhans wrote:
> On Mon, Dec 06, 2021 at 02:28:35PM +0000, Phil Endecott wrote:
> > On one of my Lightsail instances, which was created a few years
> > ago from a Debian image and has subsequently been upgraded a few
> > times, I have a /etc/sysctl.d/01_ec2.conf file which sets
> > vm.swappiness = 0. On another newer EC2 instance I don't have this
> > file.
>
> Right; we haven't shipped that file since (IIRC) jessie.
>
> > dpkg -S doesn't tell me where this file came from. I don't
> > think I created it myself!!
>
> It was installed as part of the image creation process.  Similar to
> files like /etc/hosts, it is not actually owned by a package.

I wonder if Debian needs some way to track the provenance
of files like this?

> > Does anyone recognise this? Of course the issue is that I
> > discovered the hard way that the system was not using its
> > swapfile. The file has a pile of other configuration settings
> > and I wonder if any of those are also undesirable, see below.
>
> Most of the settings from that file were lifted from Amazon Linux, IIRC,
> and are pretty reasonable for a default "server" class VM.  Even
> vm.swappiness is a reasonable default, given that there's no swap file
> created by default.  But nothing will break if you choose to remove the
> file.

I'm not sure I agree with swapiness=0 being reasonable when there
is no swapfile. If there's no swapfile, the swapiness value is
ignored. The only effect of setting swapiness=0 on a system with
no swap is to prevent any swap that is added in the future from
working. If you've added swap it's likely that you want it to work.

It would have been great if e.g. the output of "swapon" had told
me that the swap was not being used. Perhaps I'll file a bug about
that.

Anyway this is moot as the file has been removed. Though maybe you
should consider actively removing it during updates?


Thanks,

Phil.