[SSSD-users] sssd_nss writing 25 MiB to /var/lib/sss/mc every time it starts up
Hi folks! I'm chasing down some interesting behavior on our Ubuntu 20.04 VMs.
We're running sssd 2.3.3.
The people who run the underlying disks for our VM hosts have been complaining about a
spike in disk write latency every 10min.
After investigation, I figured out the issue.
By default, every 10 mins Debian systems run a cronjob, which kicks off a script called
debian-sa1. By default, this script is basically a no-op. It doesn't write to disk.
But running this cronjob (which runs as root by default) causes sssd_nss to get started up
(on Ubuntu, it's socket activated, as you probably well know).
When sssd_nss starts up, it writes the memory-mapped cache in /var/lib/sss/mc/.
For us, it writes about 25 MiB of data. (Is that a lot?)
By default, sssd_nss has a 5-minute timeout, so, if nothing else looks up any user info,
sssd_nss shuts down.
So, basically, every 10 mins, sssd_nss wakes up and writes 25 MiB. Across many thousands
of VMs, this adds up to a lot of writes!
Oh RHEL, seems like sssd_nss runs continuously, not socket-activated. I am thinking of
just doing the same on Ubuntu. This will keep the memory-mapped cache active all the time,
which is fine.
It'll prevent all those disk writes.
However, I wanted to see if you had any other suggestions. Thanks!