Pi-hole on docker crashing because of shared memory shortage

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1

I run Pi-hole in docker and I just realized that it creashed, probably many times but I did not notice because of docker restarting it (EDIT: now I see it is crashing every 15 minutes!). The question is similar to PiHole Crash - Reason shared memory tmpfs /dev/shm 100% (now closed), but in a docker context.

My version:

Docker Tag 2025.06.2Core v6.1.2FTL v6.2.3Web interface v6.2.1

The logs:

pihole-1  | 2025-07-14 20:00:35.737 UTC [50M] WARNING: Shared memory shortage (/dev/shm) ahead: 93% is used (63.0MB used, 67.1MB total, FTL uses 63.0MB)
pihole-1  | 2025-07-14 20:00:41.908 UTC [50M] WARNING: Shared memory shortage (/dev/shm) ahead: 94% is used (63.3MB used, 67.1MB total, FTL uses 63.3MB)
pihole-1  | 2025-07-14 20:00:48.581 UTC [50M] WARNING: Shared memory shortage (/dev/shm) ahead: 94% is used (63.6MB used, 67.1MB total, FTL uses 63.6MB)
pihole-1  | 2025-07-14 20:00:54.771 UTC [50M] WARNING: Shared memory shortage (/dev/shm) ahead: 95% is used (63.9MB used, 67.1MB total, FTL uses 63.9MB)
pihole-1  | 2025-07-14 20:01:02.709 UTC [50M] WARNING: Shared memory shortage (/dev/shm) ahead: 95% is used (64.2MB used, 67.1MB total, FTL uses 64.2MB)
pihole-1  | 2025-07-14 20:01:08.915 UTC [50M] WARNING: Shared memory shortage (/dev/shm) ahead: 96% is used (64.5MB used, 67.1MB total, FTL uses 64.5MB)
pihole-1  | 2025-07-14 20:01:15.746 UTC [50M] WARNING: Shared memory shortage (/dev/shm) ahead: 96% is used (64.8MB used, 67.1MB total, FTL uses 64.8MB)
pihole-1  | 2025-07-14 20:01:22.502 UTC [50M] WARNING: Shared memory shortage (/dev/shm) ahead: 96% is used (65.1MB used, 67.1MB total, FTL uses 65.1MB)
pihole-1  | 2025-07-14 20:01:28.977 UTC [50M] WARNING: Shared memory shortage (/dev/shm) ahead: 97% is used (65.4MB used, 67.1MB total, FTL uses 65.3MB)
pihole-1  | 2025-07-14 20:01:35.810 UTC [50M] WARNING: Shared memory shortage (/dev/shm) ahead: 97% is used (65.7MB used, 67.1MB total, FTL uses 65.6MB)
pihole-1  | 2025-07-14 20:01:42.584 UTC [50M] WARNING: Shared memory shortage (/dev/shm) ahead: 98% is used (65.9MB used, 67.1MB total, FTL uses 65.9MB)
pihole-1  | 2025-07-14 20:01:49.361 UTC [50M] WARNING: Shared memory shortage (/dev/shm) ahead: 98% is used (66.2MB used, 67.1MB total, FTL uses 66.2MB)
pihole-1  | 2025-07-14 20:01:55.784 UTC [50M] WARNING: Shared memory shortage (/dev/shm) ahead: 99% is used (66.5MB used, 67.1MB total, FTL uses 66.5MB)
pihole-1  | 2025-07-14 20:02:04.550 UTC [50M] WARNING: Shared memory shortage (/dev/shm) ahead: 99% is used (66.8MB used, 67.1MB total, FTL uses 66.8MB)
pihole-1  | 2025-07-14 20:02:04.555 UTC [50M] WARNING: Could not fallocate() in realloc_shm() (/app/src/shmem.c:838): No space left on device
pihole-1  | 2025-07-14 20:02:04.555 UTC [50M] CRIT: realloc_shm(): Failed to resize "/FTL-50-queries" (10) to 65175552: No space left on device (28)
pihole-1  | 
pihole-1  |   [i] pihole-FTL exited with status 1
pihole-1  | 
pihole-1  |   [i] Container will now stop or restart depending on your restart policy

I have ample RAM on my server - how can I increase the shared memory for the docker contrainer?

Debug Token:

https://tricorder.pi-hole.net/IMw6VaBL/

2

I increased the size with the docker compose entry shm_size: 900m, I will monitor how it goes

3

The default SHM allocation for Docker can be too small. We used to have a note somewhere about SHM size and changing it but I can't find it quickly.

4

A short time after the change I was at

root@srv ~# docker exec -it pihole-pihole-1 df -h /dev/shm
Filesystem      Size  Used Avail Use% Mounted on
shm             900M   65M  836M   8% /dev/shm

and now (about 20 hours later) at

root@srv ~# docker exec -it pihole-pihole-1 df -h /dev/shm
Filesystem      Size  Used Avail Use% Mounted on
shm             900M   60M  841M   7% /dev/shm

It looks like there is a stabilization at ~60-65 Mb, I will wait a few more days and set 100 Mb to be on the safe side.

5

We try to be light when using system resources and not take everything available and then warn when things are getting to the limit.

6

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