How to fix Error response from daemon: invalid tmpfs size in Docker

First, identify where the invalid size is specified. Check your docker run command or docker-compose.yml:

# For docker run, look for --tmpfs or --mount flags
docker run --tmpfs /tmp:size=invalid_value myimage

# For compose files, check tmpfs sections
cat docker-compose.yml | grep -A 5 tmpfs

Look for patterns like:
- size: "100m" (quoted string - problematic in v2)
- size: 100m (unquoted string - may not work)
- o: "size=invalid" (invalid format in options)

When using docker run with --tmpfs flag, specify size in the mount options:

# Correct format with size suffix
docker run -d --tmpfs /tmp:size=100m myimage
docker run -d --tmpfs /tmp:rw,noexec,nosuid,size=65536k myimage

# Using --mount flag (recommended)
docker run -d --mount type=tmpfs,destination=/tmp,tmpfs-size=104857600 myimage

Valid size suffixes for --tmpfs:
- k or K - kilobytes
- m or M - megabytes
- g or G - gigabytes

For --mount flag, use tmpfs-size with bytes or suffixed values.

Docker Compose v2 requires tmpfs size as an integer (bytes). Update your docker-compose.yml:

Wrong (causes error):

services:
  myapp:
    image: myimage
    volumes:
      - type: tmpfs
        target: /tmp
        tmpfs:
          size: "100m"  # String - will fail

Correct (use integer bytes):

services:
  myapp:
    image: myimage
    volumes:
      - type: tmpfs
        target: /tmp
        tmpfs:
          size: 104857600  # 100MB in bytes (no quotes!)

Common size conversions:
- 64MB = 67108864
- 100MB = 104857600
- 256MB = 268435456
- 512MB = 536870912
- 1GB = 1073741824
- 2GB = 2147483648

Docker Compose also supports a short syntax for tmpfs that allows string suffixes:

services:
  myapp:
    image: myimage
    tmpfs:
      - /tmp:size=100m,mode=1777
      - /run:size=64m

Or as a simple list (uses default size - 50% of host RAM):

services:
  myapp:
    image: myimage
    tmpfs:
      - /tmp
      - /run

The short syntax tmpfs: at service level handles size differently than the long volumes: syntax.

For reusable tmpfs volumes, create them with proper options:

# Create a 100MB tmpfs volume
docker volume create --driver local \
    --opt type=tmpfs \
    --opt device=tmpfs \
    --opt o=size=100m,uid=1000 \
    my_tmpfs_volume

# Use in docker run
docker run -v my_tmpfs_volume:/tmp myimage

In docker-compose.yml with volumes:

volumes:
  tmpfs_vol:
    driver_opts:
      type: tmpfs
      device: tmpfs
      o: "size=100m"  # String allowed here

services:
  myapp:
    image: myimage
    volumes:
      - tmpfs_vol:/tmp

Check your Docker and Docker Compose versions to understand which syntax is supported:

docker --version
docker compose version
# or for older standalone compose
docker-compose --version

Docker Compose behavior by version:
- v1 (docker-compose): Supports string notation like "100m"
- v2 (docker compose): Requires integer for tmpfs.size in long syntax

If using Docker Compose v2 and you need string notation, use the short tmpfs: syntax instead of the long volumes: syntax with type: tmpfs.