8.4.1.5 Tmpfs Memory Limits

Tmpfs memory limits control the maximum size a tmpfs mount is permitted to grow to, preventing it from consuming an unbounded amount of the host's available memory if an application writes more data to it than anticipated.

Setting an Explicit Size Limit

The size option, specified as part of the tmpfs mount configuration, caps how much data the mount can hold.

docker run -d --tmpfs /app/cache:size=100m myapp:1.0

Once this tmpfs mount's content reaches 100MB, further writes fail, exactly as they would on a disk-backed filesystem that had reached its capacity.

Why an Explicit Limit Is Important

Without a specified limit, a tmpfs mount's effective size constraint depends on broader system defaults, which may not provide the specific, predictable boundary needed to prevent this particular mount from consuming more host memory than intended.

docker run -d --tmpfs /app/cache myapp:1.0

Explicitly setting a deliberate size limit, rather than relying on whatever default behavior might otherwise apply, provides clearer, more predictable control over this mount's potential memory consumption.

Choosing an Appropriate Size for the Intended Use Case

The size limit should comfortably accommodate the application's actual expected usage of this temporary storage, while still providing a meaningful ceiling against unexpectedly excessive consumption.

docker run -d --tmpfs /tmp:size=200m myapp:1.0

Monitoring Actual Usage Against the Configured Limit

Confirming how much of the configured limit is actually being used during normal operation helps validate whether the chosen size is appropriately calibrated.

docker exec myapp df -h /app/cache

Why Tmpfs Memory Limits Matter

Setting an explicit, appropriately sized limit on a tmpfs mount is an important safeguard, preventing an application's temporary, memory-backed storage usage from growing unexpectedly large and consuming more of the host's available memory than was actually intended for this purpose.