8.4.1 Memory Backed Mounts
A focused guide to Memory Backed Mounts, connecting core concepts with practical Docker and container operations.
Memory-backed mounts, implemented through tmpfs, store data directly in the host's RAM rather than on disk, providing significantly faster read and write performance for content that genuinely doesn't need to persist, at the cost of consuming the host's available memory for as long as data remains stored there.
Why Memory Backing Provides a Performance Advantage
Reading from and writing to memory is substantially faster than reading from and writing to even fast disk storage, making a memory-backed mount well suited to performance-sensitive temporary data.
docker run -d --tmpfs /app/scratch:size=500m myapp:1.0
Application logic that frequently reads and writes temporary, scratch-space data benefits directly from this memory-backed speed.
The Trade-off: Consuming Host Memory
Unlike disk-backed storage, data stored in a memory-backed mount directly consumes the host's available RAM for as long as that data exists, meaning a sufficiently large or numerous tmpfs mount could meaningfully affect the host's overall available memory.
docker run -d --tmpfs /app/scratch:size=2g myapp:1.0
Allocating a tmpfs mount this large directly reserves that much of the host's memory for this purpose, a consideration worth weighing against the host's total available memory and other demands on it.
Setting an Explicit Size Limit
Specifying an explicit size limit for a tmpfs mount prevents it from growing unboundedly and potentially exhausting the host's available memory.
docker run -d --tmpfs /app/cache:size=100m myapp:1.0
Without an explicit limit, a tmpfs mount could in principle grow to consume a very large amount of memory if the application writes more data there than anticipated.
Why Memory-Backed Mounts Matter
Tmpfs mounts provide a deliberate trade-off — speed in exchange for memory consumption and a complete lack of persistence — that is well suited specifically to temporary, performance-sensitive data, making the explicit choice to use memory-backed storage an important, deliberate decision rather than a default.