devshm

devshm, commonly referred to as /dev/shm, is a temporary in-memory filesystem used on many Unix-like systems, notably Linux. It is mounted as a tmpfs filesystem at the path /dev/shm and serves as a location for temporary files and for POSIX shared memory objects created with shm_open or related interfaces. Data stored in /dev/shm resides in RAM (and may be swapped if swap is enabled) and is not preserved across reboots.

As a tmpfs-backed filesystem, /dev/shm uses system memory as its storage pool. The size of the mounted filesystem is configurable; many Linux distributions default to a size around half of physical RAM, but the exact limit can vary. The filesystem is typically mounted with permissions that allow broad write access (mode 1777), making it readable and writable by all users, which is suitable for collaboration but has security implications in multi-user environments. The contents are ephemeral: files disappear after a reboot, and data can be evicted under memory pressure.

Common usage includes storing large temporary files for applications, buffering data, and providing fast IPC through POSIX shared memory objects. Applications create shared memory regions in /dev/shm and access them via library calls such as shm_open. Because it is in memory, /dev/shm offers speed advantages over disk-backed storage but should not be used for persistent data.

Management tips include checking size and usage with df -h /dev/shm, remounting or resizing with mount -o remount,size=... /dev/shm, or configuring it in /etc/fstab. In containerized environments, the available space for /dev/shm may be limited and configurable per container.