tmpfs

tmpfs is a RAM-based filesystem — data stored in tmpfs lives entirely in memory (with optional swap backing). It’s extremely fast because there’s no disk I/O, and it’s automatically freed when the filesystem is unmounted or the system reboots.

What tmpfs Is

tmpfs:
  - Uses RAM + swap as backing store
  - No disk — no spinning, no seek
  - Fixed size limit (doesn't grow forever)
  - Contents lost on reboot
  - Permissions work normally (unlike /dev/shm in older kernels)

Common tmpfs Mounts

# Most Linux systems mount several things as tmpfs:
mount | grep tmpfs
# tmpfs on /run type tmpfs (rw,nosuid,nodev,mode=755)
# tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)
# tmpfs on /tmp type tmpfs (rw,nosuid,nodev)
 
# /run — runtime data (PID files, sockets)
# /dev/shm — POSIX shared memory (also used by df -h /dev/shm)
# /tmp — temporary files (often tmpfs on modern systems)

Creating a tmpfs

# Mount tmpfs manually
mount -t tmpfs -o size=1G tmpfs /mnt/tmpfs
 
# With options
mount -t tmpfs -o size=512M,mode=1777,uid=root,gid=root tmpfs /mnt/myfs
 
# Add to /etc/fstab for persistence
tmpfs  /mnt/myfs  tmpfs  defaults,size=512M,mode=1777  0 0

Options

# size — maximum size (required in fstab, optional on CLI)
mount -t tmpfs -o size=256M tmpfs /mnt
 
# nr_blocks — same as size (in 4K blocks)
# nr_inodes — max number of inodes (file count limit)
 
# mode — permissions
mount -t tmpfs -o mode=1777 tmpfs /mnt  # sticky bit
 
# uid, gid — ownership
mount -t tmpfs -o uid=1000,gid=1000 tmpfs /mnt
 
# Defaults: size=half of RAM, mode=1777, uid=0, gid=0

/dev/shm — POSIX Shared Memory

# /dev/shm is tmpfs mounted for shared memory
# POSIX shm: shmget(), shmat(), shmdt()
 
# df shows it
df -h /dev/shm
# Filesystem      Size  Used Avail Use% Mounted on
# tmpfs            7.8G     0  7.8G   0% /dev/shm
 
# Useful for:
# - tmpfs for caches (Chrome uses /dev/shm for default tmpfs)
# - tmpfs for Docker's tmpfs mounts
docker run --tmpfs /run:rw,noexec,nosuid,size=1G nginx
 
# Increase /dev/shm size (Docker with --shm-size)
docker run --shm-size 256M nginx

tmpfs for Performance

# /tmp as tmpfs (often default on modern distros):
# Check if /tmp is tmpfs:
mount | grep " /tmp "
# tmpfs on /tmp type tmpfs ...  ← yes
 
# If not, mount as tmpfs:
mount -t tmpfs -o size=10G tmpfs /tmp
 
# Browser cache as tmpfs (ram browsers):
mount -t tmpfs -o size=2G,uid=1000 tmpfs /home/darshan/cache/chromium
 
# Build cache as tmpfs:
mount -t tmpfs -o size=10G tmpfs /var/cache/build
 
# SQLite database on tmpfs (max speed):
mount -t tmpfs -o size=1G tmpfs /mnt/db
# WARNING: data lost on reboot — use for temp tables, not persistence!

Checking tmpfs Usage

# df
df -h | grep tmpfs
 
# du (be careful — walking tmpfs can be slow on large)
du -sh /tmp
 
# Check size
mount | grep " /tmp "
# tmpfs on /tmp type tmpfs (rw,nosuid,nodev,relatime,size=10240k)
 
# Resize tmpfs (if remountable):
mount -o remount,size=5G /tmp

tmpfs vs ramfs

Feature	tmpfs	ramfs
Backing store	RAM + swap	RAM only (no swap)
Size limit	Yes (enforced)	No (grows until OOM)
Fixed size	Can remount to change size	Cannot change size
OOM behavior	Writes fail at size limit	OOM killer triggers
Disk quotas	Supported	Not supported
Persistence	Lost on reboot	Lost on reboot
Swappiness	Can use swap	Cannot use swap

tmpfs and Containers

# Docker tmpfs mount
docker run --tmpfs /tmp:rw,noexec,size=1G alpine
 
# Kubernetes tmpfs volume
spec:
  containers:
  - name: app
    volumeMounts:
    - name: tmpfs
      mountPath: /tmp
  volumes:
  - name: tmpfs
    emptyDir:
      medium: Memory     # ← tmpfs
      sizeLimit: 1Gi
 
# Kubernetes memory-backed emptyDir (tmpfs):
emptyDir:
  medium: Memory
  sizeLimit: 512Mi

Security Notes

# tmpfs should usually be nosuid,nodev,noexec
mount -t tmpfs -o nosuid,noexec,nodev,mode=1777 tmpfs /mnt
 
# Common tmpfs flags:
# rw, nosuid, nodev, noexec, relatime, size=...