Where is my /tmp mounted?

Question

$ df /tmp
Filesystem     1K-blocks     Used Available Use% Mounted on
/dev/root      480589544 42607368 413546516  10% /

$ ls /dev/root
ls: cannot access /dev/root: No such file or directory

I wanted to check if my default Debian installation places /tmp in RAM or on the disk, but now am completely confused. Why would a non-existing device be reported as a filesystem type? What doe "mounted on /" mean?

Here is the output of mount:

/dev/sda1 on / type ext4 (rw,relatime,errors=remount-ro,data=ordered)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
tmpfs on /run type tmpfs (rw,nosuid,noexec,relatime,size=811520k,mode=755)
tmpfs on /run/lock type tmpfs (rw,nosuid,nodev,noexec,relatime,size=5120k)
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
devtmpfs on /dev type devtmpfs (rw,relatime,size=10240k,nr_inodes=1013960,mode=755)
tmpfs on /run/shm type tmpfs (rw,nosuid,nodev,noexec,relatime,size=1623020k)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
none_debugfs on /sys/kernel/debug type debugfs (rw,relatime)
cgroup on /sys/fs/cgroup type tmpfs (rw,relatime,size=12k)
cgmfs on /run/cgmanager/fs type tmpfs (rw,relatime,size=100k,mode=755)
systemd on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,release_agent=/run/cgmanager/agents/cgm-release-agent.systemd,name=systemd)
tmpfs on /run/user/1000 type tmpfs (rw,nosuid,nodev,relatime,size=811520k,mode=700,uid=1000,gid=1000)

Answer 1

If the output is as above, it's on the hard disk. You can get /dev/root by looking at the kernel commandline:

$ cat /proc/cmdline | grep root
BOOT_IMAGE=/boot/vmlinuz-3.19.0-32-generic root=UUID=0cde5cf9-b15d-4369-b3b1-4405204fd9ff ro

So /dev/root is equivalent to the partition with the UUID printed above; your's will differ. To look this UUID up, use

$ sudo blkid
/dev/sda1: UUID="0cde5cf9-b15d-4369-b3b1-4405204fd9ff" TYPE="ext4" 
/dev/sda5: UUID="37bc6a9c-a27f-43dc-a485-5fb1830e1e42" TYPE="swap" 
/dev/sdb1: UUID="177c3cec-5612-44a7-9716-4dcba27c69f9" TYPE="ext4" 

As you can see, the matching partition is /dev/sda1. So your /tmp is on the hard disk. Another giveaway in the output of df is the mountpoint /. If you mounted /tmp in the RAM, you'd instead get

$ df /tmp
Filesystem     1K-blocks  Used Available Use% Mounted on
tmpfs            3640904    20   3640884   1% /tmp

Answer 2

The output of df /tmp gives the answer: the “Mounted on” column lists /, so /tmp is part of the filesystem that's mounted on /, i.e. the root filesystem. It is not a separate filesystem.

To be more accurate, you should run df /tmp/: if /tmp is a symbolic link, then df /tmp lists information about the location of the symbolic link, whereas df /tmp/ lists information about the target directory.

The mention of /dev/root in the device column is due to its being listed in /etc/mtab. You can find the real device by looking in /proc/mounts with </proc/mounts awk '$2 == "/" {print $1}' or findmnt /.

Answer 3

df /path/to/target shows which filesystem contains the folder target, bytes total, bytes used, percentage available and the mount point.

In the example documented in this question, the folder /tmp/ is located within the filesystem labeled root, which is on the block device /dev/sda1.

mount | grep target will show any filesystems containing the name target as listed in /etc/mtab. If the target folder is not a separate filesystem, mount will not directly give you accurate information.

However, to know what device contains /tmp/, use df to get the filesystem name, (/dev/root), and the mount point, (/), and then search the output of mount to find the device or mount point and determine what block device holds that filesystem.

It is important to distinguish between device and filesystem because there is not always a 1:1 correlation. The block device can be partitioned and each partition will have a filesystem. Each partition can have a different type of filesystem. Each partition its own unique name and UUID and can have aliases.

Answer 4

To check this, mount is more appropriate. You can try it, and grep its output on 'tmp'

Ex:

$  mount | grep tmp
tmpfs on /dev/shm type tmpfs ...

Here it says that my partition /dev/shm is mounted with tmpfs (temporary file storage, i.e. in RAM). If your partition isn't mounted with tmpfs then it's not in volatile memory.