knowledge/technology/linux/Mount.md

---
obj: concept
rev: 2024-07-08
---

# Mount
Mounting is a fundamental concept in Linux and other Unix-like operating systems, enabling users to access and manage filesystems, storage devices, and partitions. Mounting is the process of making a filesystem accessible at a certain point in the directory tree. When you mount a storage device or a partition, you attach it to a specified directory, known as a mount point. This allows you to access the contents of the device or partition as if they were part of the overall filesystem.

## `mount` Command
Usage:
```shell
mount [options] [source] [mount_point]

# Bind Mounts
mount --bind /var/www /mnt/www

# Loop Device
mount -o loop /path/to/file.iso /mnt/iso

# List mounts
mount -l
```

The source of the filesystem can be specified in other ways too (depending on the filesystem):
- `LABEL=label`: Human readable filesystem identifier.
- `UUID=uuid`: Filesystem universally unique identifier. The format of the UUID is usually a series of hex digits separated by hyphens.
- `PARTLABEL=label`: Human readable partition identifier. This identifier is independent on filesystem and does not change by mkfs or mkswap operations. It’s supported for example for GUID Partition Tables (GPT).
- `PARTUUID=uuid`:  Partition universally unique identifier. This identifier is independent on filesystem and does not change by mkfs or mkswap operations. It’s supported for example for GUID Partition Tables (GPT).

### Options

| Option               | Description                                                                                                                                                                                                                                                    |
| -------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `-a, --all`          | Mount all filesystems in `/etc/fstab`.                                                                                                                                                                                                                         |
| `-B, --bind`         | Mount a directory somewhere else                                                                                                                                                                                                                               |
| `-m, --mkdir[=mode]` | Allow to make a target directory (mountpoint) if it does not exist yet. The default mode is `0755`.                                                                                                                                                            |
| `-o, --options opts` | Use the specified mount options. The opts argument is a comma-separated list.                                                                                                                                                                                  |
| `-r, --read-only`    | Mount the filesystem read-only. A synonym is `-o ro`.                                                                                                                                                                                                          |
| `--source device`    | This option allows you to explicitly define that the argument is the mount source.                                                                                                                                                                             |
| `--target directory` | This option allows you to explicitly define that the argument is the mount target.                                                                                                                                                                             |
| `-t, --types fstype` | The argument following `-t` is used to indicate the filesystem type. The filesystem types which are currently supported depend on the running kernel. See `/proc/filesystems` and `/lib/modules/$(uname -r)/kernel/fs` for a complete list of the filesystems. |

## `/etc/fstab`
The `/etc/fstab` file is used to define filesystems to be automatically mounted at boot time. Nowadays the entries in this file will be converted to [Systemd-Mounts](systemd/Systemd-Mounts.md).

Each file contains a mount instruction like this:
`DEVICE  MOUNT_POINT  FILESYSTEM_TYPE  OPTIONS  DUMP  PASS`

Example: `/dev/sdb1  /mnt/usb  ext4  defaults  0  2`

- `DEVICE`: The device or partition to be mounted.
- `MOUNT_POINT`: The directory where the filesystem will be mounted.
- `FILESYSTEM_TYPE`: The type of filesystem.
- `OPTIONS`: Mount options (e.g., `defaults`, `ro`, `rw`).
- `DUMP`: Used by the `dump` utility to decide if a filesystem should be backed up.
- `PASS`: Used by `fsck` to determine the order in which filesystems are checked.

## Mount Options
- `async`: All I/O to the filesystem should be done asynchronously.
- `atime`: Update inode access times on each access.
- `noatime`: Do not update inode access times on each access (useful for performance).
- `auto`: Can be mounted with the `mount -a` command.
- `noauto`: Can only be mounted explicitly.
- `defaults`: Use default options (`rw`, `suid`, `dev`, `exec`, `auto`, `nouser`, and `async`).
- `dev`: Interpret character or block special devices on the filesystem.
- `nodev`: Do not interpret character or block special devices on the filesystem.
- `diratime`: Update directory inode access times on each access.
- `nodiratime`: Do not update directory inode access times on each access.
- `dirsync`: All directory updates within the filesystem should be done synchronously.
- `exec`: Permit execution of binaries.
- `noexec`: Do not permit execution of any binaries.
- `group`: Allow an ordinary user to mount the filesystem if they are a member of the group owning the device.
- `iversion`: Increment the inode version field upon each change.
- `mand`: Allow mandatory locks on this filesystem.
- `nomand`: Do not allow mandatory locks on this filesystem.
- `relatime`: Update inode access times relative to modify or change time.
- `norelatime`: Do not use the `relatime` feature.
- `lazytime`: Only update times on the in-memory version of an inode.
- `nolazytime`: Do not use the `lazytime` feature.
- `ro`: Mount the filesystem read-only.
- `rw`: Mount the filesystem read-write.
- `suid`: Permit set-user-identifier or set-group-identifier bits to take effect.
- `nosuid`: Do not permit set-user-identifier or set-group-identifier bits to take effect.
- `silent`: Suppress some error messages.
- `loud`: Do not suppress error messages.
- `strictatime`: Strictly update inode access times on each access.

### Filesystem-Specific Options
Each filesystem type may support its own set of mount options. Here are some examples for a few common filesystems:

#### ext2, ext3, ext4
- `acl`: Enable POSIX Access Control Lists.
- `noacl`: Disable POSIX Access Control Lists.
- `user_xattr`: Enable user extended attributes.
- `nouser_xattr`: Disable user extended attributes.
- `journal`: Enable journaling (ext3/ext4).
- `nojournal`: Disable journaling (ext3/ext4).

#### btrfs
- `compress`: Enable compression.
- `compress-force`: Force compression.
- `nodatacow`: Disable copy-on-write for data blocks.
- `nodatasum`: Disable data checksumming.
- `subvol`: Mount a specific subvolume.

#### xfs
- `logbufs`: Number of in-memory log buffers.
- `logbsize`: Size of each in-memory log buffer.
- `noquota`: Disable quota enforcement.
- `quota`: Enable quota enforcement.

#### nfs
- `rsize`: Set the read buffer size.
- `wsize`: Set the write buffer size.
- `timeo`: Set the timeout value.
- `intr`: Allow interrupts on hard mounts.
- `nointr`: Do not allow interrupts on hard mounts.

#### vfat
- `shortname`: Define behavior for handling of short filenames.
- `iocharset`: Character set to use for input and output.
- `utf8`: Encode and decode 8-bit characters as UTF-8.

### Network Filesystems

#### cifs (Common Internet File System)
- `username`: Username to connect to the CIFS share.
- `password`: Password to connect to the CIFS share.
- `domain`: Domain name for authentication.
- `uid`: Set the owner of the files.
- `gid`: Set the group of the files.

#### nfs (Network File System)
- `vers`: Specify NFS version.
- `proto`: Specify transport protocol (`tcp` or `udp`).
- `mountport`: Specify port number for MOUNT service.