Disks and file system overview

This page provides an overview of the Container-Optimized OS file system and describes how to mount and format disks.

File system

The Container-Optimized OS node image file system layout is optimized to enhance node security. The boot disk space is split into three types of partitions:

Root partition, which is mounted as read-only.
Stateful partitions, which are writable and stateful.
Stateless partitions, which are writable but the contents do not persist across reboots.

When using Container-Optimized OS, be aware of the partitioning if you run your own services that have certain expectations about the file system layout outside of containers.

The root file system is mounted as read-only to protect system integrity. However, home directories and /mnt/stateful_partition are persistent and writable.

Working with the Container-Optimized OS file system

The following is a list of paths in the Container-Optimized OS node image file system, along with their properties and recommended usage:

Path	Properties	Purpose
/	read-only executable	The root filesystem is mounted as read-only to maintain integrity. The kernel verifies integrity root filesystem during boot up, and refuses to boot in case of errors.
/home /var	writable non-executable stateful	These paths are meant for storing data that persists for the lifetime of the boot disk. They are mounted from `/mnt/stateful_partition`.
/var/lib/google /var/lib/docker /var/lib/toolbox	writable executable stateful	These paths are working directories for Compute Engine packages (for example, the accounts manager service), Docker, and Toolbox respectively.
/var/lib/cloud	writable executable stateless tmpfs	This path is the working directory of the `cloud-init` package.
/etc	writable executable stateless tmpfs	Typically holds your configuration (for example, `systemd` services defined via `cloud-init`). It's a good idea to capture the desired state of your instances in `cloud-init`, as `cloud-init` is applied when an instance is newly created as well as when an instance is restarted.
/tmp	writable non-executable stateless tmpfs	Typically used as a scratch space and should not be used to store persistent data.
/mnt/disks	writable executable stateless tmpfs	You can mount persistent disks at directories under `/mnt/disks`.

Mounting and formatting disks

You can attach a persistent disk or create an instance with Local SSDs when using Container-Optimized OS. Follow the instructions on Formatting and mounting a persistent disk or Format and mount a local SSD device for the appropriate use-case.

The disks can be mounted by creating a subdirectory under /mnt/disks directory. Since /etc/ is stateless on Container-Optimized OS, you cannot use /etc/fstab to automatically fsck (file system consistency check) and mount the disks on boot. But you can achieve the same by doing those operations from the bootcmd section in your cloud-config.

The following example mounts the disk DEVICE_ID under the /mnt/disks directory.

Create a script file using the following contents:
```
#cloud-config

bootcmd:
- fsck.ext4 -tvy /dev/DEVICE_ID
- mkdir -p /mnt/disks/MNT_DIR
- mount -t ext4 -o ... /dev/DEVICE_ID /mnt/disks/MNT_DIR
```
Replace the following:
- DEVICE_ID: the device ID of the disk that you want to format and mount.
- MNT_DIR: the directory in which to mount your disk.
Refer to the Linux documentation for a full set of options supported by the fsck.ext4 and mount commands.
Update the VM metadata to include the script file by using the --metadata-from-file flag.

To create a VM, use the following command:
```
gcloud compute instances create INSTANCE_NAME \
    --metadata-from-file user-data=SCRIPT_FILE_NAME
```
To update an existing instance, use the following command:
```
gcloud compute instances add-metadata INSTANCE_NAME \
    --metadata-from-file user-data=SCRIPT_FILE_NAME
```
Replace the following:
- INSTANCE_NAME: the name of your VM instance.
- SCRIPT_FILE_NAME: the name of the metadata script file.