# Using a scratch disk with Edera

This guide covers scratch disks for **standalone Edera zones** using the `protect` CLI.

Scratch disks are temporary, high-speed storage volumes used during workload execution. They’re designed for performance—not persistence.

By default, Edera zones use a memory-backed scratch disk (`tmpfs`) layered with `overlayfs`. This works for many workloads, but it doesn’t scale well for disk-heavy jobs. For example, a workload that needs 60 GB of disk would require over 120 GB of memory and a host with at least 125 GB of available RAM.

To solve this, Edera supports multiple scratch disk backends—including loopback images and block devices—that let you decouple disk from memory.

**Scratch disks are not durable**. All data is wiped when a zone restarts or shuts down. Do not store important data here.

## Why use a scratch disk?

- **High disk demand with low memory**: Some apps need lots of disk but little memory. A scratch disk avoids overprovisioning RAM.
- **Better performance**: Block devices offer more durability and speed than memory-backed overlayfs.
- **Overlayfs workarounds**: Some containers misbehave with overlayfs. You can bypass it with a bind mount.
- **Flexible zone sizing**: Scratch disks let you tune memory and disk independently per zone.

## TL;DR

- **tmpfs (default)**: Edera allocates tmpfs using half of the zone’s memory and uses overlayfs.
- **Temporary scratch disk**: Use `--create-scratch-disk` with `protect zone launch` to create a loopback disk image. (CLI-only—Kubernetes support coming soon.)
- **Block device scratch disk**: Attach a block device; zone mounts it and formats as ext4 on each launch.
- **Bind‑mount workload directories**: Use `--mount-scratch-disk` to bypass overlayfs on specific paths.

## Background

When you run workloads in an Edera zone, they require temporary storage (a scratch disk):

- **No scratch disk (default)**

Edera allocates `tmpfs` (half the zone’s memory) and layers it with `overlayfs`.

- **Created scratch disk**

Use `--create-scratch-disk <size-in‑MB>` to create a loopback file-backed volume. Overlayfs is layered on this disk.

- **Attach scratch disk**

Use `--attach-scratch-disk <disk>` Attach a physical or virtual block device and configure Edera to use it. The disk is formatted with ext4 and overlayfs is used.

## Option 1: Temporary scratch disk

Use a loopback file-based disk image for scratch storage:

```bash
protect zone launch   --name temp-zone   --create-scratch-disk 2048
```

This mounts a temporary disk for all workloads in the zone. Overlayfs is layered on top.

## Option 2: Block device scratch disk

Use a physical or virtual block device for better performance.

### Step 1: Attach block device

Add a second volume (for example, `/dev/sdb`) to your host system and verify it’s visible:

```bash
lsblk
```

### Step 2: Configure Edera daemon

Edit `/var/lib/edera/protect/daemon.toml`:

```toml
[block.devices]
[block.devices.disk0]
path = "/dev/sdb"
```

Then restart the daemon:

```bash
sudo systemctl restart protect-daemon
```

### Step 3: Confirm device availability

```bash
protect device list
```

Make sure `disk0` appears in the output.

### Step 4: Launch a zone using the disk

```bash
protect zone launch   --name test-zone   --attach-scratch-disk disk0
```

**The device is formatted with ext4 on each launch. All existing data will be erased.**

## Bind-mount inside workloads

To bypass overlayfs and directly mount a scratch disk path:

```bash
protect workload launch   --zone test-zone   --name my-workload   --mount-scratch-disk /data   my-image:v1.0
```

To confirm the bind mount:

```bash
df -Th /data
```

If the filesystem is not `overlay`, the bind mount is active.

## Summary

| Mode                  | Backing          | Overlayfs | Persistent Disk |
|-----------------------|------------------|-----------|-----------------|
| No scratch disk       | tmpfs            | Yes       | No              |
| Created scratch disk  | Loopback image    | Yes       | No              |
| Block device disk     | Physical device   | Yes/No    | No (wiped each launch) |

## Additional notes

Scratch disks are supported starting in [Edera v1.2.0](https://docs.edera.dev/reference/release-notes/v1.2.0/)