patch-block.md

Updating block devices after boot

Firecracker offers support to update attached block devices after the microVM has been started. This is provided via PATCH /drives API which notifies Firecracker that the underlying block file has been changed on the host. It should be called when the path to the block device is changed or if the file size has been modified. It is important to note that external changes to the block device file do not automatically trigger a notification in Firecracker so the explicit PATCH API call is mandatory.

How it works

The implementation of the PATCH /drives API does not modify the host backing file. It only updates the emulation layer block device properties, path and length and then triggers a virtio device reconfiguration that is handled by the guest driver which will update the size of the raw block device. With that being said, a sequence which performs resizing/altering of the block underlying host file followed by a PATCH /drives API call is not an atomic operation as the guest can also modify the block file via emulation during the sequence, if the raw block device is mounted or accessible.

Supported use case

This feature was designed to work with a cooperative guest in order to effectively simulate hot plug/unplug functionality for block devices.

The following guarantees need to be provided:

• guest did not mount the device
• guest does not read or write from the raw block device /dev/vdX during the update sequence

Example sequence that configures a microVM with a placeholder drive and then updates it with the real one:

# Create and set up a block device.
touch ${ro_drive_path}

curl --unix-socket ${socket} -i \
     -X PUT "http://localhost/drives/scratch" \
     -H "accept: application/json" \
     -H "Content-Type: application/json" \
     -d "{
             \"drive_id\": \"scratch\",
             \"path_on_host\": \"${ro_drive_path}\",
             \"is_root_device\": false,
             \"is_read_only\": true
         }"
# Finish configuring and start the microVM. Wait for the guest to boot.

# Before mounting the block device in the guest:
# Use another backing file of different size to effectively resize the
# vm block device.
touch ${updated_ro_drive_path}
truncate --size ${new_size}M ${updated_ro_drive_path}
# Create a filesystem in it.
mkfs.ext4 ${updated_ro_drive_path}

# PATCH the block device to use the new backing file.
curl --unix-socket ${socket} -i \
     -X PATCH "http://localhost/drives/scratch" \
     -H "accept: application/json" \
     -H "Content-Type: application/json" \
     -d "{
             \"drive_id\": \"scratch\",
             \"path_on_host\": \"${updated_ro_drive_path}\"
         }"

# It's now safe to mount the block device in the guest and use it
# with the updated backing file.

Data integrity and other issues

We do not recommend using this feature outside of its supported use case scope. If the required guarantees are not provided, data integrity and potential other issues may arise depending on the actual use case. There are two major aspects that need be considered here:

Atomicity of the update sequence

If the guest has the opportunity to perform I/O against the block device during the update sequence it can either read data while it is changed or can overwrite data already written by a host process. For example a truncate operation can be undone if the guest issues a write for the last sector of the raw block device, or the guest application can become inconsistent or/and can create inconsistency in the block device itself.

In flight I/O requests

If the atomicity of the operation is guaranteed by using methods to make the microVM quiescence during the update sequence (for example pausing the microVM) the guest itself or block device can still become incosistent from in flight I/O requests in the guest that will be executed after it is resumed.