storage: create k8s emptyDir inside VM

This introduces a new storage type: local. Local storage type will
tell the kata-agent to create an empty directory in the sandbox
directory within the VM.

K8s host emptyDirs will then use the local storage type and mount it
inside each container. By doing this, we utilise the storage medium
that the sandbox uses. In most cases this will be 9p.

If the VM is using device mapper for container storage, the containers
will benefit from the better performance of device mapper for
host emptyDir.

Fixes #1472

Signed-off-by: Alex Price <aprice@atlassian.com>

pkg/katautils/create.go

@@ -164,6 +164,9 @@ func SetEphemeralStorageType(ociSpec oci.CompatOCISpec) oci.CompatOCISpec {
 		if vc.IsEphemeralStorage(mnt.Source) {
 			ociSpec.Mounts[idx].Type = "ephemeral"
 		}
+		if vc.Isk8sHostEmptyDir(mnt.Source) {
+			ociSpec.Mounts[idx].Type = "local"
+		}
 	}
 	return ociSpec
 }

virtcontainers/kata_agent.go

@@ -70,6 +70,7 @@ var (
 	kataEphemeralDevType = "ephemeral"
 	ephemeralPath        = filepath.Join(kataGuestSandboxDir, kataEphemeralDevType)
 	grpcMaxDataSize      = int64(1024 * 1024)
+	kataLocalDevType     = "local"
 )
 
 // KataAgentConfig is a structure storing information needed
@@ -1038,6 +1039,9 @@ func (k *kataAgent) createContainer(sandbox *Sandbox, c *Container) (p *Process,
 	epheStorages := k.handleEphemeralStorage(ociSpec.Mounts)
 	ctrStorages = append(ctrStorages, epheStorages...)
 
+	localStorages := k.handleLocalStorage(ociSpec.Mounts, sandbox.id)
+	ctrStorages = append(ctrStorages, localStorages...)
+
 	// We replace all OCI mount sources that match our container mount
 	// with the right source path (The guest one).
 	if err = k.replaceOCIMountSource(ociSpec, newMounts); err != nil {
@@ -1134,6 +1138,30 @@ func (k *kataAgent) handleEphemeralStorage(mounts []specs.Mount) []*grpc.Storage
 	return epheStorages
 }
 
+// handleLocalStorage handles local storage within the VM
+// by creating a directory in the VM from the source of the mount point
+func (k *kataAgent) handleLocalStorage(mounts []specs.Mount, sandboxID string) []*grpc.Storage {
+	var localStorages []*grpc.Storage
+	for idx, mnt := range mounts {
+		if mnt.Type == kataLocalDevType {
+			// Set the mount source path to a the desired directory point in the VM.
+			// In this case it is located in the sandbox directory.
+			mounts[idx].Source = filepath.Join(kataGuestSharedDir, sandboxID, kataLocalDevType, filepath.Base(mnt.Source))
+
+			// Create a storage struct so that the kata agent is able to create the
+			// directory inside the VM.
+			localStorage := &grpc.Storage{
+				Driver:     kataLocalDevType,
+				Source:     "local",
+				Fstype:     "local",
+				MountPoint: mounts[idx].Source,
+			}
+			localStorages = append(localStorages, localStorage)
+		}
+	}
+	return localStorages
+}
+
 // handleBlockVolumes handles volumes that are block devices files
 // by passing the block devices as Storage to the agent.
 func (k *kataAgent) handleBlockVolumes(c *Container) []*grpc.Storage {