Merge pull request microsoft#886 from dcantah/lcow-scratch-work

Set default scratch size for containers/UVMs and reuse LCOW scratch

hcsoci/create.go

@@ -118,10 +118,10 @@ func CreateContainer(ctx context.Context, createOptions *CreateOptions) (_ cow.C
 	}()
 
 	if coi.HostingSystem != nil {
-		n := coi.HostingSystem.ContainerCounter()
 		if coi.Spec.Linux != nil {
 			r.SetContainerRootInUVM(fmt.Sprintf(lcowRootInUVM, createOptions.ID))
 		} else {
+			n := coi.HostingSystem.ContainerCounter()
 			r.SetContainerRootInUVM(fmt.Sprintf(wcowRootInUVM, strconv.FormatUint(n, 16)))
 		}
 	}

layers/layers.go

@@ -143,9 +143,13 @@ func MountContainerLayers(ctx context.Context, layerFolders []string, guestRoot
 		}
 	}
 
-	hostPath := filepath.Join(layerFolders[len(layerFolders)-1], "sandbox.vhdx")
 	containerScratchPathInUVM := ospath.Join(uvm.OS(), guestRoot)
+	hostPath, err := getScratchVHDPath(layerFolders)
+	if err != nil {
+		return "", fmt.Errorf("failed to get scratch VHD path in layer folders: %s", err)
+	}
 	log.G(ctx).WithField("hostPath", hostPath).Debug("mounting scratch VHD")
+
 	scsiMount, err := uvm.AddSCSI(ctx, hostPath, containerScratchPathInUVM, false, uvmpkg.VMAccessTypeIndividual)
 	if err != nil {
 		return "", fmt.Errorf("failed to add SCSI scratch VHD: %s", err)
@@ -284,7 +288,10 @@ func UnmountContainerLayers(ctx context.Context, layerFolders []string, containe
 
 	// Unload the SCSI scratch path
 	if (op & UnmountOperationSCSI) == UnmountOperationSCSI {
-		hostScratchFile := filepath.Join(layerFolders[len(layerFolders)-1], "sandbox.vhdx")
+		hostScratchFile, err := getScratchVHDPath(layerFolders)
+		if err != nil {
+			return fmt.Errorf("failed to get scratch VHD path in layer folders: %s", err)
+		}
 		if err := uvm.RemoveSCSI(ctx, hostScratchFile); err != nil {
 			log.G(ctx).WithError(err).Warn("failed to remove scratch")
 			if retError == nil {
@@ -353,3 +360,18 @@ func containerRootfsPath(uvm *uvm.UtilityVM, rootPath string) string {
 	}
 	return ospath.Join(uvm.OS(), rootPath, uvmpkg.RootfsPath)
 }
+
+func getScratchVHDPath(layerFolders []string) (string, error) {
+	hostPath := filepath.Join(layerFolders[len(layerFolders)-1], "sandbox.vhdx")
+	// For LCOW, we can reuse another container's scratch space (usually the sandbox container's).
+	//
+	// When sharing a scratch space, the `hostPath` will be a symlink to the sandbox.vhdx location to use.
+	// When not sharing a scratch space, `hostPath` will be the path to the sandbox.vhdx to use.
+	//
+	// Evaluate the symlink here (if there is one).
+	hostPath, err := filepath.EvalSymlinks(hostPath)
+	if err != nil {
+		return "", fmt.Errorf("failed to eval symlinks: %s", err)
+	}
+	return hostPath, nil
+}

uvm/create_wcow.go

@@ -16,6 +16,7 @@ import (
 	hcsschema "github.com/Microsoft/hcsshim/internal/schema2"
 	"github.com/Microsoft/hcsshim/internal/schemaversion"
 	"github.com/Microsoft/hcsshim/internal/uvmfolder"
+	"github.com/Microsoft/hcsshim/internal/wclayer"
 	"github.com/Microsoft/hcsshim/internal/wcow"
 	"github.com/pkg/errors"
 	"go.opencensus.io/trace"
@@ -110,6 +111,11 @@ func CreateWCOW(ctx context.Context, opts *OptionsWCOW) (_ *UtilityVM, err error
 		if err := wcow.CreateUVMScratch(ctx, uvmFolder, scratchFolder, uvm.id); err != nil {
 			return nil, fmt.Errorf("failed to create scratch: %s", err)
 		}
+	} else {
+		// Sandbox.vhdx exists, just need to grant vm access to it.
+		if err := wclayer.GrantVmAccess(ctx, uvm.id, scratchPath); err != nil {
+			return nil, errors.Wrap(err, "failed to grant vm access to scratch")
+		}
 	}
 
 	processorTopology, err := hostProcessorInfo(ctx)