Truncate the block file during custom volume migration

internal/server/storage/drivers/generic_vfs.go

@@ -392,6 +392,14 @@ func genericVFSCreateVolumeFromMigration(d Driver, initVolume func(vol Volume) (
 				if err != nil {
 					return err
 				}
+
+				// During migration (e.g., LVM → dir), the block file may be smaller because
+				// recvBlockVol uses SparseFileWrapper, which omits trailing zero bytes and does not truncate.
+				// enlargeVolumeBlockFile ensures the block file matches the source volume size by applying truncation.
+				err = enlargeVolumeBlockFile(snapVol, pathBlock)
+				if err != nil {
+					return err
+				}
 			}
 
 			// Create the snapshot itself.
@@ -445,6 +453,14 @@ func genericVFSCreateVolumeFromMigration(d Driver, initVolume func(vol Volume) (
 			if err != nil {
 				return err
 			}
+
+			// During migration (e.g., LVM → dir), the block file may be smaller because
+			// recvBlockVol uses SparseFileWrapper, which omits trailing zero bytes and does not truncate.
+			// enlargeVolumeBlockFile ensures the block file matches the source volume size by applying truncation.
+			err = enlargeVolumeBlockFile(vol, pathBlock)
+			if err != nil {
+				return err
+			}
 		}
 
 		return nil

internal/server/storage/drivers/utils.go

@@ -24,6 +24,7 @@ import (
 	"github.com/lxc/incus/v6/shared/idmap"
 	"github.com/lxc/incus/v6/shared/logger"
 	"github.com/lxc/incus/v6/shared/subprocess"
+	"github.com/lxc/incus/v6/shared/units"
 	"github.com/lxc/incus/v6/shared/util"
 )
 
@@ -389,6 +390,34 @@ func ensureVolumeBlockFile(vol Volume, path string, sizeBytes int64, allowUnsafe
 	return false, nil
 }
 
+// enlargeVolumeBlockFile enlarges the raw block file for a volume to the specified size.
+func enlargeVolumeBlockFile(vol Volume, path string) error {
+	if linux.IsBlockdevPath(path) {
+		return nil
+	}
+
+	volSize, err := units.ParseByteSizeString(vol.ConfigSize())
+	if err != nil {
+		return err
+	}
+
+	actualSize, err := BlockDiskSizeBytes(path)
+	if err != nil {
+		return err
+	}
+
+	if volSize < actualSize {
+		return fmt.Errorf("Block volumes cannot be shrunk: %w", ErrCannotBeShrunk)
+	}
+
+	err = ensureSparseFile(path, volSize)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
 // mkfsOptions represents options for filesystem creation.
 type mkfsOptions struct {
 	Label string