diff --git a/core/src/test/java/org/elasticsearch/index/shard/NewPathForShardTest.java b/core/src/test/java/org/elasticsearch/index/shard/NewPathForShardTest.java
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+++ b/core/src/test/java/org/elasticsearch/index/shard/NewPathForShardTest.java
@@ -0,0 +1,188 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.elasticsearch.index.shard;
+
+import com.carrotsearch.randomizedtesting.annotations.Repeat;
+
+import org.apache.lucene.mockfile.FilterFileSystem;
+import org.apache.lucene.mockfile.FilterFileSystemProvider;
+import org.apache.lucene.mockfile.FilterPath;
+import org.elasticsearch.cluster.metadata.IndexMetaData;
+import org.elasticsearch.common.io.PathUtils;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.env.Environment;
+import org.elasticsearch.env.NodeEnvironment;
+import org.elasticsearch.test.ESTestCase;
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import java.io.IOException;
+import java.lang.reflect.Field;
+import java.nio.file.FileStore;
+import java.nio.file.FileSystem;
+import java.nio.file.FileSystems;
+import java.nio.file.Path;
+import java.nio.file.attribute.FileAttributeView;
+import java.nio.file.attribute.FileStoreAttributeView;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Set;
+
+import static org.elasticsearch.common.settings.Settings.settingsBuilder;
+
+public class NewPathForShardTest extends ESTestCase {
+
+    // Sneakiness to install a mock filesystem to pretend how much free space we have on each path.data:
+
+    private static MockFileStore aFileStore = new MockFileStore("mocka");
+    private static MockFileStore bFileStore = new MockFileStore("mockb");
+    private static FileSystem origFileSystem;
+
+    @BeforeClass
+    public static void installMockUsableSpaceFS() throws Exception {
+        // Necessary so when Environment.clinit runs, to gather all FileStores, it sees ours:
+        Field field = PathUtils.class.getDeclaredField("DEFAULT");
+        field.setAccessible(true);
+        // nocommit can't double Filter maybe?
+        // origFileSystem = (FileSystem) field.get(null);
+        origFileSystem = FileSystems.getDefault();
+        FileSystem mock = new MockUsableSpaceFileSystemProvider().getFileSystem(getBaseTempDirForTestClass().toUri());
+        field.set(null, mock);
+        assertEquals(mock, PathUtils.getDefaultFileSystem());
+    }
+
+    @AfterClass
+    public static void removeMockUsableSpaceFS() throws Exception {
+        Field field = PathUtils.class.getDeclaredField("DEFAULT");
+        field.setAccessible(true);
+        field.set(null, origFileSystem);
+        origFileSystem = null;
+        aFileStore = null;
+        bFileStore = null;
+    }
+
+    /** Mock file system that fakes usable space for each FileStore */
+    static class MockUsableSpaceFileSystemProvider extends FilterFileSystemProvider {
+    
+        public MockUsableSpaceFileSystemProvider() {
+            super("mockusablespace://", FileSystems.getDefault());
+            final List<FileStore> fileStores = new ArrayList<>();
+            fileStores.add(aFileStore);
+            fileStores.add(bFileStore);
+            fileSystem = new FilterFileSystem(this, origFileSystem) {
+                    @Override
+                    public Iterable<FileStore> getFileStores() {
+                        return fileStores;
+                    }
+                };
+        }
+
+        @Override
+        public FileStore getFileStore(Path path) throws IOException {
+            if (path.toString().contains("/a/")) {
+                return aFileStore;
+            } else {
+                return bFileStore;
+            }
+        }
+    }
+
+    static class MockFileStore extends FileStore {
+
+        public long usableSpace;
+
+        private final String desc;
+
+        public MockFileStore(String desc) {
+            this.desc = desc;
+        }
+    
+        @Override
+        public String type() {
+            return "mock";
+        }
+
+        @Override
+        public String name() {
+            return "mock";
+        }
+
+        @Override
+        public String toString() {
+            return desc;
+        }
+
+        // TODO: we can enable mocking of these when we need them later:
+
+        @Override
+        public boolean isReadOnly() {
+            return false;
+        }
+
+        @Override
+        public long getTotalSpace() throws IOException {
+            return usableSpace*3;
+        }
+
+        @Override
+        public long getUsableSpace() throws IOException {
+            return usableSpace;
+        }
+
+        @Override
+        public long getUnallocatedSpace() throws IOException {
+            return usableSpace*2;
+        }
+
+        @Override
+        public boolean supportsFileAttributeView(Class<? extends FileAttributeView> type) {
+            return false;
+        }
+
+        @Override
+        public boolean supportsFileAttributeView(String name) {
+            return false;
+        }
+
+        @Override
+        public <V extends FileStoreAttributeView> V getFileStoreAttributeView(Class<V> type) {
+            return null;
+        }
+
+        @Override
+        public Object getAttribute(String attribute) throws IOException {
+            return null;
+        }
+    }
+
+    public void testSelectNewPathForShard() throws Exception {
+        Path path = createTempDir();
+
+        String[] paths = new String[2];
+        paths[0] = path.resolve("a").toString();
+        paths[1] = path.resolve("b").toString();
+
+        Settings settings = Settings.builder()
+            .put("path.home", path)
+            .putArray("path.data", paths).build();
+        NodeEnvironment nodeEnv = new NodeEnvironment(settings, new Environment(settings));
+    }
+}