diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/dfs/DfsGarbageCollectorTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/dfs/DfsGarbageCollectorTest.java
index ab998951f..405e12677 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/dfs/DfsGarbageCollectorTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/dfs/DfsGarbageCollectorTest.java
@@ -15,6 +15,7 @@
 import static org.junit.Assert.fail;
 
 import java.io.IOException;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.concurrent.TimeUnit;
 
@@ -1100,6 +1101,70 @@ public void commitGraphWithoutGCrestPack() throws Exception {
 		}
 	}
 
+	@Test
+	public void objectSizeIdx_reachableBlob_bigEnough_indexed() throws Exception {
+		String master = "refs/heads/master";
+		RevCommit root = git.branch(master).commit().message("root").noParents()
+				.create();
+		RevBlob headsBlob = git.blob("twelve bytes");
+		git.branch(master).commit()
+				.message("commit on head")
+				.add("file.txt", headsBlob)
+				.parent(root)
+				.create();
+
+		gcWithObjectSizeIndex(10);
+
+		DfsReader reader = odb.newReader();
+		DfsPackFile gcPack = findFirstBySource(odb.getPacks(), GC);
+		assertTrue(gcPack.hasObjectSizeIndex(reader));
+		assertEquals(12, gcPack.getIndexedObjectSize(reader, headsBlob));
+	}
+
+	@Test
+	public void objectSizeIdx_reachableBlob_tooSmall_notIndexed() throws Exception {
+		String master = "refs/heads/master";
+		RevCommit root = git.branch(master).commit().message("root").noParents()
+				.create();
+		RevBlob tooSmallBlob = git.blob("small");
+		git.branch(master).commit()
+				.message("commit on head")
+				.add("small.txt", tooSmallBlob)
+				.parent(root)
+				.create();
+
+		gcWithObjectSizeIndex(10);
+
+		DfsReader reader = odb.newReader();
+		DfsPackFile gcPack = findFirstBySource(odb.getPacks(), GC);
+		assertTrue(gcPack.hasObjectSizeIndex(reader));
+		assertEquals(-1, gcPack.getIndexedObjectSize(reader, tooSmallBlob));
+	}
+
+	@Test
+	public void objectSizeIndex_unreachableGarbage_noIdx() throws Exception {
+		String master = "refs/heads/master";
+		RevCommit root = git.branch(master).commit().message("root").noParents()
+				.create();
+		git.branch(master).commit()
+				.message("commit on head")
+				.add("file.txt", git.blob("a blob"))
+				.parent(root)
+				.create();
+		git.update(master, root); // blob is unreachable
+		gcWithObjectSizeIndex(0);
+
+		DfsReader reader = odb.newReader();
+		DfsPackFile gcRestPack = findFirstBySource(odb.getPacks(), UNREACHABLE_GARBAGE);
+		assertFalse(gcRestPack.hasObjectSizeIndex(reader));
+	}
+
+	private static DfsPackFile findFirstBySource(DfsPackFile[] packs, PackSource source) {
+		return Arrays.stream(packs)
+				.filter(p -> p.getPackDescription().getPackSource() == source)
+				.findFirst().get();
+	}
+
 	private TestRepository<InMemoryRepository>.CommitBuilder commit() {
 		return git.commit();
 	}
@@ -1110,6 +1175,12 @@ private void gcWithCommitGraph() throws IOException {
 		run(gc);
 	}
 
+	private void gcWithObjectSizeIndex(int threshold) throws IOException {
+		DfsGarbageCollector gc = new DfsGarbageCollector(repo);
+		gc.getPackConfig().setMinBytesForObjSizeIndex(threshold);
+		run(gc);
+	}
+
 	private void gcNoTtl() throws IOException {
 		DfsGarbageCollector gc = new DfsGarbageCollector(repo);
 		gc.setGarbageTtl(0, TimeUnit.MILLISECONDS); // disable TTL
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsGarbageCollector.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsGarbageCollector.java
index 92e23b8b4..9f415782f 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsGarbageCollector.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsGarbageCollector.java
@@ -20,6 +20,7 @@
 import static org.eclipse.jgit.internal.storage.pack.PackExt.BITMAP_INDEX;
 import static org.eclipse.jgit.internal.storage.pack.PackExt.COMMIT_GRAPH;
 import static org.eclipse.jgit.internal.storage.pack.PackExt.INDEX;
+import static org.eclipse.jgit.internal.storage.pack.PackExt.OBJECT_SIZE_INDEX;
 import static org.eclipse.jgit.internal.storage.pack.PackExt.PACK;
 import static org.eclipse.jgit.internal.storage.pack.PackExt.REFTABLE;
 import static org.eclipse.jgit.internal.storage.pack.PackWriter.NONE;
@@ -681,6 +682,17 @@ private DfsPackDescription writePack(PackSource source, PackWriter pw,
 			pack.setIndexVersion(pw.getIndexVersion());
 		}
 
+		if (source != UNREACHABLE_GARBAGE && packConfig.getMinBytesForObjSizeIndex() >= 0) {
+			try (DfsOutputStream out = objdb.writeFile(pack,
+					OBJECT_SIZE_INDEX)) {
+				CountingOutputStream cnt = new CountingOutputStream(out);
+				pw.writeObjectSizeIndex(cnt);
+				pack.addFileExt(OBJECT_SIZE_INDEX);
+				pack.setFileSize(OBJECT_SIZE_INDEX, cnt.getCount());
+				pack.setBlockSize(OBJECT_SIZE_INDEX, out.blockSize());
+			}
+		}
+
 		if (pw.prepareBitmapIndex(pm)) {
 			try (DfsOutputStream out = objdb.writeFile(pack, BITMAP_INDEX)) {
 				CountingOutputStream cnt = new CountingOutputStream(out);