Index: oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/segment/file/FileStore.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/segment/file/FileStore.java	(revision 62de029fbeb1700886799e4eb417fcc6a55c698a)
+++ oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/segment/file/FileStore.java	(revision )
@@ -204,7 +204,7 @@
         }
         this.writeFile = new File(
                 directory,
-                String.format(FILE_NAME_FORMAT, writeNumber, "a"));
+                String.format(FILE_NAME_FORMAT, writeNumber, "-0"));
         this.writer = new TarWriter(writeFile);
 
         LinkedList<String> heads = newLinkedList();
@@ -694,7 +694,7 @@
                 writeNumber++;
                 writeFile = new File(
                         directory,
-                        String.format(FILE_NAME_FORMAT, writeNumber, "a"));
+                        String.format(FILE_NAME_FORMAT, writeNumber, "-0"));
                 writer = new TarWriter(writeFile);
             }
         } catch (IOException e) {
Index: oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/segment/file/TarReader.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/segment/file/TarReader.java	(revision 62de029fbeb1700886799e4eb417fcc6a55c698a)
+++ oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/segment/file/TarReader.java	(revision )
@@ -23,6 +23,7 @@
 import static com.google.common.collect.Maps.newLinkedHashMap;
 import static com.google.common.collect.Maps.newTreeMap;
 import static com.google.common.collect.Sets.newHashSetWithExpectedSize;
+import static java.lang.Character.isDigit;
 import static java.util.Collections.singletonList;
 import static org.apache.jackrabbit.oak.plugins.segment.Segment.REF_COUNT_OFFSET;
 import static org.apache.jackrabbit.oak.plugins.segment.SegmentId.isDataSegmentId;
@@ -685,17 +686,10 @@
             return this;
         }
 
-        String name = file.getName();
-        int pos = name.length() - "a.tar".length();
-        char generation = name.charAt(pos);
-        if (generation == 'z') {
-            // no garbage collection after reaching generation z
+        File newFile = nextGeneration();
+        if (newFile == null) {
             return this;
         }
-
-        File newFile = new File(
-                file.getParentFile(),
-                name.substring(0, pos) + (char) (generation + 1) + ".tar");
         TarWriter writer = new TarWriter(newFile);
         for (int i = 0; i < sorted.length; i++) {
             TarEntry entry = sorted[i];
@@ -716,6 +710,27 @@
             log.warn("Failed to open cleaned up tar file {}", file);
             return this;
         }
+    }
+
+    private File nextGeneration() {
+        String name = file.getName();
+        int pos = name.lastIndexOf('-');
+        if (pos == -1) {
+            log.warn("Skipping garbage collection for tar file without generation suffix: {}", name);
+            return null;
+        }
+        String base = name.substring(0, pos + 1);
+
+        int generation = 0;
+        char c;
+        while (++pos < name.length() && (c = name.charAt(pos)) != '.') {
+            if (isDigit(c)) {
+                generation = generation * 10 + (c - '0');
+            } else {
+                log.warn("Skipping garbage collection for tar file with invalid generation suffix: {}", name);
+            }
+        }
+        return new File(file.getParentFile(), base + ++generation + ".tar");
     }
 
     File close() throws IOException {