MVStore: auto-save (every few MBs and every 1-2 seconds)

h2/src/main/org/h2/mvstore/MVMap.java

@@ -48,6 +48,9 @@ public class MVMap<K, V> extends AbstractMap<K, V>
     private boolean closed;
     private boolean readOnly;
 
+    private volatile boolean writing;
+    private volatile int writeCount;
+
     protected MVMap(DataType keyType, DataType valueType) {
         this.keyType = keyType;
         this.valueType = valueType;
@@ -91,13 +94,17 @@ public class MVMap<K, V> extends AbstractMap<K, V>
      */
     @SuppressWarnings("unchecked")
     public V put(K key, V value) {
-        checkWrite();
+        beforeWrite();
+        try {
             long writeVersion = store.getCurrentVersion();
             Page p = copyOnWrite(root, writeVersion);
             p = splitRootIfNeeded(p, writeVersion);
             Object result = put(p, writeVersion, key, value);
             newRoot(p);
             return (V) result;
+        } finally {
+            afterWrite();
+        }
     }
 
     /**
@@ -488,9 +495,13 @@ public class MVMap<K, V> extends AbstractMap<K, V>
      * Remove all entries.
      */
     public void clear() {
-        checkWrite();
+        beforeWrite();
+        try {
             root.removeAllRecursive();
             newRoot(Page.createEmpty(this, store.getCurrentVersion()));
+        } finally {
+            afterWrite();
+        }
     }
 
     /**
@@ -498,11 +509,16 @@ public class MVMap<K, V> extends AbstractMap<K, V>
      */
     public void removeMap() {
         checkOpen();
-        if (this != store.getMetaMap()) {
-            checkWrite();
+        if (this == store.getMetaMap()) {
+            return;
+        }
+        beforeWrite();
+        try {
             root.removeAllRecursive();
             store.removeMap(id);
             close();
+        } finally {
+            afterWrite();
         }
     }
 
@@ -527,13 +543,17 @@ public class MVMap<K, V> extends AbstractMap<K, V>
      * @return the old value if the key existed, or null otherwise
      */
     public V remove(Object key) {
-        checkWrite();
+        beforeWrite();
+        try {
             long writeVersion = store.getCurrentVersion();
             Page p = copyOnWrite(root, writeVersion);
             @SuppressWarnings("unchecked")
             V result = (V) remove(p, writeVersion, key);
             newRoot(p);
             return result;
+        } finally {
+            afterWrite();
+        }
     }
 
     /**
@@ -616,7 +636,7 @@ public class MVMap<K, V> extends AbstractMap<K, V>
                 result = p.getValue(index);
                 p.remove(index);
                 if (p.getKeyCount() == 0) {
-                    removePage(p);
+                    removePage(p.getPos());
                 }
             }
             return result;
@@ -638,7 +658,7 @@ public class MVMap<K, V> extends AbstractMap<K, V>
             if (p.getKeyCount() == 0) {
                 p.setChild(index, c);
                 p.setCounts(index, c);
-                removePage(p);
+                removePage(p.getPos());
             } else {
                 p.remove(index);
             }
@@ -673,15 +693,6 @@ public class MVMap<K, V> extends AbstractMap<K, V>
         }
     }
 
-    /**
-     * Check whether this map has any unsaved changes.
-     *
-     * @return true if there are unsaved changes.
-     */
-    public boolean hasUnsavedChanges() {
-        return !oldRoots.isEmpty();
-    }
-
     /**
      * Compare two keys.
      *
@@ -819,7 +830,8 @@ public class MVMap<K, V> extends AbstractMap<K, V>
      * @param version the version
      */
     void rollbackTo(long version) {
-        checkWrite();
+        beforeWrite();
+        try {
             removeUnusedOldVersions();
             if (version <= createVersion) {
                 // the map is removed later
@@ -837,12 +849,15 @@ public class MVMap<K, V> extends AbstractMap<K, V>
                     }
                 }
             }
+        } finally {
+            afterWrite();
+        }
     }
 
     /**
      * Forget all old versions.
      */
-    void removeAllOldVersions() {
+    private void removeAllOldVersions() {
         // create a new instance
         // because another thread might iterate over it
         oldRoots = new ArrayList<Page>();
@@ -887,16 +902,44 @@ public class MVMap<K, V> extends AbstractMap<K, V>
     }
 
     /**
-     * Check whether writing is allowed.
+     * This method is called before writing to the map. The default
+     * implementation checks whether writing is allowed.
      *
-     * @throws IllegalStateException if the map is read-only
+     * @throws UnsupportedOperationException if the map is read-only
      */
-    protected void checkWrite() {
+    protected void beforeWrite() {
         if (readOnly) {
             checkOpen();
             throw DataUtils.newUnsupportedOperationException(
                     "This map is read-only");
         }
+        writing = true;
+        store.beforeWrite();
+    }
+
+    /**
+     * This method is called after writing to the map.
+     */
+    protected void afterWrite() {
+        writeCount++;
+        writing = false;
+    }
+
+    void waitUntilWritten(long version) {
+        if (root.getVersion() < version) {
+            // a write will create a new version
+            return;
+        }
+        // wait until writing is done,
+        // but only for the current write operation
+        // a bit like a spin lock
+        int w = writeCount;
+        while (writing) {
+            if (writeCount > w) {
+                return;
+            }
+            Thread.yield();
+        }
     }
 
     public int hashCode() {
@@ -926,8 +969,8 @@ public class MVMap<K, V> extends AbstractMap<K, V>
      *
      * @param p the page
      */
-    protected void removePage(Page p) {
-        store.removePage(p.getPos());
+    protected void removePage(long pos) {
+        store.removePage(this, pos);
     }
 
     /**
@@ -1051,15 +1094,18 @@ public class MVMap<K, V> extends AbstractMap<K, V>
      * @param newMapName the name name
      */
     public void renameMap(String newMapName) {
-        checkWrite();
+        beforeWrite();
+        try {
             store.renameMap(this, newMapName);
+        } finally {
+            afterWrite();
+        }
     }
 
     public String toString() {
         return asString(null);
     }
 
-
     /**
      * A builder for maps.
      *

h2/src/main/org/h2/mvstore/MVMapConcurrent.java

@@ -34,24 +34,32 @@ public class MVMapConcurrent<K, V> extends MVMap<K, V> {
 
     @SuppressWarnings("unchecked")
     public V put(K key, V value) {
-        checkWrite();
-        V result = get(key);
-        if (value.equals(result)) {
-            return result;
-        }
+        beforeWrite();
+        try {
+            // even if the result is the same, we still update the value
+            // (otherwise compact doesn't work)
+            get(key);
             long writeVersion = store.getCurrentVersion();
             synchronized (this) {
                 Page p = copyOnWrite(root, writeVersion);
                 p = splitRootIfNeeded(p, writeVersion);
-            result = (V) put(p, writeVersion, key, value);
+                V result = (V) put(p, writeVersion, key, value);
                 newRoot(p);
-        }
                 return result;
             }
+        } finally {
+            afterWrite();
+        }
+    }
+
+    void waitUntilWritten(long version) {
+        // no need to wait
+    }
 
     @SuppressWarnings("unchecked")
     public V remove(Object key) {
-        checkWrite();
+        beforeWrite();
+        try {
             V result = get(key);
             if (result == null) {
                 return null;
@@ -63,6 +71,9 @@ public class MVMapConcurrent<K, V> extends MVMap<K, V> {
                 newRoot(p);
             }
             return result;
+        } finally {
+            afterWrite();
+        }
     }
 
     /**

h2/src/main/org/h2/mvstore/MVStore.java

@@ -18,6 +18,7 @@ import java.util.Comparator;
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 import org.h2.compress.CompressLZF;
 import org.h2.compress.Compressor;
 import org.h2.mvstore.cache.CacheLongKeyLIRS;
@@ -42,6 +43,16 @@ H:3,...
 
 TODO:
 
+- getTime: use milliseconds, not seconds (no division, finer granurality)
+- naming: hasUnsavedChanges() versus store(): hasUnstoredChanges?
+- test rollback of meta table: it is changed after save; could rollback be a problem?
+- async store: write test cases; should fail at freedChunks
+- async store of current root is illegal, except with MVMapConcurrent
+- auto-store needs to be reverted on startup
+- auto-store synchronously if too many unstored pages (8 MB)
+- auto-store in background thread after 1-2 second by default
+- auto-store in background thread if more than 4 MB of unstored pages
+- auto-store: use notify to wake up background thread?
 - automated 'kill process' and 'power failure' test
 - mvcc with multiple transactions
 - update checkstyle
@@ -64,16 +75,11 @@ TODO:
 - store number of write operations per page (maybe defragment
 -- if much different than count)
 - r-tree: nearest neighbor search
-- use FileChannel by default (nio file system), but:
--- an interrupt closes the FileChannel
-- auto-save temporary data if it uses too much memory,
--- but revert it on startup if needed.
 - chunk metadata: do not store default values
 - support maps without values (just existence of the key)
 - support maps without keys (counted b-tree features)
 - use a small object cache (StringCache), test on Android
 - dump values
-- tool to import / manipulate CSV files (maybe concurrently)
 - map split / merge (fast if no overlap)
 - auto-save if there are too many changes (required for StreamStore)
 - StreamStore optimization: avoid copying bytes
@@ -87,8 +93,7 @@ TODO:
 -- to support concurrent updates and writes, and very large maps
 - implement an off-heap file system
 - remove change cursor, or add support for writing to branches
-- file encryption: try using multiple threads
-- file encryption: add a fast, insecure algorithm
+- support pluggable logging or remove log
 
 */
 
@@ -111,6 +116,8 @@ public class MVStore {
     private static final int FORMAT_WRITE = 1;
     private static final int FORMAT_READ = 1;
 
+    volatile boolean closed;
+
     private final String fileName;
     private final char[] filePassword;
 
@@ -129,22 +136,25 @@ public class MVStore {
     private final CacheLongKeyLIRS<Page> cache;
 
     private int lastChunkId;
-    private final HashMap<Integer, Chunk> chunks = New.hashMap();
+
+    /**
+     * The map of chunks.
+     */
+    private final ConcurrentHashMap<Integer, Chunk> chunks =
+            new ConcurrentHashMap<Integer, Chunk>();
 
     /**
      * The map of temporarily freed entries in the chunks. The key is the
      * unsaved version, the value is the map of chunks. The maps of chunks
      * contains the number of freed entries per chunk.
+     * Access is synchronized.
      */
     private final HashMap<Long, HashMap<Integer, Chunk>> freedChunks = New.hashMap();
 
-    private MVMap<String, String> meta;
-    private final HashMap<Integer, MVMap<?, ?>> maps = New.hashMap();
+    private MVMapConcurrent<String, String> meta;
 
-    /**
-     * The set of maps with potentially unsaved changes.
-     */
-    private final HashMap<Integer, MVMap<?, ?>> mapsChanged = New.hashMap();
+    private final ConcurrentHashMap<Integer, MVMap<?, ?>> maps =
+            new ConcurrentHashMap<Integer, MVMap<?, ?>>();
 
     private HashMap<String, String> fileHeader = New.hashMap();
 
@@ -162,9 +172,11 @@ public class MVStore {
     private final boolean compress;
 
     private long currentVersion;
+    private long lastStoredVersion;
     private int fileReadCount;
     private int fileWriteCount;
     private int unsavedPageCount;
+    private int maxUnsavedPages;
 
     /**
      * The time the store was created, in seconds since 1970.
@@ -172,24 +184,41 @@ public class MVStore {
     private long creationTime;
     private int retentionTime = 45;
 
-    private boolean closed;
+    private long lastStoreTime;
+    private Thread backgroundThread;
+
+    /**
+     * The version of the current store operation (if any).
+     */
+    private long currentStoreVersion = -1;
+
+    private volatile boolean metaChanged;
 
     MVStore(HashMap<String, Object> config) {
         String f = (String) config.get("fileName");
         if (f != null && !f.startsWith("nio:")) {
-            // nio is used by default
+            // NIO is used by default
+            // the following line is to ensure the NIO file system is compiled
             FilePathNio.class.getName();
             f = "nio:" + f;
         }
         this.fileName = f;
-        this.readOnly = "r".equals(config.get("openMode"));
-        this.compress = "1".equals(config.get("compress"));
+        this.readOnly = config.containsKey("readOnly");
+        this.compress = config.containsKey("compress");
         if (fileName != null) {
-            Object s = config.get("cacheSize");
-            int mb = s == null ? 16 : Integer.parseInt(s.toString());
+            Object o = config.get("cacheSize");
+            int mb = o == null ? 16 : (Integer) o;
+            int maxMemoryBytes = mb * 1024 * 1024;
+            int averageMemory = pageSize / 2;
+            int segmentCount = 16;
+            int stackMoveDistance = maxMemoryBytes / averageMemory * 2 / 100;
             cache = new CacheLongKeyLIRS<Page>(
-                    mb * 1024 * 1024, 2048, 16, mb * 1024 * 1024 / 2048 * 2 / 100);
+                    maxMemoryBytes, averageMemory, segmentCount, stackMoveDistance);
             filePassword = (char[]) config.get("encrypt");
+            o = config.get("writeBufferSize");
+            mb = o == null ? 4 : (Integer) o;
+            int writeBufferSize =  mb * 1024 * 1024;
+            maxUnsavedPages = writeBufferSize / pageSize;
         } else {
             cache = null;
             filePassword = null;
@@ -314,7 +343,7 @@ public class MVStore {
 
     private MVMap<String, String> getMetaMap(long version) {
         Chunk c = getChunkForVersion(version);
-        DataUtils.checkArgument(c != null, "Unknown version {}", version);
+        DataUtils.checkArgument(c != null, "Unknown version {0}", version);
         c = readChunkHeader(c.start);
         MVMap<String, String> oldMeta = meta.openReadOnly();
         oldMeta.setRootPos(c.metaRootPos, version);
@@ -343,7 +372,6 @@ public class MVStore {
         meta.remove("map." + id);
         meta.remove("name." + name);
         meta.remove("root." + id);
-        mapsChanged.remove(id);
         maps.remove(id);
     }
 
@@ -353,7 +381,9 @@ public class MVStore {
      * @param map the map
      */
     void markChanged(MVMap<?, ?> map) {
-        mapsChanged.put(map.getId(), map);
+        if (map == meta) {
+            metaChanged = true;
+        }
     }
 
     private void markMetaChanged() {
@@ -366,7 +396,7 @@ public class MVStore {
      * Open the store.
      */
     void open() {
-        meta = new MVMap<String, String>(StringDataType.INSTANCE, StringDataType.INSTANCE);
+        meta = new MVMapConcurrent<String, String>(StringDataType.INSTANCE, StringDataType.INSTANCE);
         HashMap<String, String> c = New.hashMap();
         c.put("id", "0");
         c.put("createVersion", Long.toString(currentVersion));
@@ -390,6 +420,15 @@ public class MVStore {
                 Arrays.fill(filePassword, (char) 0);
             }
         }
+        lastStoreTime = getTime();
+        // if we use auto-save, also start the background thread
+        if (maxUnsavedPages > 0) {
+            Writer w = new Writer(this);
+            Thread t = new Thread(w, "MVStore writer " + fileName);
+            t.setDaemon(true);
+            t.start();
+            backgroundThread = t;
+        }
     }
 
     /**
@@ -428,6 +467,7 @@ public class MVStore {
             if (fileSize == 0) {
                 creationTime = 0;
                 creationTime = getTime();
+                lastStoreTime = creationTime;
                 fileHeader.put("H", "3");
                 fileHeader.put("blockSize", "" + BLOCK_SIZE);
                 fileHeader.put("format", "" + FORMAT_WRITE);
@@ -534,6 +574,7 @@ public class MVStore {
         if (currentVersion < 0) {
             throw DataUtils.newIllegalStateException("File header is corrupt");
         }
+        lastStoredVersion = -1;
     }
 
     private byte[] getFileHeaderBytes() {
@@ -547,7 +588,7 @@ public class MVStore {
         DataUtils.appendMap(buff, "fletcher", Integer.toHexString(checksum));
         bytes = DataUtils.utf8Encode(buff.toString());
         DataUtils.checkArgument(bytes.length <= BLOCK_SIZE,
-                "File header too large: {}", buff);
+                "File header too large: {0}", buff);
         return bytes;
     }
 
@@ -570,9 +611,21 @@ public class MVStore {
         close(true);
     }
 
-    private void close(boolean shrinkIfPossible) {
+    private synchronized void close(boolean shrinkIfPossible) {
         closed = true;
-        if (file != null) {
+        if (file == null) {
+            return;
+        }
+        if (backgroundThread != null) {
+            Thread t = backgroundThread;
+            backgroundThread = null;
+            t.interrupt();
+            try {
+                t.join();
+            } catch (Exception e) {
+                // ignore
+            }
+        }
         try {
             if (shrinkIfPossible) {
                 shrinkFileIfPossible(0);
@@ -590,7 +643,6 @@ public class MVStore {
             chunks.clear();
             cache.clear();
             maps.clear();
-                mapsChanged.clear();
         } catch (Exception e) {
             throw DataUtils.newIllegalStateException(
                     "Closing failed for file {0}", fileName, e);
@@ -598,7 +650,6 @@ public class MVStore {
             file = null;
         }
     }
-    }
 
     /**
      * Get the chunk for the given position.
@@ -623,24 +674,41 @@ public class MVStore {
      * Commit all changes and persist them to disk. This method does nothing if
      * there are no unsaved changes, otherwise it increments the current version
      * and stores the data (for file based stores).
+     * <p>
+     * One store operation may run at any time.
      *
      * @return the new version (incremented if there were changes)
      */
     public long store() {
+        return store(false);
+    }
+
+    /**
+     * Store changes.
+     *
+     * @param temp whether the changes should be rolled back after opening
+     * @return the new version (incremented if there were changes)
+     */
+    private synchronized long store(boolean temp) {
         checkOpen();
+        if (currentStoreVersion >= 0) {
+            // store is possibly called within store, if the meta map changed
+            return currentVersion;
+        }
         if (!hasUnsavedChanges()) {
             return currentVersion;
         }
-
         int currentUnsavedPageCount = unsavedPageCount;
-        long storeVersion = currentVersion;
+        long storeVersion = currentStoreVersion = currentVersion;
         long version = incrementVersion();
-        long time = getTime();
 
         if (file == null) {
             return version;
         }
 
+        long time = getTime();
+        lastStoreTime = time;
+
         // the last chunk was not completely correct in the last store()
         // this needs to be updated now (it's better not to update right after
         // storing, because that would modify the meta map again)
@@ -661,21 +729,29 @@ public class MVStore {
         c.version = version;
         chunks.put(c.id, c);
         meta.put("chunk." + c.id, c.asString());
-
-        for (MVMap<?, ?> m : mapsChanged.values()) {
-            if (m == meta || !m.hasUnsavedChanges()) {
-                continue;
+        ArrayList<MVMap<?, ?>> list = New.arrayList(maps.values());
+        ArrayList<MVMap<?, ?>> changed = New.arrayList();
+        for (MVMap<?, ?> m : list) {
+            if (m != meta) {
+                long v = m.getVersion();
+                if (v >= 0 && m.getVersion() >= lastStoredVersion) {
+                    changed.add(m.openVersion(storeVersion));
+                }
             }
-            Page p = m.openVersion(storeVersion).getRoot();
+        }
+        for (MVMap<?, ?> m : changed) {
+            Page p = m.getRoot();
             if (p.getTotalCount() == 0) {
                 meta.put("root." + m.getId(), "0");
             } else {
-                meta.put("root." + m.getId(), String.valueOf(Long.MAX_VALUE));
+                meta.put("root." + m.getId(), String.valueOf(Integer.MAX_VALUE));
             }
         }
-        applyFreedChunks();
+        applyFreedChunks(storeVersion);
         ArrayList<Integer> removedChunks = New.arrayList();
-        do {
+        // do it twice, because changing the meta table
+        // could cause a chunk to get empty
+        for (int i = 0; i < 2; i++) {
             for (Chunk x : chunks.values()) {
                 if (x.maxLengthLive == 0 && canOverwriteChunk(x, time)) {
                     meta.remove("chunk." + x.id);
@@ -683,9 +759,9 @@ public class MVStore {
                 } else {
                     meta.put("chunk." + x.id, x.asString());
                 }
-                applyFreedChunks();
+                applyFreedChunks(storeVersion);
+            }
         }
-        } while (freedChunks.size() > 0);
         ByteBuffer buff;
         if (writeBuffer != null) {
             buff = writeBuffer;
@@ -697,11 +773,8 @@ public class MVStore {
         c.writeHeader(buff);
         c.maxLength = 0;
         c.maxLengthLive = 0;
-        for (MVMap<?, ?> m : mapsChanged.values()) {
-            if (m == meta || !m.hasUnsavedChanges()) {
-                continue;
-            }
-            Page p = m.openVersion(storeVersion).getRoot();
+        for (MVMap<?, ?> m : changed) {
+            Page p = m.getRoot();
             if (p.getTotalCount() > 0) {
                 buff = p.writeUnsavedRecursive(c, buff);
                 long root = p.getPos();
@@ -728,7 +801,7 @@ public class MVStore {
 
         long fileLength = getFileLengthUsed();
         long filePos = reuseSpace ? allocateChunk(length) : fileLength;
-        boolean atEnd = filePos + length >= fileLength;
+        boolean storeAtEndOfFile = filePos + length >= fileLength;
 
         // need to keep old chunks
         // until they are are no longer referenced
@@ -744,7 +817,7 @@ public class MVStore {
         buff.position(0);
         c.writeHeader(buff);
         rootChunkStart = filePos;
-        revertTemp();
+        revertTemp(storeVersion);
 
         buff.position(buff.limit() - BLOCK_SIZE);
         byte[] header = getFileHeaderBytes();
@@ -761,12 +834,16 @@ public class MVStore {
         }
 
         // overwrite the header if required
-        if (!atEnd) {
+        if (!storeAtEndOfFile) {
             writeFileHeader();
             shrinkFileIfPossible(1);
         }
         // some pages might have been changed in the meantime (in the newest version)
         unsavedPageCount = Math.max(0, unsavedPageCount - currentUnsavedPageCount);
+        currentStoreVersion = -1;
+        metaChanged = false;
+        lastStoredVersion = storeVersion;
+
         return version;
     }
 
@@ -778,9 +855,14 @@ public class MVStore {
         return (System.currentTimeMillis() / 1000) - creationTime;
     }
 
-    private void applyFreedChunks() {
-        // TODO support concurrent operations
-        for (HashMap<Integer, Chunk> freed : freedChunks.values()) {
+    private void applyFreedChunks(long storeVersion) {
+        synchronized (freedChunks) {
+            for (Iterator<Long> it = freedChunks.keySet().iterator(); it.hasNext();) {
+                long v = it.next();
+                if (v > storeVersion) {
+                    continue;
+                }
+                Map<Integer, Chunk> freed = freedChunks.get(v);
                 for (Chunk f : freed.values()) {
                     Chunk c = chunks.get(f.id);
                     c.maxLengthLive += f.maxLengthLive;
@@ -789,8 +871,9 @@ public class MVStore {
                                 "Corrupt max length {0}", c.maxLengthLive);
                     }
                 }
+                it.remove();
+            }
         }
-        freedChunks.clear();
     }
 
     /**
@@ -870,11 +953,12 @@ public class MVStore {
      */
     public boolean hasUnsavedChanges() {
         checkOpen();
-        if (mapsChanged.size() == 0) {
-            return false;
+        if (metaChanged) {
+            return true;
         }
-        for (MVMap<?, ?> m : mapsChanged.values()) {
-            if (m == meta || m.hasUnsavedChanges()) {
+        for (MVMap<?, ?> m : maps.values()) {
+            long v = m.getVersion();
+            if (v >= 0 && v >= lastStoredVersion) {
                 return true;
             }
         }
@@ -1051,9 +1135,10 @@ public class MVStore {
     /**
      * Remove a page.
      *
+     * @param map the map the page belongs to
      * @param pos the position of the page
      */
-    void removePage(long pos) {
+    void removePage(MVMap<?, ?> map, long pos) {
         // we need to keep temporary pages,
         // to support reading old versions and rollback
         if (pos == 0) {
@@ -1065,10 +1150,19 @@ public class MVStore {
         // but we don't optimize for rollback
         cache.remove(pos);
         Chunk c = getChunk(pos);
-        HashMap<Integer, Chunk>freed = freedChunks.get(currentVersion);
+        long version = currentVersion;
+        if (map == meta && currentStoreVersion >= 0) {
+            // if the meta map is modified while storing,
+            // then this freed page needs to be registered
+            // with the stored chunk, so that the old chunk
+            // can be re-used
+            version = currentStoreVersion;
+        }
+        synchronized (freedChunks) {
+            HashMap<Integer, Chunk>freed = freedChunks.get(version);
             if (freed == null) {
                 freed = New.hashMap();
-            freedChunks.put(currentVersion, freed);
+                freedChunks.put(version, freed);
             }
             Chunk f = freed.get(c.id);
             if (f == null) {
@@ -1077,6 +1171,7 @@ public class MVStore {
             }
             f.maxLengthLive -= DataUtils.getPageMaxLength(pos);
         }
+    }
 
     /**
      * Log the string, if logging is enabled.
@@ -1146,10 +1241,10 @@ public class MVStore {
      * How long to retain old, persisted chunks, in seconds. Chunks that are
      * older than this many seconds may be overwritten once they contain no live
      * data. The default is 45 seconds. It is assumed that a file system and
-     * hard disk will flush all write buffers after this many seconds at the
-     * latest. Using a lower value might be dangerous, unless the file system
-     * and hard disk flush the buffers earlier. To manually flush the buffers,
-     * use <code>MVStore.getFile().force(true)</code>, however please note that
+     * hard disk will flush all write buffers within this many seconds. Using a
+     * lower value might be dangerous, unless the file system and hard disk
+     * flush the buffers earlier. To manually flush the buffers, use
+     * <code>MVStore.getFile().force(true)</code>, however please note that
      * according to various tests this does not always work as expected.
      * <p>
      * This setting is not persisted.
@@ -1232,6 +1327,15 @@ public class MVStore {
         unsavedPageCount++;
     }
 
+    /**
+     * This method is called before writing to a map.
+     */
+    void beforeWrite() {
+        if (unsavedPageCount > maxUnsavedPages && maxUnsavedPages > 0) {
+            store();
+        }
+    }
+
     /**
      * Get the store version. The store version is usually used to upgrade the
      * structure of the store after upgrading the application. Initially the
@@ -1264,12 +1368,12 @@ public class MVStore {
      *
      * @param version the version to revert to
      */
-    public void rollbackTo(long version) {
+    public synchronized void rollbackTo(long version) {
         checkOpen();
         DataUtils.checkArgument(
                 isKnownVersion(version),
                 "Unknown version {0}", version);
-        for (MVMap<?, ?> m : mapsChanged.values()) {
+        for (MVMap<?, ?> m : maps.values()) {
             m.rollbackTo(version);
         }
         for (long v = currentVersion; v >= version; v--) {
@@ -1279,11 +1383,12 @@ public class MVStore {
             freedChunks.remove(v);
         }
         meta.rollbackTo(version);
+        metaChanged = false;
         boolean loadFromFile = false;
         Chunk last = chunks.get(lastChunkId);
         if (last != null) {
             if (last.version >= version) {
-                revertTemp();
+                revertTemp(version);
                 loadFromFile = true;
                 do {
                     last = chunks.remove(lastChunkId);
@@ -1313,19 +1418,27 @@ public class MVStore {
                 if (loadFromFile) {
                     String r = meta.get("root." + id);
                     long root = r == null ? 0 : Long.parseLong(r);
-                    m.setRootPos(root, version);
+                    m.setRootPos(root, -1);
                 }
             }
         }
+//        this.lastStoredVersion = version - 1;
         this.currentVersion = version;
     }
 
-    private void revertTemp() {
-        freedChunks.clear();
-        for (MVMap<?, ?> m : mapsChanged.values()) {
-            m.removeAllOldVersions();
+    private void revertTemp(long storeVersion) {
+        synchronized (freedChunks) {
+            for (Iterator<Long> it = freedChunks.keySet().iterator(); it.hasNext();) {
+                long v = it.next();
+                if (v > storeVersion) {
+                    continue;
+                }
+                it.remove();
+            }
+        }
+        for (MVMap<?, ?> m : maps.values()) {
+            m.removeUnusedOldVersions();
         }
-        mapsChanged.clear();
     }
 
     /**
@@ -1430,6 +1543,42 @@ public class MVStore {
         return DataUtils.parseMap(m).get("name");
     }
 
+    void storeIfNeeded() {
+        if (closed || unsavedPageCount == 0) {
+            return;
+        }
+        long time = getTime();
+        if (time <= lastStoreTime + 1) {
+            return;
+        }
+        store();
+    }
+
+    /**
+     * A background writer to automatically store changes every two seconds.
+     */
+    private static class Writer implements Runnable {
+
+        private final MVStore store;
+
+        Writer(MVStore store) {
+            this.store = store;
+        }
+
+        @Override
+        public void run() {
+            while (!store.closed) {
+                store.storeIfNeeded();
+                try {
+                    Thread.sleep(1000);
+                } catch (InterruptedException e) {
+                    // ignore
+                }
+            }
+        }
+
+    }
+
     /**
      * A builder for an MVStore.
      */
@@ -1481,7 +1630,7 @@ public class MVStore {
          * @return this
          */
         public Builder readOnly() {
-            return set("openMode", "r");
+            return set("readOnly", 1);
         }
 
         /**
@@ -1491,7 +1640,21 @@ public class MVStore {
          * @return this
          */
         public Builder cacheSizeMB(int mb) {
-            return set("cacheSize", Integer.toString(mb));
+            return set("cacheSize", mb);
+        }
+
+        /**
+         * Set the size of the write buffer in MB. The default is 4 MB. Changes
+         * are automatically stored if the buffer grows larger than this, and
+         * after 2 seconds (whichever occurs earlier).
+         * <p>
+         * To disable automatically storing, set the buffer size to 0.
+         *
+         * @param mb the write buffer size
+         * @return this
+         */
+        public Builder writeBufferSizeMB(int mb) {
+            return set("writeBufferSize", mb);
         }
 
         /**
@@ -1502,7 +1665,7 @@ public class MVStore {
          * @return this
          */
         public Builder compressData() {
-            return set("compress", "1");
+            return set("compress", 1);
         }
 
         /**

h2/src/main/org/h2/mvstore/Page.java

@@ -267,7 +267,7 @@ public class Page {
      * @return a page with the given version
      */
     public Page copy(long version) {
-        map.getStore().removePage(pos);
+        map.removePage(pos);
         Page newPage = create(map, version,
                 keyCount, keys, values, children, childrenPages,
                 counts, totalCount,
@@ -541,14 +541,14 @@ public class Page {
                     long c = children[i];
                     int type = DataUtils.getPageType(c);
                     if (type == DataUtils.PAGE_TYPE_LEAF) {
-                        map.getStore().removePage(c);
+                        map.removePage(c);
                     } else {
                         map.readPage(c).removeAllRecursive();
                     }
                 }
             }
         }
-        map.getStore().removePage(pos);
+        map.removePage(pos);
     }
 
     /**

h2/src/main/org/h2/mvstore/rtree/MVRTreeMap.java

@@ -150,7 +150,7 @@ public class MVRTreeMap<V> extends MVMap<SpatialKey, V> {
                     result = p.getValue(i);
                     p.remove(i);
                     if (p.getKeyCount() == 0) {
-                        removePage(p);
+                        removePage(p.getPos());
                     }
                     break;
                 }
@@ -170,7 +170,7 @@ public class MVRTreeMap<V> extends MVMap<SpatialKey, V> {
                     // this child was deleted
                     p.remove(i);
                     if (p.getKeyCount() == 0) {
-                        removePage(p);
+                        removePage(p.getPos());
                     }
                     break;
                 }
@@ -211,7 +211,8 @@ public class MVRTreeMap<V> extends MVMap<SpatialKey, V> {
     }
 
     private Object putOrAdd(SpatialKey key, V value, boolean alwaysAdd) {
-        checkWrite();
+        beforeWrite();
+        try {
             long writeVersion = store.getCurrentVersion();
             Page p = copyOnWrite(root, writeVersion);
             Object result;
@@ -239,6 +240,9 @@ public class MVRTreeMap<V> extends MVMap<SpatialKey, V> {
             }
             newRoot(p);
             return result;
+        } finally {
+            afterWrite();
+        }
     }
 
     /**

h2/src/test/org/h2/test/store/TestMVStore.java

@@ -206,7 +206,7 @@ public class TestMVStore extends TestBase {
         s.store();
         s.close();
         int[] expectedReadsForCacheSize = {
-                3412, 2590, 1924, 1440, 1102, 956, 918
+                3407, 2590, 1924, 1440, 1106, 956, 918
         };
         for (int cacheSize = 0; cacheSize <= 6; cacheSize += 4) {
             s = new MVStore.Builder().
@@ -723,17 +723,22 @@ public class TestMVStore extends TestBase {
         m.put("1", "Hello");
         assertEquals(1, s.incrementVersion());
         s.rollbackTo(1);
+        assertEquals(1, s.getCurrentVersion());
         assertEquals("Hello", m.get("1"));
+
         long v2 = s.store();
         assertEquals(2, v2);
         assertEquals(2, s.getCurrentVersion());
         assertFalse(s.hasUnsavedChanges());
+        assertEquals("Hello", m.get("1"));
         s.close();
 
         s = openStore(fileName);
         assertEquals(2, s.getCurrentVersion());
         meta = s.getMetaMap();
         m = s.openMap("data");
+        assertFalse(s.hasUnsavedChanges());
+        assertEquals("Hello", m.get("1"));
         m0 = s.openMap("data0");
         MVMap<String, String> m1 = s.openMap("data1");
         m.put("1", "Hallo");