A persistent multi-version map: split based on size (memory), not number of…

A persistent multi-version map: split based on size (memory), not number of entries; allow to estimate the memory used by unsaved changes

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

@@ -164,18 +164,20 @@ public class MVRTreeMap<K, V> extends MVMap<K, V> {
         Page p = root.copyOnWrite(writeVersion);
         Object result;
         if (alwaysAdd || get(key) == null) {
-            if (p.getKeyCount() > store.getMaxPageSize()) {
+            if (p.getMemory() > store.getMaxPageSize() && p.getKeyCount() > 1) {
                 // only possible if this is the root, else we would have split earlier
                 // (this requires maxPageSize is fixed)
                 long totalCount = p.getTotalCount();
                 Page split = split(p, writeVersion);
-                Object[] keys = { getBounds(p), getBounds(split) };
+                Object k1 = getBounds(p);
+                Object k2 = getBounds(split);
+                Object[] keys = { k1, k2 };
                 long[] children = { p.getPos(), split.getPos(), 0 };
                 Page[] childrenPages = { p, split, null };
                 long[] counts = { p.getTotalCount(), split.getTotalCount(), 0 };
                 p = Page.create(this, writeVersion, 2,
                         keys, null, children, childrenPages, counts,
-                        totalCount, 0);
+                        totalCount, 0, 0);
                 // now p is a node; continues
             }
             add(p, writeVersion, key, value);
@@ -244,7 +246,7 @@ public class MVRTreeMap<K, V> extends MVMap<K, V> {
             }
         }
         Page c = p.getChildPage(index).copyOnWrite(writeVersion);
-        if (c.getKeyCount() >= store.getMaxPageSize()) {
+        if (c.getMemory() > store.getMaxPageSize() && c.getKeyCount() > 1) {
             // split on the way down
             Page split = split(c, writeVersion);
             p = p.copyOnWrite(writeVersion);
@@ -365,7 +367,7 @@ public class MVRTreeMap<K, V> extends MVMap<K, V> {
         long[] c = leaf ? null : new long[1];
         Page[] cp = leaf ? null : new Page[1];
         return Page.create(this, writeVersion, 0,
-                new Object[4], values, c, cp, c, 0, 0);
+                new Object[4], values, c, cp, c, 0, 0, 0);
     }
 
     private static void move(Page source, Page target, int sourceIndex) {

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

@@ -64,7 +64,7 @@ public class TestMVRTree extends TestMVStore {
             SpatialKey k = new SpatialKey(i, x - p, x + p, y - p, y + p);
             r.add(k, "" + i);
             if (i > 0 && (i % len / 10) == 0) {
-                s.store();
+                s.save();
             }
             if (i > 0 && (i % 10000) == 0) {
                 render(r, getBaseDir() + "/test.png");
@@ -72,7 +72,7 @@ public class TestMVRTree extends TestMVStore {
         }
         // System.out.println(prof.getTop(5));
         // System.out.println("add: " + (System.currentTimeMillis() - t));
-        s.store();
+        s.save();
         s.close();
         s = openStore(fileName);
         r = s.openMap("data", "r", "s2", "");

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

@@ -185,7 +185,7 @@ public class TestMVStore extends TestBase {
         assertEquals(null, s.getSetting("hello"));
         s.setSetting("test", "Hello");
         assertEquals("Hello", s.getSetting("test"));
-        s.store();
+        s.save();
         s.close();
         s = MVStore.open(fileName);
         assertEquals(1, s.getCurrentVersion());
@@ -227,7 +227,7 @@ public class TestMVStore extends TestBase {
         map.put(1,  "Hello");
         map.put("2", 200);
         map.put(new Object[1], new Object[]{1, "2"});
-        s.store();
+        s.save();
         s.close();
         s = MVStore.open(fileName, new TestMapFactory());
         map = s.openMap("test");
@@ -273,7 +273,7 @@ public class TestMVStore extends TestBase {
                 map.openVersion(oldVersion);
 
         // store the newest data to disk
-        s.store();
+        s.save();
 
         // print the old version (can be done
         // concurrently with further modifications)
@@ -301,7 +301,7 @@ public class TestMVStore extends TestBase {
                 for (int i = 0; i < 3; i++) {
                     Integer x = m.get("value");
                     m.put("value", x == null ? 0 : x + 1);
-                    s.store();
+                    s.save();
                 }
                 s.close();
             }
@@ -320,7 +320,7 @@ public class TestMVStore extends TestBase {
             m.put(i, "Hi");
         }
         s.incrementVersion();
-        s.store();
+        s.save();
         for (int i = 20; i < 40; i++) {
             assertEquals("Hi", m.put(i, "Hello"));
         }
@@ -373,7 +373,7 @@ public class TestMVStore extends TestBase {
         assertTrue(mOld.isReadOnly());
         s.getCurrentVersion();
         s.setRetainChunk(0);
-        long old2 = s.store();
+        long old2 = s.save();
 
         // the old version is still available
         assertEquals("Hello", mOld.get("1"));
@@ -381,7 +381,7 @@ public class TestMVStore extends TestBase {
 
         m.put("1",  "Hi");
         assertEquals("Welt", m.remove("2"));
-        s.store();
+        s.save();
         s.close();
 
         s = openStore(fileName);
@@ -404,17 +404,17 @@ public class TestMVStore extends TestBase {
         for (int i = 0; i < 1000; i++) {
             m.put(i, "Hello World");
         }
-        s.store();
+        s.save();
         s.close();
         long len = FileUtils.size(fileName);
         s = openStore(fileName);
         m = s.openMap("data", Integer.class, String.class);
         m.clear();
-        s.store();
+        s.save();
         s.compact(100);
         s.close();
         long len2 = FileUtils.size(fileName);
-        assertTrue(len2 < len);
+        assertTrue("len2: " + len2 + " len: " + len, len2 < len);
     }
 
     private void testFastDelete() {
@@ -423,14 +423,15 @@ public class TestMVStore extends TestBase {
         MVStore s;
         MVMap<Integer, String> m;
         s = openStore(fileName);
-        s.setMaxPageSize(100);
+        s.setMaxPageSize(700);
         m = s.openMap("data", Integer.class, String.class);
         for (int i = 0; i < 1000; i++) {
             m.put(i, "Hello World");
             assertEquals(i + 1, m.size());
         }
         assertEquals(1000, m.size());
-        s.store();
+        assertEquals(281, s.getUnsavedPageCount());
+        s.save();
         assertEquals(3, s.getWriteCount());
         s.close();
 
@@ -438,9 +439,9 @@ public class TestMVStore extends TestBase {
         m = s.openMap("data", Integer.class, String.class);
         m.clear();
         assertEquals(0, m.size());
-        s.store();
+        s.save();
         // ensure only nodes are read, but not leaves
-        assertEquals(4, s.getReadCount());
+        assertEquals(34, s.getReadCount());
         assertEquals(2, s.getWriteCount());
         s.close();
     }
@@ -462,7 +463,7 @@ public class TestMVStore extends TestBase {
         assertEquals(1, s.incrementVersion());
         s.rollbackTo(1);
         assertEquals("Hello", m.get("1"));
-        long v2 = s.store();
+        long v2 = s.save();
         assertEquals(2, v2);
         assertEquals(2, s.getCurrentVersion());
         assertFalse(s.hasUnsavedChanges());
@@ -486,7 +487,7 @@ public class TestMVStore extends TestBase {
         assertNull(meta.get("map.data1"));
         assertNull(m0.get("1"));
         assertEquals("Hello", m.get("1"));
-        assertEquals(2, s.store());
+        assertEquals(2, s.save());
         s.close();
 
         s = openStore(fileName);
@@ -504,7 +505,7 @@ public class TestMVStore extends TestBase {
         m.put("1",  "Hallo");
         s.incrementVersion();
         assertEquals(3, s.getCurrentVersion());
-        long v4 = s.store();
+        long v4 = s.save();
         assertEquals(4, v4);
         assertEquals(4, s.getCurrentVersion());
         s.close();
@@ -513,7 +514,7 @@ public class TestMVStore extends TestBase {
         s.setRetainChunk(0);
         m = s.openMap("data", String.class, String.class);
         m.put("1",  "Hello");
-        s.store();
+        s.save();
         s.close();
 
         s = openStore(fileName);
@@ -582,11 +583,11 @@ public class TestMVStore extends TestBase {
         MVMap<String, String> data = s.openMap("data", String.class, String.class);
         data.put("1", "Hello");
         data.put("2", "World");
-        s.store();
+        s.save();
         assertEquals("1/0///", m.get("map.data"));
         assertTrue(m.containsKey("chunk.1"));
         assertEquals("Hello", data.put("1", "Hallo"));
-        s.store();
+        s.save();
         assertEquals("1/0///", m.get("map.data"));
         assertTrue(m.get("root.1").length() > 0);
         assertTrue(m.containsKey("chunk.1"));
@@ -648,10 +649,10 @@ public class TestMVStore extends TestBase {
                 m.put(i, o);
                 i++;
                 if (i % 10000 == 0) {
-                    s.store();
+                    s.save();
                 }
             }
-            s.store();
+            s.save();
             s.close();
             // System.out.println(prof.getTop(5));
             // System.out.println("store time " + (System.currentTimeMillis() - t));
@@ -679,7 +680,7 @@ public class TestMVStore extends TestBase {
         // p = new Profiler();
         //p.startCollecting();
         // t = System.currentTimeMillis();
-        s.store();
+        s.save();
         // System.out.println("store: " + (System.currentTimeMillis() - t));
         // System.out.println(p.getTop(5));
         assertEquals("hello 0", m.remove(0));
@@ -687,7 +688,7 @@ public class TestMVStore extends TestBase {
         for (int i = 1; i < count; i++) {
             assertEquals("hello " + i, m.get(i));
         }
-        s.store();
+        s.save();
         s.close();
         s = openStore(fileName);
         m = s.openMap("data", Integer.class, String.class);
@@ -698,7 +699,7 @@ public class TestMVStore extends TestBase {
         for (int i = 1; i < count; i++) {
             m.remove(i);
         }
-        s.store();
+        s.save();
         assertNull(m.get(0));
         for (int i = 0; i < count; i++) {
             assertNull(m.get(i));
@@ -713,10 +714,10 @@ public class TestMVStore extends TestBase {
         for (int j = 0; j < 20; j++) {
             MVStore s = openStore(fileName);
             MVMap<Integer, String> m = s.openMap("data", Integer.class, String.class);
-            for (int i = 0; i < 10; i++) {
+            for (int i = 0; i < 100; i++) {
                 m.put(j + i, "Hello " + j);
             }
-            s.store();
+            s.save();
             s.compact(80);
             s.close();
             long len = FileUtils.size(fileName);
@@ -734,7 +735,7 @@ public class TestMVStore extends TestBase {
         for (int i = 0; i < 100; i++) {
             m.remove(i);
         }
-        s.store();
+        s.save();
         s.compact(80);
         s.close();
         // len = FileUtils.size(fileName);
@@ -757,12 +758,12 @@ public class TestMVStore extends TestBase {
             for (int i = 0; i < 10; i++) {
                 m.put(i, "Hello");
             }
-            s.store();
+            s.save();
             for (int i = 0; i < 10; i++) {
                 assertEquals("Hello", m.get(i));
                 assertEquals("Hello", m.remove(i));
             }
-            s.store();
+            s.save();
             s.close();
             long len = FileUtils.size(fileName);
             if (initialLength == 0) {
@@ -865,7 +866,7 @@ public class TestMVStore extends TestBase {
         si.put("Test", 10);
         MVMap<String, String> ss = s.openMap("stringString", String.class, String.class);
         ss.put("Hello", "World");
-        s.store();
+        s.save();
         s.close();
         s = openStore(fileName);
         is = s.openMap("intString", Integer.class, String.class);
@@ -889,7 +890,7 @@ public class TestMVStore extends TestBase {
         for (int i = 0; i < 10; i++) {
             m.put(i, "hello " + i);
         }
-        s.store();
+        s.save();
         it = m.keyIterator(null);
         it.next();
         assertThrows(UnsupportedOperationException.class, it).remove();
@@ -920,7 +921,7 @@ public class TestMVStore extends TestBase {
         for (int i = 0; i < 3; i++) {
             m.put(i, "hello " + i);
         }
-        s.store();
+        s.save();
         // closing twice should be fine
         s.close();
         s.close();
@@ -934,14 +935,14 @@ public class TestMVStore extends TestBase {
         for (int i = 0; i < 3; i++) {
             m.put(i, "hello " + i);
         }
-        s.store();
+        s.save();
         assertEquals("hello 0", m.remove(0));
 
         assertNull(m.get(0));
         for (int i = 1; i < 3; i++) {
             assertEquals("hello " + i, m.get(i));
         }
-        s.store();
+        s.save();
         s.close();
 
         s = openStore(fileName);
@@ -961,7 +962,7 @@ public class TestMVStore extends TestBase {
      */
     protected static MVStore openStore(String fileName) {
         MVStore store = MVStore.open(fileName, new TestMapFactory());
-        store.setMaxPageSize(10);
+        store.setMaxPageSize(1000);
         return store;
     }
 

h2/src/tools/org/h2/dev/store/btree/Chunk.java

@@ -19,6 +19,7 @@ import java.util.HashMap;
  * 1 byte: 'c'
  * 4 bytes: length
  * 4 bytes: chunk id (an incrementing number)
+ * 4 bytes: pageCount
  * 8 bytes: metaRootPos
  * 8 bytes: maxLengthLive
  * [ Page ] *
@@ -40,6 +41,11 @@ public class Chunk {
      */
     int length;
 
+    /**
+     * The number of pages.
+     */
+    int pageCount;
+
     /**
      * The sum of the max length of all pages.
      */
@@ -75,10 +81,12 @@ public class Chunk {
         }
         int length = buff.getInt();
         int chunkId = buff.getInt();
+        int pageCount = buff.getInt();
         long metaRootPos = buff.getLong();
         long maxLengthLive = buff.getLong();
         Chunk c = new Chunk(chunkId);
         c.length = length;
+        c.pageCount = pageCount;
         c.start = start;
         c.metaRootPos = metaRootPos;
         c.maxLengthLive = maxLengthLive;
@@ -89,6 +97,7 @@ public class Chunk {
         buff.put((byte) 'c');
         buff.putInt(length);
         buff.putInt(id);
+        buff.putInt(pageCount);
         buff.putLong(metaRootPos);
         buff.putLong(maxLengthLive);
     }
@@ -105,6 +114,7 @@ public class Chunk {
         Chunk c = new Chunk(id);
         c.start = Long.parseLong(map.get("start"));
         c.length = Integer.parseInt(map.get("length"));
+        c.pageCount = Integer.parseInt(map.get("pageCount"));
         c.maxLength = Long.parseLong(map.get("maxLength"));
         c.maxLengthLive = Long.parseLong(map.get("maxLengthLive"));
         c.metaRootPos = Long.parseLong(map.get("metaRoot"));
@@ -129,6 +139,7 @@ public class Chunk {
                 "id:" + id + "," +
                 "start:" + start + "," +
                 "length:" + length + "," +
+                "pageCount:" + pageCount + "," +
                 "maxLength:" + maxLength + "," +
                 "maxLengthLive:" + maxLengthLive + "," +
                 "metaRoot:" + metaRootPos + "," +

h2/src/tools/org/h2/dev/store/btree/DataUtils.java

@@ -56,6 +56,16 @@ public class DataUtils {
      */
     public static final long COMPRESSED_VAR_LONG_MAX = 0x1ffffffffffffL;
 
+    /**
+     * The estimated number of bytes used per page object.
+     */
+    public static final int PAGE_MEMORY = 128;
+
+    /**
+     * The estimated number of bytes used per child entry.
+     */
+    public static final int PAGE_MEMORY_CHILD = 16;
+
     /**
      * Get the length of the variable size int.
      *

h2/src/tools/org/h2/dev/store/btree/Dump.java

@@ -79,11 +79,13 @@ public class Dump {
                 }
                 int chunkLength = block.getInt();
                 int chunkId = block.getInt();
+                int pageCount = block.getInt();
                 long metaRootPos = block.getLong();
                 long maxLengthLive = block.getLong();
                 writer.println("    chunk " + chunkId +
                         " at " + pos +
                         " length " + chunkLength +
+                        " pageCount " + pageCount +
                         " root " + metaRootPos +
                         " maxLengthLive " + maxLengthLive);
                 ByteBuffer chunk = ByteBuffer.allocate(chunkLength);

h2/src/tools/org/h2/dev/store/btree/MVMap.java

@@ -55,6 +55,7 @@ public class MVMap<K, V> extends AbstractMap<K, V>
         this.valueType = valueType;
         this.createVersion = createVersion;
         this.root = Page.createEmpty(this,  createVersion - 1);
+        store.registerUnsavedPage();
     }
 
     /**
@@ -69,7 +70,7 @@ public class MVMap<K, V> extends AbstractMap<K, V>
         checkWrite();
         long writeVersion = store.getCurrentVersion();
         Page p = root.copyOnWrite(writeVersion);
-        if (p.getKeyCount() > store.getMaxPageSize()) {
+        if (p.getMemory() > store.getMaxPageSize() && p.getKeyCount() > 1) {
             int at = p.getKeyCount() / 2;
             long totalCount = p.getTotalCount();
             Object k = p.getKey(at);
@@ -79,7 +80,8 @@ public class MVMap<K, V> extends AbstractMap<K, V>
             Page[] childrenPages = { p, split };
             long[] counts = { p.getTotalCount(), split.getTotalCount() };
             p = Page.create(this, writeVersion, 1,
-                    keys, null, children, childrenPages, counts, totalCount, 0);
+                    keys, null, children, childrenPages, counts, totalCount, 0, 0);
+            store.registerUnsavedPage();
             // now p is a node; insert continues
         }
         Object result = put(p, writeVersion, key, value);
@@ -114,7 +116,7 @@ public class MVMap<K, V> extends AbstractMap<K, V>
             index++;
         }
         Page c = p.getChildPage(index).copyOnWrite(writeVersion);
-        if (c.getKeyCount() >= store.getMaxPageSize()) {
+        if (c.getMemory() > store.getMaxPageSize() && c.getKeyCount() > 1) {
             // split on the way down
             int at = c.getKeyCount() / 2;
             Object k = c.getKey(at);
@@ -575,23 +577,6 @@ public class MVMap<K, V> extends AbstractMap<K, V>
         }
         Page cOld = p.getChildPage(index);
         Page c = cOld.copyOnWrite(writeVersion);
-
-        int todoMerge;
-//        if (c.getKeyCount() < store.getMaxPageSize() / 2) {
-//            if (p.getChildPageCount() == 1) {
-//                int todo;
-//                // replace this node with the child
-//            } else if (index > 0) {
-//                int indexSibling = index - 1;
-//                Page sOld = p.getChildPage(indexSibling);
-//                merge(cOld, sOld);
-//                p.remove(indexSibling);
-//            } else {
-//                int indexSibling = index + 1;
-//                Page sOld = p.getChildPage(indexSibling);
-//            }
-//        }
-
         long oldCount = c.getTotalCount();
         result = remove(c, writeVersion, key);
         if (oldCount == c.getTotalCount()) {
@@ -611,29 +596,6 @@ public class MVMap<K, V> extends AbstractMap<K, V>
         return result;
     }
 
-    int todoMerge;
-//    private boolean merge(Page a, Page b, boolean left) {
-//        boolean leaf = a.isLeaf();
-//        if (leaf != b.isLeaf()) {
-//            return false;
-//        }
-//        if (left) {
-//            int moved = 0;
-//                while (a.getKeyCount() < b.getKeyCount() - 1) {
-//                    if (leaf) {
-//                        Object k = b.getKey(0);
-//                        Object v = b.getValue(0);
-//                        b.remove(0);
-//                        a.insertLeaf(a.getKeyCount(), k, v);
-//                    } else {
-//
-//                    }
-//                    moved++;
-//                }
-//            }
-//        }
-//    }
-
     protected void setRoot(Page newRoot) {
         if (root != newRoot) {
             removeUnusedOldVersions();

h2/src/tools/org/h2/dev/store/btree/MVStore.java

@@ -35,13 +35,12 @@ header:
 H:3,blockSize=4096,...
 
 TODO:
-- merge pages if small
+- support custom fields in the file header (auto-server ip address,...)
 - support stores that span multiple files (chunks stored in other files)
 - triggers
 - r-tree: add missing features (NN search for example)
 - pluggable cache (specially for in-memory file systems)
 - maybe store the factory class in the file header
-- support custom fields in the header (auto-server ip address,...)
 - auto-server: store port in the header
 - recovery: keep some old chunks; don't overwritten for 1 minute
 - pluggable caching (specially for in-memory file systems)
@@ -57,11 +56,12 @@ TODO:
 - support background writes (concurrent modification & store)
 - limited support for writing to old versions (branches)
 - support concurrent operations (including file I/O)
-- maxPageSize should be size in bytes (not it is actually maxPageEntryCount)
 - on insert, if the child page is already full, don't load and modify it - split directly
 - performance test with encrypting file system
 - possibly split chunk data into immutable and mutable
 - compact: avoid processing pages using a counting bloom filter
+- defragment (re-creating maps, specially those with small pages)
+- write using ByteArrayOutputStream; remove DataType.getMaxLength
 
 */
 
@@ -85,7 +85,7 @@ public class MVStore {
 
     private final int readCacheSize = 2 * 1024 * 1024;
 
-    private int maxPageSize = 30;
+    private int maxPageSize = 4 * 1024;
 
     private FileChannel file;
     private long fileSize;
@@ -123,6 +123,7 @@ public class MVStore {
     private long currentVersion;
     private int readCount;
     private int writeCount;
+    private int unsavedPageCount;
 
     private MVStore(String fileName, MapFactory mapFactory) {
         this.fileName = fileName;
@@ -502,7 +503,7 @@ public class MVStore {
      *
      * @return the new version (incremented if there were changes)
      */
-    public long store() {
+    public long save() {
         if (!hasUnsavedChanges()) {
             return currentVersion;
         }
@@ -560,7 +561,7 @@ public class MVStore {
             }
             Page p = m.getRoot();
             if (p.getTotalCount() > 0) {
-                long root = p.writeTempRecursive(c, buff);
+                long root = p.writeUnsavedRecursive(c, buff);
                 meta.put("root." + m.getId(), "" + root);
             }
         }
@@ -569,7 +570,7 @@ public class MVStore {
 
         // this will modify maxLengthLive, but
         // the correct value is written in the chunk header
-        meta.getRoot().writeTempRecursive(c, buff);
+        meta.getRoot().writeUnsavedRecursive(c, buff);
 
         buff.flip();
         int length = buff.limit();
@@ -604,6 +605,7 @@ public class MVStore {
         // write the new version (after the commit)
         writeHeader();
         shrinkFileIfPossible(1);
+        unsavedPageCount = 0;
         return version;
     }
 
@@ -799,7 +801,7 @@ public class MVStore {
             copyLive(c, old);
         }
 
-        store();
+        save();
         return true;
     }
 
@@ -885,24 +887,26 @@ public class MVStore {
     void removePage(long pos) {
         // we need to keep temporary pages,
         // to support reading old versions and rollback
-        if (pos > 0) {
-            // this could result in a cache miss
-            // if the operation is rolled back,
-            // but we don't optimize for rollback
-            cache.remove(pos);
-            Chunk c = getChunk(pos);
-            HashMap<Integer, Chunk>freed = freedChunks.get(currentVersion);
-            if (freed == null) {
-                freed = New.hashMap();
-                freedChunks.put(currentVersion, freed);
-            }
-            Chunk f = freed.get(c.id);
-            if (f == null) {
-                f = new Chunk(c.id);
-                freed.put(c.id, f);
-            }
-            f.maxLengthLive -= DataUtils.getPageMaxLength(pos);
+        if (pos == 0) {
+            unsavedPageCount--;
+            return;
+        }
+        // this could result in a cache miss
+        // if the operation is rolled back,
+        // but we don't optimize for rollback
+        cache.remove(pos);
+        Chunk c = getChunk(pos);
+        HashMap<Integer, Chunk>freed = freedChunks.get(currentVersion);
+        if (freed == null) {
+            freed = New.hashMap();
+            freedChunks.put(currentVersion, freed);
+        }
+        Chunk f = freed.get(c.id);
+        if (f == null) {
+            f = new Chunk(c.id);
+            freed.put(c.id, f);
         }
+        f.maxLengthLive -= DataUtils.getPageMaxLength(pos);
     }
 
     /**
@@ -915,14 +919,22 @@ public class MVStore {
         // System.out.println(string);
     }
 
+    /**
+     * Set the maximum amount of memory a page should contain, in bytes. Larger
+     * pages are split. The default is 4 KB. This is not a limit in the page
+     * size, as pages with one entry can be larger. As a rule of thumb, pages
+     * should not be larger than 1 MB, for caching to work efficiently.
+     *
+     * @param maxPageSize the page size
+     */
     public void setMaxPageSize(int maxPageSize) {
         this.maxPageSize = maxPageSize;
     }
 
     /**
-     * The maximum number of key-value pairs in a page.
+     * Get the maximum page size, in bytes.
      *
-     * @return the maximum number of entries
+     * @return the maximum page size
      */
     public int getMaxPageSize() {
         return maxPageSize;
@@ -1011,6 +1023,21 @@ public class MVStore {
         return true;
     }
 
+    /**
+     * Get the estimated number of unsaved pages. The returned value is not
+     * accurate, specially after rollbacks, but can be used to estimate the
+     * memory usage for unsaved data.
+     *
+     * @return the number of unsaved pages
+     */
+    public int getUnsavedPageCount() {
+        return unsavedPageCount;
+    }
+
+    void registerUnsavedPage() {
+        unsavedPageCount++;
+    }
+
     public int getStoreVersion() {
         String x = getSetting("storeVersion");
         return x == null ? 0 : Integer.parseInt(x);

h2/src/tools/org/h2/dev/store/btree/Page.java

@@ -51,6 +51,11 @@ public class Page {
      */
     private int sharedFlags;
 
+    /**
+     * The estimated memory used.
+     */
+    private int memory;
+
     private Object[] keys;
     private Object[] values;
     private long[] children;
@@ -72,7 +77,7 @@ public class Page {
     public static Page createEmpty(MVMap<?, ?> map, long version) {
         return create(map, version, 0,
                 EMPTY_OBJECT_ARRAY, EMPTY_OBJECT_ARRAY,
-                null, null, null, 0, 0);
+                null, null, null, 0, 0, DataUtils.PAGE_MEMORY);
     }
 
     /**
@@ -93,7 +98,7 @@ public class Page {
     public static Page create(MVMap<?, ?> map, long version,
             int keyCount, Object[] keys,
             Object[] values, long[] children, Page[] childrenPages, long[] counts,
-            long totalCount, int sharedFlags) {
+            long totalCount, int sharedFlags, int memory) {
         Page p = new Page(map, version);
         // the position is 0
         p.keys = keys;
@@ -104,6 +109,7 @@ public class Page {
         p.counts = counts;
         p.totalCount = totalCount;
         p.sharedFlags = sharedFlags;
+        p.memory = memory == 0 ? p.calculateMemory() : memory;
         return p;
     }
 
@@ -246,7 +252,9 @@ public class Page {
         Page newPage = create(map, writeVersion,
                 keyCount, keys, values, children, childrenPages,
                 counts, totalCount,
-                SHARED_KEYS | SHARED_VALUES | SHARED_CHILDREN | SHARED_COUNTS);
+                SHARED_KEYS | SHARED_VALUES | SHARED_CHILDREN | SHARED_COUNTS,
+                memory);
+        map.getStore().registerUnsavedPage();
         newPage.cachedCompare = cachedCompare;
         return newPage;
     }
@@ -328,7 +336,10 @@ public class Page {
         totalCount = a;
         Page newPage = create(map, version, b,
                 bKeys, bValues, null, null, null,
-                bKeys.length, 0);
+                bKeys.length, 0, 0);
+        map.getStore().registerUnsavedPage();
+        memory = calculateMemory();
+        newPage.memory = newPage.calculateMemory();
         return newPage;
     }
 
@@ -372,7 +383,10 @@ public class Page {
         }
         Page newPage = create(map, version, b - 1,
                 bKeys, null, bChildren, bChildrenPages,
-                bCounts, t, 0);
+                bCounts, t, 0, 0);
+        map.getStore().registerUnsavedPage();
+        memory = calculateMemory();
+        newPage.memory = newPage.calculateMemory();
         return newPage;
     }
 
@@ -441,6 +455,11 @@ public class Page {
             keys = Arrays.copyOf(keys, keys.length);
             sharedFlags &= ~SHARED_KEYS;
         }
+        Object old = keys[index];
+        if (old != null) {
+            memory -= map.getKeyType().getMemory(old);
+        }
+        memory += map.getKeyType().getMemory(key);
         keys[index] = key;
     }
 
@@ -457,6 +476,8 @@ public class Page {
             values = Arrays.copyOf(values, values.length);
             sharedFlags &= ~SHARED_VALUES;
         }
+        memory -= map.getValueType().getMemory(old);
+        memory += map.getValueType().getMemory(value);
         values[index] = value;
         return old;
     }
@@ -511,6 +532,8 @@ public class Page {
         keyCount++;
         sharedFlags &= ~(SHARED_KEYS | SHARED_VALUES);
         totalCount++;
+        memory += map.getKeyType().getMemory(key);
+        memory += map.getValueType().getMemory(value);
     }
 
     /**
@@ -545,6 +568,8 @@ public class Page {
 
         sharedFlags &= ~(SHARED_KEYS | SHARED_CHILDREN | SHARED_COUNTS);
         totalCount += childPage.getTotalCount();
+        memory += map.getKeyType().getMemory(key);
+        memory += DataUtils.PAGE_MEMORY_CHILD;
     }
 
     /**
@@ -554,6 +579,8 @@ public class Page {
      */
     public void remove(int index) {
         int keyIndex = index >= keyCount ? index - 1 : index;
+        Object old = keys[keyIndex];
+        memory -= map.getKeyType().getMemory(old);
         if ((sharedFlags & SHARED_KEYS) == 0 && keys.length > keyCount - 4) {
             if (keyIndex < keyCount - 1) {
                 System.arraycopy(keys, keyIndex + 1, keys, keyIndex, keyCount - keyIndex - 1);
@@ -567,6 +594,8 @@ public class Page {
         }
 
         if (values != null) {
+            old = values[index];
+            memory -= map.getValueType().getMemory(old);
             if ((sharedFlags & SHARED_VALUES) == 0 && values.length > keyCount - 4) {
                 if (index < keyCount - 1) {
                     System.arraycopy(values, index + 1, values, index, keyCount - index - 1);
@@ -582,6 +611,7 @@ public class Page {
         }
         keyCount--;
         if (children != null) {
+            memory -= DataUtils.PAGE_MEMORY_CHILD;
             long countOffset = counts[index];
 
             long[] newChildren = new long[children.length - 1];
@@ -638,8 +668,10 @@ public class Page {
             compressor.expand(comp, 0, compLen, exp, 0, exp.length);
             buff = ByteBuffer.wrap(exp);
         }
+        DataType keyType = map.getKeyType();
         for (int i = 0; i < len; i++) {
-            keys[i] = map.getKeyType().read(buff);
+            Object k = keyType.read(buff);
+            keys[i] = k;
         }
         if (node) {
             children = new long[len + 1];
@@ -657,11 +689,14 @@ public class Page {
             totalCount = total;
         } else {
             values = new Object[len];
+            DataType valueType = map.getValueType();
             for (int i = 0; i < len; i++) {
-                values[i] = map.getValueType().read(buff);
+                Object v = valueType.read(buff);
+                values[i] = v;
             }
             totalCount = len;
         }
+        memory = calculateMemory();
     }
 
     /**
@@ -722,6 +757,7 @@ public class Page {
         long max = DataUtils.getPageMaxLength(pos);
         chunk.maxLength += max;
         chunk.maxLengthLive += max;
+        chunk.pageCount++;
     }
 
     /**
@@ -769,13 +805,13 @@ public class Page {
      * @param buff the target buffer
      * @return the page id
      */
-    long writeTempRecursive(Chunk chunk, ByteBuffer buff) {
+    long writeUnsavedRecursive(Chunk chunk, ByteBuffer buff) {
         if (!isLeaf()) {
             int len = children.length;
             for (int i = 0; i < len; i++) {
                 Page p = childrenPages[i];
                 if (p != null) {
-                    children[i] = p.writeTempRecursive(chunk, buff);
+                    children[i] = p.writeUnsavedRecursive(chunk, buff);
                     childrenPages[i] = null;
                 }
             }
@@ -809,4 +845,28 @@ public class Page {
         return pos != 0 ? (int) (pos | (pos >>> 32)) : super.hashCode();
     }
 
+    public int getMemory() {
+        if (MVStore.ASSERT) {
+            if (memory != calculateMemory()) {
+                throw new RuntimeException("Memory calculation error");
+            }
+        }
+        return memory;
+    }
+
+    private int calculateMemory() {
+        int mem = DataUtils.PAGE_MEMORY;
+        for (int i = 0; i < keyCount; i++) {
+            mem += map.getKeyType().getMemory(keys[i]);
+        }
+        if (this.isLeaf()) {
+            for (int i = 0; i < keyCount; i++) {
+                mem += map.getValueType().getMemory(values[i]);
+            }
+        } else {
+            mem += this.getChildPageCount() * DataUtils.PAGE_MEMORY_CHILD;
+        }
+        return mem;
+    }
+
 }