avoid copying data from ByteBuffer to another ByteBuffer if possible, specially…

avoid copying data from ByteBuffer to another ByteBuffer if possible,  specially for the OffHeapStore

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

@@ -46,9 +46,11 @@ public class FileStore {
         return fileName;
     }
     
-    public void readFully(long pos, ByteBuffer dst) {
+    public ByteBuffer readFully(long pos, int len) {
         readCount++;
+        ByteBuffer dst = ByteBuffer.allocate(len);
         DataUtils.readFully(file, pos, dst);
+        return dst;
     }
 
     public void writeFully(long pos, ByteBuffer src) {

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

@@ -112,8 +112,6 @@ MVStore:
 - simple rollback method (rollback to last committed version)
 - MVMap to implement SortedMap, then NavigableMap
 - Test with OSGi
-- avoid copying data from ByteBuffer to another ByteBuffer if possible,
-    specially for the OffHeapStore
 - storage that splits database into multiple files, 
     to speed up compact and allow using trim 
     (by truncating / deleting empty files)
@@ -555,12 +553,11 @@ public class MVStore {
         // we don't know which chunk is the newest
         long newestChunk = -1;
         // read the last block of the file, and then the two first blocks
+        ByteBuffer buffLastBlock = fileStore.readFully(fileStore.size() - BLOCK_SIZE, BLOCK_SIZE);
+        ByteBuffer buffFirst2Blocks = fileStore.readFully(0, BLOCK_SIZE * 2);
         ByteBuffer buff = ByteBuffer.allocate(3 * BLOCK_SIZE);
-        buff.limit(BLOCK_SIZE);
-        fileStore.readFully(fileStore.size() - BLOCK_SIZE, buff);
-        buff.limit(3 * BLOCK_SIZE);
-        buff.position(BLOCK_SIZE);
-        fileStore.readFully(0, buff);
+        buff.put(buffLastBlock);
+        buff.put(buffFirst2Blocks);
         for (int i = 0; i < 3 * BLOCK_SIZE; i += BLOCK_SIZE) {
             String s = new String(buff.array(), i, BLOCK_SIZE, DataUtils.UTF8)
                     .trim();
@@ -1112,9 +1109,7 @@ public class MVStore {
     }
 
     private Chunk readChunkHeader(long start) {
-        ByteBuffer buff = ByteBuffer.allocate(40);
-        fileStore.readFully(start, buff);
-        buff.rewind();
+        ByteBuffer buff = fileStore.readFully(start, 40);
         return Chunk.fromHeader(buff, start);
     }
 
@@ -1164,7 +1159,8 @@ public class MVStore {
             int length = MathUtils.roundUpInt(c.length, BLOCK_SIZE) + BLOCK_SIZE;
             buff = DataUtils.ensureCapacity(buff, length);
             buff.limit(length);
-            fileStore.readFully(c.start, buff);
+            ByteBuffer buff2 = fileStore.readFully(c.start, length);
+            buff.put(buff2);
             long end = getEndPosition();
             fileStore.markUsed(end, length);
             fileStore.free(c.start, length);
@@ -1196,7 +1192,8 @@ public class MVStore {
             int length = MathUtils.roundUpInt(c.length, BLOCK_SIZE) + BLOCK_SIZE;
             buff = DataUtils.ensureCapacity(buff, length);
             buff.limit(length);
-            fileStore.readFully(c.start, buff);
+            ByteBuffer buff2 = fileStore.readFully(c.start, length);
+            buff.put(buff2);
             long pos = fileStore.allocate(length);
             fileStore.free(c.start, length);
             buff.position(0);
@@ -1325,8 +1322,7 @@ public class MVStore {
     }
 
     private void copyLive(Chunk chunk, ArrayList<Chunk> old) {
-        ByteBuffer buff = ByteBuffer.allocate(chunk.length);
-        fileStore.readFully(chunk.start, buff);
+        ByteBuffer buff = fileStore.readFully(chunk.start, chunk.length);
         Chunk.fromHeader(buff, chunk.start);
         int chunkLength = chunk.length;
         markMetaChanged();

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

@@ -30,20 +30,22 @@ public class OffHeapStore extends FileStore {
     }
     
     @Override
-    public void readFully(long pos, ByteBuffer dst) {
-        Entry<Long, ByteBuffer> mem = memory.floorEntry(pos);
-        if (mem == null) {
+    public ByteBuffer readFully(long pos, int len) {
+        Entry<Long, ByteBuffer> memEntry = memory.floorEntry(pos);
+        if (memEntry == null) {
             throw DataUtils.newIllegalStateException(DataUtils.ERROR_READING_FAILED, 
                     "Could not read from position {0}", pos);
         }
         readCount++;
-        ByteBuffer buff = mem.getValue();
-        ByteBuffer read = buff.duplicate();
-        int offset = (int) (pos - mem.getKey());
-        read.position(offset);
-        read.limit(dst.remaining() + offset);
-        dst.put(read);
-        dst.rewind();
+        ByteBuffer buff = memEntry.getValue();
+        int oldLimit = buff.limit();
+        int offset = (int) (pos - memEntry.getKey());
+        buff.position(offset);
+        buff.limit(len + offset);
+        ByteBuffer read = buff.slice();
+        buff.position(0);
+        buff.limit(oldLimit);
+        return read;
     }
     
     @Override

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

@@ -169,13 +169,11 @@ public class Page {
         maxLength = (int) Math.min(fileSize - filePos, maxLength);
         int length = maxLength;
         if (maxLength == Integer.MAX_VALUE) {
-            buff = ByteBuffer.allocate(128);
-            fileStore.readFully(filePos, buff);
+            buff = fileStore.readFully(filePos, 128);
             maxLength = buff.getInt();
             // read the first bytes again
         }
-        buff = ByteBuffer.allocate(length);
-        fileStore.readFully(filePos, buff);
+        buff = fileStore.readFully(filePos, length);
         Page p = new Page(map, 0);
         p.pos = pos;
         int chunkId = DataUtils.getPageChunkId(pos);

h2/src/main/org/h2/mvstore/cache/FilePathCache.java

@@ -87,8 +87,7 @@ public class FilePathCache extends FilePathWrapper {
                 }
             }
             len = Math.min(len, dst.remaining());
-            System.arraycopy(buff.array(), off, dst.array(), dst.position(), len);
-            dst.position(dst.position() + len);
+            dst.put(buff.array(), off, len);
             return len;
         }