diff --git a/src/main.cpp b/src/main.cpp
index 3151a806d..9edd9a299 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1533,7 +1533,7 @@ bool CBlock::DisconnectBlock(CValidationState &state, CBlockIndex *pindex, CCoin
     }
 }
 
-void static FlushBlockFile()
+void static FlushBlockFile(bool fFinalize = false)
 {
     LOCK(cs_LastBlockFile);
 
@@ -1541,12 +1541,16 @@ void static FlushBlockFile()
 
     FILE *fileOld = OpenBlockFile(posOld);
     if (fileOld) {
+        if (fFinalize)
+            TruncateFile(fileOld, infoLastBlockFile.nSize);
         FileCommit(fileOld);
         fclose(fileOld);
     }
 
     fileOld = OpenUndoFile(posOld);
     if (fileOld) {
+        if (fFinalize)
+            TruncateFile(fileOld, infoLastBlockFile.nUndoSize);
         FileCommit(fileOld);
         fclose(fileOld);
     }
@@ -1965,7 +1969,7 @@ bool FindBlockPos(CValidationState &state, CDiskBlockPos &pos, unsigned int nAdd
     } else {
         while (infoLastBlockFile.nSize + nAddSize >= MAX_BLOCKFILE_SIZE) {
             printf("Leaving block file %i: %s\n", nLastBlockFile, infoLastBlockFile.ToString().c_str());
-            FlushBlockFile();
+            FlushBlockFile(true);
             nLastBlockFile++;
             infoLastBlockFile.SetNull();
             pblocktree->ReadBlockFileInfo(nLastBlockFile, infoLastBlockFile); // check whether data for the new file somehow already exist; can fail just fine
diff --git a/src/util.cpp b/src/util.cpp
index 4eff6ce71..fc3e846a6 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -3,6 +3,15 @@
 // Distributed under the MIT/X11 software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#ifndef WIN32
+// for posix_fallocate
+#ifdef __linux__
+#define _POSIX_C_SOURCE 200112L
+#endif
+#include <fcntl.h>
+#include <sys/stat.h>
+#endif
+
 #include "util.h"
 #include "sync.h"
 #include "version.h"
@@ -1152,9 +1161,46 @@ int GetFilesize(FILE* file)
     return nFilesize;
 }
 
+bool TruncateFile(FILE *file, unsigned int length) {
+#if defined(WIN32)
+    return _chsize(_fileno(file), length) == 0;
+#else
+    return ftruncate(fileno(file), length) == 0;
+#endif
+}
+
 // this function tries to make a particular range of a file allocated (corresponding to disk space)
 // it is advisory, and the range specified in the arguments will never contain live data
 void AllocateFileRange(FILE *file, unsigned int offset, unsigned int length) {
+#if defined(WIN32)
+    // Windows-specific version
+    HANDLE hFile = (HANDLE)_get_osfhandle(_fileno(file));
+    LARGE_INTEGER nFileSize;
+    int64 nEndPos = (int64)offset + length;
+    nFileSize.u.LowPart = nEndPos & 0xFFFFFFFF;
+    nFileSize.u.HighPart = nEndPos >> 32;
+    SetFilePointerEx(hFile, nFileSize, 0, FILE_BEGIN);
+    SetEndOfFile(hFile);
+#elif defined(MAC_OSX)
+    // OSX specific version
+    fstore_t fst;
+    fst.fst_flags = F_ALLOCATECONTIG;
+    fst.fst_posmode = F_PEOFPOSMODE;
+    fst.fst_offset = 0;
+    fst.fst_length = (off_t)offset + length;
+    fst.fst_bytesalloc = 0;
+    if (fcntl(fileno(file), F_PREALLOCATE, &fst) == -1) {
+        fst.fst_flags = F_ALLOCATEALL;
+        fcntl(fileno(file), F_PREALLOCATE, &fst);
+    }
+    ftruncate(fileno(file), fst.fst_length);
+#elif defined(__linux__)
+    // Version using posix_fallocate
+    off_t nEndPos = (off_t)offset + length;
+    posix_fallocate(fileno(file), 0, nEndPos);
+#else
+    // Fallback version
+    // TODO: just write one byte per block
     static const char buf[65536] = {};
     fseek(file, offset, SEEK_SET);
     while (length > 0) {
@@ -1164,6 +1210,7 @@ void AllocateFileRange(FILE *file, unsigned int offset, unsigned int length) {
         fwrite(buf, 1, now, file); // allowed to fail; this function is advisory anyway
         length -= now;
     }
+#endif
 }
 
 void ShrinkDebugFile()
diff --git a/src/util.h b/src/util.h
index a6b88206e..d129d1369 100644
--- a/src/util.h
+++ b/src/util.h
@@ -193,6 +193,7 @@ bool WildcardMatch(const char* psz, const char* mask);
 bool WildcardMatch(const std::string& str, const std::string& mask);
 void FileCommit(FILE *fileout);
 int GetFilesize(FILE* file);
+bool TruncateFile(FILE *file, unsigned int length);
 void AllocateFileRange(FILE *file, unsigned int offset, unsigned int length);
 bool RenameOver(boost::filesystem::path src, boost::filesystem::path dest);
 boost::filesystem::path GetDefaultDataDir();