Index: src/os_unix.c
==================================================================
--- src/os_unix.c
+++ src/os_unix.c
@@ -3716,20 +3716,20 @@
       ** on systems that do not have a real fallocate() system call.
       */
       int nBlk = buf.st_blksize;  /* File-system block size */
       i64 iWrite;                 /* Next offset to write to */
 
-      if( robust_ftruncate(pFile->h, nSize) ){
-        pFile->lastErrno = errno;
-        return unixLogError(SQLITE_IOERR_TRUNCATE, "ftruncate", pFile->zPath);
-      }
       iWrite = ((buf.st_size + 2*nBlk - 1)/nBlk)*nBlk-1;
       while( iWrite<nSize ){
         int nWrite = seekAndWrite(pFile, iWrite, "", 1);
         if( nWrite!=1 ) return SQLITE_IOERR_WRITE;
         iWrite += nBlk;
       }
+      if( robust_ftruncate(pFile->h, nSize) ){
+        pFile->lastErrno = errno;
+        return unixLogError(SQLITE_IOERR_TRUNCATE, "ftruncate", pFile->zPath);
+      }
 #endif
     }
   }
 
 #if SQLITE_MAX_MMAP_SIZE>0

Index: src/vdbesort.c
==================================================================
--- src/vdbesort.c
+++ src/vdbesort.c
@@ -1130,16 +1130,16 @@
 ** Whether or not the file does end up memory mapped of course depends on
 ** the specific VFS implementation.
 */
 static void vdbeSorterExtendFile(sqlite3 *db, sqlite3_file *pFd, i64 nByte){
   if( nByte<=(i64)(db->nMaxSorterMmap) && pFd->pMethods->iVersion>=3 ){
-    int rc = sqlite3OsTruncate(pFd, nByte);
-    if( rc==SQLITE_OK ){
-      void *p = 0;
-      sqlite3OsFetch(pFd, 0, (int)nByte, &p);
-      sqlite3OsUnfetch(pFd, 0, p);
-    }
+    void *p = 0;
+    int chunksize = 4*1024;
+    sqlite3OsFileControlHint(pFd, SQLITE_FCNTL_CHUNK_SIZE, &chunksize);
+    sqlite3OsFileControlHint(pFd, SQLITE_FCNTL_SIZE_HINT, &nByte);
+    sqlite3OsFetch(pFd, 0, (int)nByte, &p);
+    sqlite3OsUnfetch(pFd, 0, p);
   }
 }
 #else
 # define vdbeSorterExtendFile(x,y,z)
 #endif