Added flushing OS cache to medium

LiteDB/Engine/Disks/FileDiskService.cs

@@ -208,6 +208,10 @@ public void WriteJournal(ICollection<byte[]> pages)
                 _journal.Write(buffer, 0, BasePage.PAGE_SIZE);
             }
 
+#if !NET35
+            _journal.Flush(true);
+#endif
+
             // journal file will be unlocked only in ClearJournal
         }
 
@@ -267,6 +271,16 @@ public IEnumerable<byte[]> ReadJournal()
             _journal.TryUnlock(0, 1);
         }
 
+        /// <summary>
+        /// Ensures all pages from the OS cache are persisted on medium
+        /// </summary>
+        public void Flush()
+        {
+#if !NET35
+            _stream.Flush(true);
+#endif
+        }
+
         /// <summary>
         /// Clear journal file (set size to 0 length)
         /// </summary>

LiteDB/Engine/Disks/IDiskService.cs

@@ -51,6 +51,11 @@ public interface IDiskService : IDisposable
         /// </summary>
         void ClearJournal();
 
+        /// <summary>
+        /// Ensures all pages from the OS cache are persisted on medium
+        /// </summary>
+        void Flush();
+
         /// <summary>
         /// Lock datafile returning lock position
         /// </summary>

LiteDB/Engine/Disks/StreamDiskService.cs

@@ -149,6 +149,13 @@ public void Unlock(LockState state)
         {
         }
 
+        /// <summary>
+        /// No flush implemented
+        /// </summary>
+        public void Flush()
+        {
+        }
+
         #endregion
 
     }

LiteDB/Engine/Services/TransactionService.cs

@@ -80,6 +80,9 @@ public void Commit()
                 // mark all dirty pages in clean pages (all are persisted in disk and are valid pages)
                 _cache.MarkDirtyAsClean();
 
+                // ensure all pages from OS cache has been persisted on medium
+                _disk.Flush();
+
                 // discard journal file
                 _disk.ClearJournal();
             }