Enable configuration of NuCache BTree block size

src/Umbraco.Web/PublishedCache/NuCache/DataSource/BTree.cs

@@ -1,4 +1,5 @@
-using CSharpTest.Net.Collections;
+using System.Configuration;
+using CSharpTest.Net.Collections;
 using CSharpTest.Net.Serialization;
 
 namespace Umbraco.Web.PublishedCache.NuCache.DataSource
@@ -14,6 +15,12 @@ namespace Umbraco.Web.PublishedCache.NuCache.DataSource
                 CreateFile = exists ? CreatePolicy.IfNeeded : CreatePolicy.Always,
                 FileName = filepath,
 
+                // read or write but do NOT keep in memory
+                CachePolicy = CachePolicy.None,
+
+                // default is 4096, min 2^9 = 512, max 2^16 = 64K
+                FileBlockSize = GetBlockSize(),
+
                 // other options?
             };
 
@@ -25,6 +32,28 @@ namespace Umbraco.Web.PublishedCache.NuCache.DataSource
             return tree;
         }
 
+        private static int GetBlockSize()
+        {
+            var blockSize = 4096;
+
+            var appSetting = ConfigurationManager.AppSettings["Umbraco.Web.PublishedCache.NuCache.BTree.BlockSize"];
+            if (appSetting == null)
+                return blockSize;
+
+            if (!int.TryParse(appSetting, out blockSize))
+                throw new ConfigurationErrorsException($"Invalid block size value \"{appSetting}\": not a number.");
+
+            var bit = 0;
+            for (var i = blockSize; i != 1; i >>= 1)
+                bit++;
+            if (1 << bit != blockSize)
+                throw new ConfigurationErrorsException($"Invalid block size value \"{blockSize}\": must be a power of two.");
+            if (blockSize < 512 || blockSize > 65536)
+                throw new ConfigurationErrorsException($"Invalid block size value \"{blockSize}\": must be >= 512 and <= 65536.");
+
+            return blockSize;
+        }
+
         /*
         class ListOfIntSerializer : ISerializer<List<int>>
         {