2 files changed, 35 insertions, 4 deletions

diff --git a/guides/source/4_0_release_notes.md b/guides/source/4_0_release_notes.md
index 006d0cda92..8be7a86d20 100644
--- a/guides/source/4_0_release_notes.md
+++ b/guides/source/4_0_release_notes.md
@@ -68,19 +68,19 @@ Major Features
  * **Strong parameters** ([commit](https://github.com/rails/rails/commit/a8f6d5c6450a7fe058348a7f10a908352bb6c7fc)) - Only allow whitelisted parameters to update model objects (`params.permit(:title, :text)`).
  * **Routing concerns** ([commit](https://github.com/rails/rails/commit/0dd24728a088fcb4ae616bb5d62734aca5276b1b)) - In the routing DSL, factor out common subroutes (`comments` from `/posts/1/comments` and `/videos/1/comments`).
  * **ActionController::Live** ([commit](https://github.com/rails/rails/commit/af0a9f9eefaee3a8120cfd8d05cbc431af376da3)) - Stream JSON with `response.stream`.
- * **Declarative ETags** ([commit](https://github.com/rails/rails/commit/ed5c938fa36995f06d4917d9543ba78ed506bb8d)) - Set ETag and Last-Modified headers using `etag` and `fresh_when`.
+ * **Declarative ETags** ([commit](https://github.com/rails/rails/commit/ed5c938fa36995f06d4917d9543ba78ed506bb8d)) - Add controller-level etag additions that will be part of the action etag computation
  * **[Russian doll caching](http://37signals.com/svn/posts/3113-how-key-based-cache-expiration-works)** ([commit](https://github.com/rails/rails/commit/4154bf012d2bec2aae79e4a49aa94a70d3e91d49)) - Cache nested fragments of views. Each fragment expires based on a set of dependencies (a cache key). The cache key is usually a template version number and a model object.
  * **Turbolinks** ([commit](https://github.com/rails/rails/commit/e35d8b18d0649c0ecc58f6b73df6b3c8d0c6bb74)) - Serve only one initial HTML page. When the user navigates to another page, use pushState to update the URL and use AJAX to update the title and body.
- * **Decouple ActionView from ActionController** ([commit](https://github.com/rails/rails/commit/78b0934dd1bb84e8f093fb8ef95ca99b297b51cd)) - ActionView is moved outside of ActionPack.
+ * **Decouple ActionView from ActionController** ([commit](https://github.com/rails/rails/commit/78b0934dd1bb84e8f093fb8ef95ca99b297b51cd)) - ActionView was decoupled from ActionPack and will be moved to a separated gem in Rails 4.1.
  * **Do not depend on ActiveModel** ([commit](https://github.com/rails/rails/commit/166dbaa7526a96fdf046f093f25b0a134b277a68)) - ActionPack no longer depends on ActiveModel.
 
 ### General
 
- * **ActiveModel::Model** ([commit](https://github.com/rails/rails/commit/3b822e91d1a6c4eab0064989bbd07aae3a6d0d08)) - `ActiveModel::Model` is extracted from ActiveRecord. `ActiveModel::Model` provides validations and `form_for` for normal Ruby objects.
+ * **ActiveModel::Model** ([commit](https://github.com/rails/rails/commit/3b822e91d1a6c4eab0064989bbd07aae3a6d0d08)) - `ActiveModel::Model`, a mixin to make normal Ruby objects to work with ActionPack out of box (ex. for `form_for`)
  * **New scope API** ([commit](https://github.com/rails/rails/commit/50cbc03d18c5984347965a94027879623fc44cce)) - Scopes must always use callables.
  * **Schema cache dump** ([commit](https://github.com/rails/rails/commit/5ca4fc95818047108e69e22d200e7a4a22969477)) - To improve Rails boot time, instead of loading the schema directly from the database, load the schema from a dump file.
  * **Support for specifying transaction isolation level** ([commit](https://github.com/rails/rails/commit/392eeecc11a291e406db927a18b75f41b2658253)) - Choose whether repeatable reads or improved performance (less locking) is more important.
- * **Dalli** ([commit](https://github.com/rails/rails/commit/82663306f428a5bbc90c511458432afb26d2f238)) - For the memcache session store, use the Dalli memcache client.
+ * **Dalli** ([commit](https://github.com/rails/rails/commit/82663306f428a5bbc90c511458432afb26d2f238)) - Use Dalli memcache client for the memcache store.
  * **Notifications start & finish** ([commit](https://github.com/rails/rails/commit/f08f8750a512f741acb004d0cebe210c5f949f28)) - Active Support instrumentation reports start and finish notifications to subscribers.
  * **Thread safe by default** ([commit](https://github.com/rails/rails/commit/5d416b907864d99af55ebaa400fff217e17570cd)) - Rails can run in threaded app servers without additional configuration. Note: Check that the gems you are using are threadsafe.
  * **PATCH verb** ([commit](https://github.com/rails/rails/commit/eed9f2539e3ab5a68e798802f464b8e4e95e619e)) - In Rails, PATCH replaces PUT. PATCH is used for partial updates of resources.
diff --git a/guides/source/debugging_rails_applications.md b/guides/source/debugging_rails_applications.md
index 77a2dd4b18..50ee934b87 100644
--- a/guides/source/debugging_rails_applications.md
+++ b/guides/source/debugging_rails_applications.md
@@ -209,6 +209,37 @@ logger.tagged("BCX", "Jason") { logger.info "Stuff" }                   # Logs "
 logger.tagged("BCX") { logger.tagged("Jason") { logger.info "Stuff" } } # Logs "[BCX] [Jason] Stuff"
 ```
 
+### Impact of Logs on Performance
+Logging will always have a small impact on performance of your rails app, 
+        particularly when logging to disk.However, there are a few subtleties:
+
+Using the `:debug` level will have a greater performance penalty than `:fatal`,
+      as a far greater number of strings are being evaluated and written to the
+      log output (e.g. disk).
+
+Another potential pitfall is that if you have many calls to `Logger` like this
+      in your code:
+
+```ruby
+logger.debug "Person attributes hash: #{@person.attributes.inspect}"
+```
+
+In the above example, There will be a performance impact even if the allowed 
+output level doesn't include debug. The reason is that Ruby has to evaluate 
+these strings, which includes instantiating the somewhat heavy `String` object 
+and interpolating the variables, and which takes time.
+Therefore, it's recommended to pass blocks to the logger methods, as these are 
+only evaluated if the output level is the same or included in the allowed level 
+(i.e. lazy loading). The same code rewritten would be:
+
+```ruby
+logger.debug {"Person attibutes hash: #{@person.attributes.inspect}"}
+```
+
+The contents of the block, and therefore the string interpolation, is only 
+evaluated if debug is enabled. This performance savings is only really 
+noticeable with large amounts of logging, but it's a good practice to employ.
+
 Debugging with the `debugger` gem
 ---------------------------------