diff --git a/lib/logstash/outputs/file.rb b/lib/logstash/outputs/file.rb
index ede10a5..f5aaf12 100644
--- a/lib/logstash/outputs/file.rb
+++ b/lib/logstash/outputs/file.rb
@@ -207,13 +207,14 @@ def extract_file_root
   # the back-bone of @flusher, our periodic-flushing interval.
   private
   def flush_pending_files
-    @io_mutex.synchronize do
+    pending_files = @io_mutex.synchronize do
       @logger.debug("Starting flush cycle")
-
-      @files.each do |path, fd|
-        @logger.debug("Flushing file", :path => path, :fd => fd)
-        fd.flush
-      end
+      # flushing all the files is slow, so we should do that without blocking
+      @files.clone
+    end
+    pending_files.each do |path, fd|
+      @logger.debug("Flushing file", :path => path, :fd => fd)
+      fd.flush
     end
   rescue => e
     # squash exceptions caught while flushing after logging them
@@ -374,6 +375,7 @@ def run
 class IOWriter
   def initialize(io, buffer_size, recreate)
     @io = io
+    @mutex = Mutex.new
     @buffers = []
     @size = 0
     @limit = buffer_size
@@ -395,14 +397,16 @@ def reopen
 
   public
   def write(*args)
-    args.each do |arg|
-      @size += arg.bytesize
-      @buffers.push(arg)
-    end
-    if (@limit >= 0) and (@size > @limit)
-      do_write
+    @mutex.synchronize do
+      args.each do |arg|
+        @size += arg.bytesize
+        @buffers.push(arg)
+      end
+      if (@limit >= 0) and (@size > @limit)
+        do_write
+      end
+      @active = true
     end
-    @active = true
   end
 
   private
@@ -417,23 +421,29 @@ def do_write
 
   public
   def flush
-    do_write
-    @io.flush
-    if @io.class == Zlib::GzipWriter
-      @io.to_io.flush
+    @mutex.synchronize do
+      do_write
+      @io.flush
+      if @io.class == Zlib::GzipWriter
+        @io.to_io.flush
+      end
     end
   end
 
   public
   def truncate(length)
-    reopen
-    @io.truncate(length)
+    @mutex.synchronize do
+      reopen
+      @io.truncate(length)
+    end
   end
 
   public
   def close
     flush
-    @io.close
+    @mutex.synchronize do
+      @io.close
+    end
   end
   def method_missing(method_name, *args, &block)
     if @io.respond_to?(method_name)