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+
+## Overview
+
+I/O Replay is an I/O benchmarking tool for Linux based operating systems which captures I/O operations on a (possibly production) server in order to replay the exact same I/O operations on a load test machine.
+
+I/O Replay is operated in 5 steps:
+
+1. Capture: Record all I/O operations over a given period of time to a capture log.
+2. Initialize: Copy the log to a load test machine and initialize the load test environment.
+3. Replay: Drop all OS caches and replay all I/O operations.
+4. Analyze: Look at the OS and hardware stats (throughput, I/O ops, load average) from the run phase and draw conclusions. The aim is to identify possible I/O bottlenecks.
+5. Repeat: Repeat 2-4 times but adjust OS and hardware settings in order to improve I/O performance.
+
+Examples of OS and hardware settings and adjustments:
+
+* Change of system parameters (file system mount options, file system caching, file system type, file system creation flags).
+* Replay the I/O at different speed(s).
+* Replay the I/O with modified pattern(s) (e.g. remove reads from the replay journal).
+* Replay the I/O on different types of hardware.
+
+The file system fragmentation (depending on the file system type and utilisation) might affect I/O performance as well. Therefore, replaying the I/O will not give the exact same result as on a production system. But it provides a pretty good way to determine I/O bottlenecks. As a rule of thumb file system fragmentation will not be an issue, unless the file system begins to fill up. Modern file systems (such as Ext4) will slowly start to suffer from fragmentation and slow down then.
+
+## Benefits
+
+In contrast to traditional I/O benchmarking tools, I/O Replay reproduces real production I/O, and does not rely on a pre-defined set of I/O operations.
+
+Also, I/O Replay only requires a server machine for capturing and another server machine for replaying. A traditional load test environment would usually be a distributed system which can consist of many components and machines. Such a distributed system can become quite complex which makes it difficult to isolate possible I/O bottlenecks. For example in order to trigger I/O events a client application would usually have to call a remote server application. The remote server application itself would query a database and the database would trigger the actual I/O operations in Linux. Furthermore, it is not easy to switch forth and back between hardware and OS settings. For example without a backup and restore procedure a database would most likely be corrupt after reformatting the data partitions with a different file system type.
+
+The benefits of I/O replay are:
+
+* It is easy to determine whether a new hardware type is suitable for an already existing application.
+* It is easy to change OS and hardware for performance tests and optimizations.
+* Findings can be applied to production machines in order to optimize OS configuration and to save hardware costs.
+* Benchmarks are based on production I/O patterns and not on artificial I/O patterns.
+* Log files can be modified to see whether a change in the application behavior would improve I/O performance (without actually touching the application code)
+* Log files could be generated synthetically in order to find out how a new application would perform (even if there isn't any code for the new application yet)
+* It identifies possible flaws in the applications (e.g. Java programs which produce I/O operations on the server machines). Findings can be reported to the corresponding developers so that changes can be introduced to improve the applications I/O performance.
+* It captures I/O in Linux Kernel space (very efficient, no system slowdowns even under heavy I/O load)
+* It replays I/O via a tool developed in C with as little overhead as possible.
+
+# Send in patches
+
+Patches of any kind (bug fixes, new features...) are welcome! I/O Replay is new software and not everything might be perfect yet. Also, I/O Replay is used for a very specific use case at Mimecast. It may need tuning or extension for your use case. It will grow and mature over time.
+
+This is also potentially a great tool just for analysing (not replaying) the I/O, therefore it would be a great opportunity to add more features related to that (e.g. more stats, filters, etc.).
+
+Future work will also include file hole support and I/O support for memory mapped files.
+
+# Getting started
+
+I/O Replay consists of a set of SystemTap kernel modules (capturing I/O) and the tool ``ioreplay`` (replaying I/O). Usually you want to capture I/O from a production machine and want to replay it on a separate load testing machine.
+
+## System requirements
+
+I/O replay has been tested on
+
+* CentOS 7.4 64Bit (latest version, all packages up to date, booted into the installed Kernel)
+* SystemTap (from the default CentOS repository)
+* GCC C-Compiler (from the default CentOS repository)
+
+Before proceeding please ensure that the latest CentOS 7 kernel is installed and running on all machines involved. It should also be ensured that the capture machine and the load test machine have the same mount points mounted. This is to ensure that I/O is being replayed on the corresponding data drives on the load test machine.
+
+## Compiling and installing ioreplay
+
+I/O Replay has to be installed on all machines involved. To install I/O Replay perform the following steps:
+
+```sh
+sudo yum install gcc systemtap yum-utils kernel-devel-$(uname -r)
+sudo debuginfo-install kernel-$(uname -r)
+make && sudo make install
+export PATH=$PATH:/opt/ioreplay/bin
+```
+
+This will install the ``ioreplay`` utility to ``/opt/ioreplay/bin/`` and the SystemTap kernel modules to ``/opt/ioreplay/systemtap/``. Run ``ioreplay -h`` to print out a brief help.
+
+However, best practise is not to install any compilers on a production machine. You can either compile I/O Replay from scratch on all machines involved like shown above or only compile it on a build machine and distribute the ``/opt/ioreplay`` directory to the remaining machines. In the latter case you will also need to install the ``systemtap-runtime`` package as an additional dependency.
+
+In case you decided to deinstall I/O Replay you can do so by running
+
+```sh
+sudo ioreplay -P # purges all test files created by ioreplay
+sudo make uninstall
+```
+
+# Operating I/O Replay
+
+## 1. Capture
+
+The following steps are required to capture all I/O operation of the entire (Linux) system to the file ``io.capture``. For efficiency and security it is only capturing the meta data (amount of bytes written and read) and not the actual data itself. It is also capturing the system time in microseconds and the process IDs (PIDs) and thread IDs (TIDs) used as well as all relevant options and flags of the corresponding I/O syscalls. It will stop capturing automatically after 60 minutes:
+
+ * 1) Stop all applications on the machine. Otherwise the kernel module won't recognize any already opened file handles. Stopping the applications before starting with the capture is essential for tracing the flags in how the files were opened. All I/O operations on unknown file handles will be ignored otherwise.
+ * 2) Run:
+
+```sh
+sudo ioreplay -c ~/io.capture
+```
+
+ * 3) Start all applications again.
+ * 4) To stop capturing I/O type Ctrl+C. Alternatively one hour for the Kernel module to auto exit.
+
+To capture only I/O caused by Java process run:
+
+```sh
+sudo ioreplay -c ~/io.capture -m javaioreplay.ko
+```
+
+To capture the I/O of a specific process run the following respectively:
+
+```sh
+sudo ioreplay -c ~/io.capture -m targetedioreplay.ko -p PID
+```
+
+The resulting capture log looks like this and can be multiple GB in size:
+
+```sh
+t=1511381122062;:,i=7764:8093;:,o=open;:,d=162;:,p=///usr/local/mimecast/someapp/somesubdir/vd11-9:1;:,f=0;:,m=438;:,
+t=1511381122062;:,i=7764:8093;:,o=fstat;:,d=162;:,s=0;:,
+t=1511381122062;:,i=7764:8093;:,o=read;:,d=162;:,b=12;:,
+t=1511381122062;:,i=7764:8093;:,o=fstat;:,d=162;:,s=0;:,
+t=1511381122062;:,i=7764:8093;:,o=lseek;:,d=162;:,O=0;:,W=1;:,b=12;:,
+t=1511381 |