Question: ```
I0909 22:47:01.240753 21904 sched.cpp:635] Scheduler::statusUpdate took    23007ns
I0909 22:47:01.240617 21904 master.cpp:3600] Sending 1 offers to framework 20140909-224659-16842879-44263-21883-0000

```

What is I0909? What is the date/timestamp format used? What pattern does it use ?

Answer: 
This is a default format of glog libraray

```
 [IWEF]mmdd hh:mm:ss.uuuuuu threadid file:line]

```

For example 

```
I0909 22:47:01.240753 21904 sched.cpp:635] Scheduler::statusUpdate took    23007ns

```

* I -- Level
* 09 -- month
* 09 -- date
* 22 -- hour
* 47 -- minute
* 01 -- second
* 240753 -- micro second
* 21904 -- thread id
* sched.cpp -- source file
* 635 -- source file line
* Scheduler::statusUpdate took 23007ns -- message
Now, @haosdent has already provided the concrete answer. Just for the sake of completeness, we're using [glog](https://github.com/google/glog) in Apache Mesos for logging.