More random write performance data

[Steven Pratt <slpratt@austin.ibm.com>]

Given the anomalies we were seeing on random write workloads, I decided 
to simplify the test and do single threaded odirect random write.  This 
should eliminate the locking issue as well as any pdflush bursty 
behavior.  What I got was not quite what I expected.

The most interesting graph is probably #12,  DM write throughput.  We 
see a baseline of ~7MB/sec with spikes every 30 seconds.  I assume the 
spike are meta data related as the io is being done from user space at a 
steady constant rate.  The really odd thing is that for the entire 
almost 2 hour duration, the amplitude of the spike continues to climb, 
meaning the amount of meta data need to be flushed to disk is ever 
increasing.

http://btrfs.boxacle.net/repository/raid/longrun/btrfs-longrun-1thread/btrfs1.ffsb.random_writes__threads_0001.09-04-08_13.05.54/analysis/iostat-processed.001/chart.html

Looking at graph #8 DM IO/sec, we see that there is even a pattern 
within the pattern of spikes.  It # of IOs in each spike appears to 
change at each interval and repeats over a set of 7, 30 second intervals.

Also, we see that we average 12MB/sec of data written out, for 5MB/sec 
of benchmark throughput.

I have queued up a run without checksums and cow to see how much this 
overhead is reduced.

Steve