leveldb writes blocks occassionarily

Original [issue 164](https://code.google.com/p/leveldb/issues/detail?id=164) created by wuzuyang on 2013-05-03T06:55:01.000Z:

What steps will reproduce the problem?
1. download and compile leveldb-1.9.0
2. write a test program test.cpp(in attachment), which makes only Put() operations.
3. compile and run the test

What is the expected output? What do you see instead?

All write operations should be quick enougth(less than 100ms), there should not print any slow log.

Here the outputs:

nums: 1000
83: slow log: 7151.075
169: slow log: 106.461
403: slow log: 393.466
487: slow log: 112.986
563: slow log: 117.801

the slowest write operation takes 7151.075 ms!

What version of the product are you using? On what operating system?

Linux xen05.wd.corp.qihoo.net 2.6.18-164.el5xen # 1 SMP Thu Sep 3 04:03:03 EDT 2009 x86_64 x86_64 x86_64 GNU/Linux

Please provide any additional information below.

How to compile test.cpp: g++ test.cpp -I../leveldb-1.9.0/include ../leveldb-1.9.0/libleveldb.a -pthread
Run it: ./a.out