This program is to check a filesystem for data-integrity. 

Example: fstest -p 99 /mnt/my-test-filesystem/ >fstest.out 2>fstest.err


You should redirect stdout and stderr into different files, as fstest writes
stats about the number of written and read files minutely to stdout. You
might easily miss critical errors written to stderr, if both streams
are directed into the same file.

What is ql-fstest doing:
It consists of two threads, a writer and a reader. The writer thread
writes random numbers as pattern to files. The filename (using the number
converted to hex) corresponds to that pattern.
Initially the reader thread stays a few files behind the writer thread
and checks if the pattern is correct.
In order to avoid cache effects, we use posix_fadvise() and try to tell the kernel
to remove data from the page cache once the file was written. 
(Same applies for reads). Additionally the reader is a few files behind the 
writer, which should make sure, the cache is filled with other data, if the 
kernel should ignore the posix_fadvise() command.

Once the filesystem filled up to the maximum given level, the writer thread 
will start to delete files in order to write new data. But before it deletes a 
file, it will itself again check if the file still has correct data (unless the file
was already checked 10 times by the reader thread before).
As the writer thread now does reads and writes itself, the reader quickly catches up
and in order to give it some work, the reader thread will restart to read files from 
index 0 once it has read the last file written before the filesystem was full.

Note: In error case it continues to check remaining files by default and
does not terminate.