[PATCH] Fix fileoffset option

[PATCH] Fix fileoffset option

1. The job file we prepared:
----------offset-----------------
[global]
filename=./temp/HOWTO
nrfiles=1
rw=read
size=8K
offset=16k
bs=1k
thread
loops=10
[offset]
description="Option 'offset' doesn't work."
---------------------------------
Besides, we copied the file HOWTO to the directory ./temp/.
So we thought fio would read 8K text sequentially from the point 16K off
the beginning of HOWTO.

we run fio with gdb to see what was read out after the first io, and we
got this:
---------------------------------
... ...
(gdb) bt
from /lib/tls/i686/cmov/libpthread.so.0
(gdb) x/4w io_u->xfer_buf
0x8073f08:      0x6c626154      0x666f2065      0x6e6f6320
0x746e6574
(gdb)
---------------------------------
The above was performed after the first io finished by td_io_queue.
The contents read out as we can see was the ASCII codes for 'Table of
content', which was right from the beginning of file HOWTO.
That means fio read from the beginning but not the point 16K off the
beginning.Is that right?

2. Reason for the problem:
It seems the offset is saved in td->o.start_offset. But it isn't passed
to  f->file_offset.And the setting up of f->io_size doesn't refer to
f->file_offset.

--------------------------------------------------------

And now we can check it with gdb:
--------------------------------------------------------
... ...
(gdb) bt
from /lib/tls/i686/cmov/libpthread.so.0
(gdb) x/4w io_u->xfer_buf
0x8073f08:      0x206b636f      0x7420666f      0x66206568
0x20656c69
(gdb)
---------------------------------------------------------
After the first io, I got '0x206b636f 0x7420666f 0x66206568 0x20656c69'
read out.And they represent the string 'ock of the file '.Then i search
the string in HOWTO and got only one line containing the string:
'norandommap Normally fio will cover every block of the file when doing'
I removed the text following that line in HOWTO, and then I found that
the size of the rest text in HOWTO was 16K, which equaled to the offset
we set in job file.
So I think the option 'offset' does work after the changes.

In fact, I am not sure of the fix, but i do hope it can help.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>