Reuse filled pattern

Reuse filled pattern

I made changes to fio so we wld re-use the already populated io_u
buffer (when there is a non-random pattern) during writes. That way
only the header will be re-calculated for every I/O. This way the
buffer wld get populated in the beginning and as long as the
subsequent ios using the same io_u structure are writes and have same
or less block size, it wld get re-used. If any of the subsequent i/o
is a read or has a block size greater than the pre-filled one, then
the buffer is invalidated and will be re-filled at the next write.

Reason for this risky change: (Performance)
I tested this change on a tmpfs(with no swap backing), with the
following config file:
[sscan_write]
filename=/mytmpfs/datafile.tmp
rw=write
bs=64k
size=3G
ioengine=libaio
iodepth=1024
iodepth_low=512
runtime=10800
bwavgtime=5000
thread=1
do_verify=0
verify=meta
verify_pattern=0x55aaa55a
verify_interval=4k
continue_on_error=1

fio-1-41-6 gave 306MB/s and the new change had a performance of 1546MB/s

Side effects/Risks:
There is a risk with this fix, that if the buffer gets corrupted then
the subsequent writes will also be corrupt. I think for both
sequential writes and random writes (with verify, where the I/O log is
replayed) we shld be able to find the first I/O that started with the
corruption and if the buffer is getting corrupted, there are other
issues here.

Testing:
I have tested this fix with sequential write(verify)/random read write
mix combination(with verify).

I think I have taken care of most of the case, but please let me know
if there is anything I have missed. I have attached the patch along
with this email. I think the performance improvement outweighs the
risk associated with the fix. But I will let you decide if you wld
like to pick it up.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>