Where do these disk reads come from?

Question

I've written an application that heartbeats using single-block writes to disk using O_WRONLY|O_SYNC|O_DIRECT.

Is there something I need to do to disable the buffer cache for this device?

During testing there are no readers. I can simulate it as follows:

# dd of=/dev/sda1 bs=512 count=1 if=/dev/zero oflag=direct,dsync

Per "iostat 1" something is reading from the device every time I write.

The behavior does not change if I add "skip=1000", etc.
The behavior changes a little bit if I use /dev/sda or /dev/sda1:

sda:

Linux 2.6.32-431.5.1.el6.x86_64 (wiley)  07/31/2014     _x86_64_ (8 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.07    0.01    0.07    0.04    0.00   99.80

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sda              86.00       672.00         1.00        672          1

sda1:

sda              83.00       648.00         1.00        648          1

Answer 1

Aha!

raw(8) as seen here:

https://superuser.com/questions/242928/disable-linux-read-and-write-file-cache-on-partition/464382#464382

write:

# dd of=/dev/raw/raw1 bs=512 count=1 if=/dev/zero oflag=direct,dsync

iostat:

sda               2.00         0.00         1.00          0          1