On Tue, Dec 11, 2012 at 10:17:07PM +1100, Dave Chinner wrote:

On Tue, Dec 11, 2012 at 04:45:21PM +0800, Zheng Liu wrote:

quoted

Hi all,

At Tao Bao we meet a problem in our product system which causes a huge latency
because of stable page write.  This problem is easy to reproduce in a testing
environment, and I can reproduce it in my desktop with a SATA disk.  Here is the
fio config file that is used to reproduce this problem.

config file
-----------
[global]
iodepth=1
directory=/mnt/sda3
direct=0
group_reporting
thread
fallocate=0
runtime=120

[log-append]
ioengine=sync
rw=write
bs=1k

Sub-page sized IO. That's guaranteed to have noticable IO latency
anomalies, regardless of stable data pages. If you are just
doing appending writes, then you can easily buffer them till you
have a page of data to write and avoid the problem altogether.

Hi Dave,

Thanks for your reply.  I agree with you that the application still
could be improved.  But, unfortunately, the real life is that we cannot
change the application.  We couldn't convince the application developers
that they need to change their program to adapt to the latest kernel
because the latest kernel has a new feature that is not useful for them.
That is unacceptable for them. :-(

quoted

Hence, I wonder whether or not we could revert stable page write temporarily.
After it is improved, we could add it back again.

The plan is to turn it off for filesystems/devices that don't
require it. That list of devices will grow in future, so you
probably should plan to handle latencies in the application
properly...

I wonder whether we can provide a sysctl to turn on/off stable page
write.  At least we need to give sysadmin an opportunity to control it.

Regards,
                                                - Zheng

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com
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