[CentOS] Filesystem writes unexpectedly slow (CentOS 6.4)

[Peter Kjellström]

On Mon, 13 Oct 2014 20:15:11 -0500
Joakim Ziegler <joakim at terminalmx.com> wrote:

...
> So, if I use iozone -a to test write speeds on the raw device, I get
> results in the 500-800MB/sec range, depending on write sizes, which
> is about what I'd expect.
> 
> However, when I have an ext4 filesystem on this device, mounted with
> noatime and data=writeback, (the filesystem is completely empty) and
> I test with dd, the results are less encouraging:

...

> Now, I'm sure there can be many reasons for this, but I wonder where
> I should start looking to debug this.
 
First I'd suggest comparing apples to apples. That is try doing the dd
test on the raw device and compare to dd on ext4.

Then you may want to try changing io scheduler from the default cfq to
deadline. This typically works better for many raid controllers but
ymmv.

Also testing with xfs instead of ext4 is probably worth it. xfs usually
outperform ext4 in streaming writes (like dd). Of course this raises
the question of whether that dd is a useful metric for your actual
load... xfs may infact be needed (3T * 7 = 21 TB > ext4 max (if I
remember correctly, refer to rh online data for rhel6 to make sure)).

Good luck,
 Peter K