On 14/10/14, 6:45, Peter Kjellström wrote:
On Mon, 13 Oct 2014 20:15:11 -0500
Joakim Ziegler <joakim@xxxxxxxxxxxxxx> 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)).
Upgrading to 6.5 with its new kernel did not fix the problem. I will be doing
some more testing. The strange thing is, I have a near-identical machine also
running CentOS 6.5, also with ext4 on the same controller (and another, newer
Areca controller), and there it's extremely fast, on the fastest controller
there, dd hits around 2GB/sec sustained over 200 GB of data on a 24-disk RAID6
(both systems have 96GB of RAM each).
And yes, I've formatted with a newer version of e2fsprogs than is included with
the distro, to get 16TB+ support, although in the case of the device I'm
currently testing, it actually has two partitions, so I wouldn't have needed to.
I'll do a bit more testing and come back with my results.
--
Joakim Ziegler - Supervisor de postproducción - Terminal
joakim@xxxxxxxxxxxxxx - 044 55 2971 8514 - 5264 0864
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