Re: RAID 5,6 sequential writing seems slower in newer kernels

On Mon, Dec 7, 2015 at 8:29 AM, Robert Kierski [off-list ref] wrote:

I need some clarification on what data you want collected.... do you want iostat -xm collected before and after?  Or at a periodic interval (1 sec)?

I'm measuring sustained performance.  This means I’m runnig for a period of time -- 1 minute.  If you want iostat collected on an interval, that would probably be more data than one would want posted to this mailing list.

But... yes, this is a RAID6.

I've switched to HDD's as storage as there is concern on the list that the DDR disk devices could be causing unintended behavior in RAID6.  With 6 HDD's, each able to do 270 MB/s on their outer edge, I'm getting RAID6 throughput of about 1080 MB/s when I use a thread count of 4, using FIO.

-----Original Message-----
From: Dallas Clement [mailto:dallas.a.clement@gmail.com]
Sent: Friday, December 04, 2015 10:08 AM
To: Robert Kierski
Cc: Phil Turmel; linux-raid@vger.kernel.org
Subject: Re: RAID 5,6 sequential writing seems slower in newer kernels

Robert, is this with a RAID 6 array?  Also, what are the actual throughput numbers you are getting?  Would it be possible for you to capture the iostat -xm output and report the individual disk wMB/s and also the disk utilization for RAID 6?

Hi Robert.  Thanks for posting these results.  Your fio direct I/O
results look pretty darn good.  Were you able to confirm with hdparm
-t that you really can get 270 MB/s on your disks in reality?  Also,
what formula are you using to calculate best and worst case RAID 6
read and write speeds?

However, when I increase TC to 5, my TP is only 750 MB/s.  When I increase TC to 32, my TP is about 250 MB/s.

Now this is pretty disappointing.  I agree, 12 cores should be able to
easily handle this number of threads.
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