[CentOS] Slow RAID Check/high %iowait during check after updgrade from CentOS 6.5 -> CentOS 7.2

[m.roth at 5-cent.us]

Kelly Lesperance wrote:
> I did some additional testing - I stopped Kafka on the host, and kicked
> off a disk check, and it ran at the expected speed overnight. I started
> kafka this morning, and the raid check's speed immediately dropped down to
> ~2000K/Sec.
>
> I then enabled the write-back cache on the drives (hdparm -W1 /dev/sd*).
> The raid check is now running between 100000K/Sec and 200000K/Sec, and has
> been for several hours (it fluctuates, but seems to stay within that
> range). Write-back cache is NOT enabled for the drives on the hosts we
> haven't upgraded yet, but the speeds are similar (I kicked off a raid
> check on one of our CentOS 6 hosts as well, the window seems to be 150000
> - 200000K/Sec on that host).
<snip>
Perhaps I missed where you answered this: is this software RAID, or
hardware? And I think you said you're upgrading existing boxes?

      mark