Just curious really. Having recently swapped my HDDs to my new server, I'd thought I'd better run a parity check just to make sure all is well and to check my new cooling system. I notice the speed (according to unmenu) started at about 110,000 MB/sec and  was failrly consistent over about the 5 minutes that I watched it.  I've been keeping an eye on it periodically and noticed that it got progressively slower. About half way through it had slowed to about 80,000 MB/sec and now at 96% complete it's running at about 59,000 MB/sec. No errors have been reported. Drive temperatures have been consistent throughout with the parity at 34 degC =/- 1 deg and the two data discs at 29 dg C +/- 1 deg. Just curious to know if this is normal and if so why?

In the old days of hard drives, every track on a hard disk stored the same amount of data.  Since the inner tracks are shorter than the outer tracks, this meant that the capacity of the disk was limited by the amount that could be reliably stored on that innermost track.

 

But times have changed, and disk technology now allows different tracks to contain different numbers of sectors.  Because the outer tracks are longer, they store more data.  So a single revolution of the disk reads more data on an outer track than on an inner track.

 

This is called zone bit recording, and is explained here.

Many thanks. That explains it.