Parity rebuild slow through HBA, fast through motherboard but slowing down

[popmac]

I've been using a trial of unRAID for the past little bit then decided I'm happy with it so I bought a couple new drives and a license. Today I went to replace the old parity drive and it was going between 8 and 10 MB/s. I did a bit of reading about this and saw people recommending to plug into the motherboard instead of through my HBA, so I tried that and it immediately jumped up to 150 MB/s. After a few hours it slows down again though, going down to 80 MB/s.
I decided to cancel out of the parity rebuild, disable my plugins that aren't in use and try again and it's the same as previous - started out at 150 MB/s, now two hours in and am down to 122 MB/s and falling.

Any ideas of what's going on here? I can plug all the drives direct into the motherboard to see if that fixes the problem, but I currently have eight drives (two waiting to be added) and six ports, so that's not a long term solution. Also seeing as it takes a few hours to see if it's dropping I'd like to get some insight before trying that if possible.

 

Attached is the diagnostics, and the build is:

 

X9SCL motherboard

E3-1220L CPU

4 gb ram

H310 card flashed to IT mode

8 TB Ironwolf (new parity)

Five 2 TB WD Reds

Then I have the old 2 TB parity drive sitting unassigned (not formatted yet) and another 8 TB Ironwolf precleared and unassigned until this finishes up.

theintersect-diagnostics-20200418-1950.zip

[trurl]

1 hour ago, popmac said:

going between 8 and 10 MB/s

Can't say what was happening since syslog is only since reboot. Those speeds sound like you may have had connection problems.

 

1 hour ago, popmac said:

started out at 150 MB/s, now two hours in and am down to 122 MB/s and falling.

1 hour ago, popmac said:

Five 2 TB WD Reds

It starts with the longer outer tracks, so more data per revolution, and then continues to the shorter inner tracks, so less data per revolution. Completely normal for the outer tracks to be faster data rate than the inner tracks.

 

Also, those smaller disks will be slower than larger disks due to lower data density. Once it gets past the 2TB mark I would expect it to speed up again as it finishes checking the rest of the 8TB parity, then get slower as it goes. All about the data density.

 

[popmac]

Ah that makes sense for the slowing speed. I'm going to let this go until the parity rebuild is done tomorrow, and I've turned off everything accessing the array (Plex, etc) to avoid any additional slowdowns. Once everything is back up I'll plug the parity drive into the card again. Is there a way I can test speed after that and report back?

[trurl]

6 minutes ago, popmac said:

Is there a way I can test speed after that and report back?

 

[popmac]

Awesome, thanks. I'll post back tomorrow with the results.

[popmac]

Looks like you were right on all accounts! It did speed up after about 2.5TB or so and finished this morning, then I plugged the drive back into the card and got full speed... but another drive dropped down to next to nothing. Unplugged that one and plugged it back in and now everything is performing as it should! So it seems like these cables are just finicky and I'll replace them if they give me problems again.