Hardware RAID on the Raspberry Pi CM4

A few months ago, I posted a video titled Enterprise SAS RAID on the Raspberry Pi... but I never actually showed a SAS drive in it. And soon after, I posted another video, The Fastest SATA RAID on a Raspberry Pi.

Broadcom MegaRAID SAS storage controller HBA with HP 10K drives and Raspberry Pi Compute Module 4

Well now I have actual enterprise SAS drives running on a hardware RAID controller on a Raspberry Pi, and it's faster than the 'fastest' SATA RAID array I set up in that other video.

A Broadcom engineer named Josh watched my earlier videos and realized the ancient LSI card I was testing would not likely work with the ARM processor in the Pi, so he was able to send two pieces of kit my way:

Figuring out why an external USB hard drive won't spin down on my Mac

I am using a 2011 Mac mini as a backup server for all the data I store on iCloud, and for the first few days while I was setting up the Mac, I noticed the 4 TB and 2 TB external USB drives I had plugged in would spin down after a few minutes, and I would have blissful silence as long as there wasn't an active operation on that Mac (which should be fairly rare; just hourly Time Machine backups and periodic SSD activity since the iCloud libraries are all on SSD).

However, after a few weeks, I noticed that at least one of the two hard drives runs continuously, 24x7. Something on the Mac mini must keep hitting the drive and preventing it from spinning down.

To see what was happening, I used sudo fs_usage | grep VOLUME (in my case, VOLUME is 4\ TB\ Utility) to monitor what processes were accessing the drive, and what files they were accessing. After a few minutes watching (and doing nothing else on the computer, to make sure I wasn't causing any extra filesystem seeks), there were a couple regular culprits: