So, the 8TB drives are fine if you're doing sequential writes, mainly doing write-only workloads, or have a massive buffer in front.
In my experience, the PCIE drives they mention (the DC P3700) are incredible. They're blazing fast (though the 3D X-Point stuff is obviously faster), have big supercaps (in case of power failure), and really low latency for an SSD (150 microseconds or so). They're a pretty suitable alternative to in-memory logbuffers where latency is less crucial. Much cheaper than the equivalent memory too. Having a few of these in front of a RAID 10 made up of 8TB drives will work just fine.
FWIW your experience with a RAID 6 of big disks is unsurprising. Raid 6 is almost always a bad choice nowadays since disks are so big. Terrible recovery times and middling performance.
True about RAID6, I was just commenting on performance. A 2TB will spank an 8TB in any configuration.
I would think that GitLab's workload is mostly random, which would pose a problem for larger drives. The SSDs are a great idea, but I've only seen 8TB drives used when there are 2 to 3 tiers; with 8TB drives being all the way on the bottom. I'm not sure how effective having a single SSD as a cache drive for 24TBs of 8TB disks will be.
In my experience, the PCIE drives they mention (the DC P3700) are incredible. They're blazing fast (though the 3D X-Point stuff is obviously faster), have big supercaps (in case of power failure), and really low latency for an SSD (150 microseconds or so). They're a pretty suitable alternative to in-memory logbuffers where latency is less crucial. Much cheaper than the equivalent memory too. Having a few of these in front of a RAID 10 made up of 8TB drives will work just fine.
FWIW your experience with a RAID 6 of big disks is unsurprising. Raid 6 is almost always a bad choice nowadays since disks are so big. Terrible recovery times and middling performance.