> Is it necessary to fully replicate the persistent store onto the striped SSD array? I admire such a simple solution, but I wonder if something like an LRU cache would achieve similar speedups while using fewer resources. On the other hand, it could be a small cost to pay for a more consistent and predictable workload.

One of the reasons an LRU cache like dm-cache wasn't feasible was because we had a higher than acceptable bad sector read rate which would cause a cache like dm-cache to bubble up a block device error up to the database. The database would then shut itself down when it encountered an disk-level error.

> How does md handle a synchronous write in a heterogenous mirror? Does it wait for both devices to be written? Yes, md waits for both mirrors to be written.

> I'm also curious how this solution compares to allocating more ram to the servers, and either letting the database software use this for caching, or even creating a ramdisk and putting that in raid1 with the persistent storage. Since the SSDs are being treated as volatile anyways. I assume it would be prohibitively expensive to replicate the entire persistent store into main memory. Yeah, we're talking many terabytes.

> I'd also be interested to know how this compares with replacing the entire persistent disk / SSD system with zfs over a few SSDs (which would also allow snapshoting). Of course it is probably a huge feature to be able to have snapshots be integrated into your cloud... Would love if we could've used ZFS, but Scylla requires XFS.