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hard drives [e5490541]

1. Spinning Rust [e5f32c22]

Operating systems implement all sorts of spinning rust scheduling, since they can make a good guess about how long operations will take (compared to jobs)
- SSTF - shortest seek time first, but vulnerable to starvation due to a stream of nearby jobs
- SCAN - loops back and forth, enqueueing for the next scan
- SPTF - Shortest access first
Disks can also merge IOs together

2. NVMe [7de46ff9]

NVMe drives will lie about write speeds under low traffic
NVMe drives are good for append-operations (aka structured logs)
linux has nvme to deal with this
in theory, the drives and the controllers are different, but in practice, these are sold as one whole thing
nvmexpress.org/wp-content/uploads/NVM-Express-1_4c-2021.06.28-Ratified.pdf
Leverages PCIe bus
NVMe driver replaces the old block layer requester

2.1. Writes and Reads

NVMe writes and reads are done through two circular buffers, a submission queue and a completion queue.
When the host system has a command to send to the SSD, it places the command in the submission queue and then rings the doorbell. This updates the tailpointer of the queue
Similar thing happens for completion queues, the drive updates the head doorbell when something is done

2.2. VS SATA and SCSI

NVMe supports 64k queues with 64k commands per queue, so you have extra parallelism
www.architecting.it/blog/performance-analysis-sas-sata-nvme/