Io

A disk that spikes with no obvious writer is hard to chase; iotop maps throughput straight back to the task doing the damage. System-wide tools show the symptom — high await, saturated utilization — but never the suspect: fifty processes share the disk and the average blames all of them equally. The question that ends the investigation is always per-process — who, how much, since when — and iotop answers exactly that, live, from kernel counters no application can hide from. ...

blk-mq gives the block layer a software staging queue per CPU and a hardware dispatch queue per device queue, with the I/O scheduler between them. A request takes the shortest path — straight to the hardware queue — unless a scheduler is attached or it can be merged, in which case it is staged in software first. That scheduler is the only place in the stack where the kernel decides the order your requests reach the device. ...