I/O Systems and Secondary Storage
I/O Systems and Secondary Storage
I/O System Hardware Environment
I/O devices are typically either block devices, which transfer data in fixed-size blocks, or character devices, which transfer data one character at a time in a stream.
Device controllers are the hardware (a mini-computer) that connects the CPU to the I/O devices. They are responsible for sending commands to the device, and for receiving and sending data to and from the device.
The I/O devices communicate via controller registers/memory-mapped I/O, or direct memory access (DMA).
Old computers typically had a single bus (the system bus) that connects the CPU, memory, and I/O devices. The topology of the computer was similar to old ethernet networks with a single broadcast domain.
More modern and performant systems have multiple buses. The PCI bus is a high-speed backbone that all the other busses branch off of (memory, SCSI, USB, etc.).
The I/O system needs to be able to handle a wide variety of devices with different data transfer rates, data formats, and control mechanisms.
I/O System's Role and Structure
Needs to provide:
- A uniform interface to many devices, as well as device specific interfaces when necessary.
- Device-system communication and interaction through device drivers.
- Every device is given a unique id so it can be referenced by applications.
Organization
- Programmed I/O with polling: CPU issues an I/O command for the process, and the process busy waits until the I/O is complete. Inefficient because the CPU is tied up waiting for the I/O to complete.
- Interrupt-driven I/O: CPU issues an I/O command and continues to execute. When the I/O is complete, the device sends an interrupt to the CPU, which then handles the I/O completion. The process may or may not be blocked while waiting for the I/O to complete, but the processor is not tied up waiting for the I/O to complete.
- Direct Memory Access (DMA): The DMA module controls exchange of data between I/O module and main memory using physical addresses. The processor requests transfer of a block of data from DMA, and is interrupted when the transfer is complete. This saves the processor from having to handle the data transfer.
Secondary Storage
Anything outside of primary memory (RAM) is considered secondary storage. This includes hard drives, SSDs, and other storage devices. SS doesn't allow direct execution of instructions or data access via load/store instructions, and is instead accessed via I/O operations.
Secondary storage is... - Non-volatile: Data is retained even when the power is off. - Very slow compared to primary storage. - Failure-prone. - Giant and relatively cheap compared to primary storage. 2024 numbers: - 2 TB HDD for $73 ($0.04/GB) - 30 TB HDD for $700 - 500 GB SSD for $50 ($10/GB) - 100 TB SSD for $40,000
Memory Hierarchy
| Level | Speed | Cost | Size | Volatility |
|---|---|---|---|---|
| Registers | Fastest | Most Expensive | Smallest | Volatile |
| L1 Cache | Fast | Expensive | Small | Volatile |
| L2 Cache | Fast | Less Expensive | Still not a lot | Volatile |
| Main Memory | Slower | Less Expensive | Larger | Volatile |
| Secondary Storage | Slow | Cheap | Largest | Non-Volatile |
| Tertiary Storage | Slowest | Least Expensive | Largest | Non-Volatile |
HDDs and SDDs
HDDs are mechanical devices that store data on spinning disks. They have a read/write head that moves across the disk to read and write data. They are slow, but have a large capacity and are relatively cheap.
SSDs are solid-state devices that store data on flash memory. They are much faster than HDDs, but are more expensive. They are also more reliable and consume less power.
Disks and the OS
Disks are messy, slow, error-prone, horrible devices, and its the OS's job to make them look like a nice, clean, fast, reliable, and easy-to-use device.
The OS typically provides different levels of disk access to different clients, including: - physical block access: the ability to read and write blocks of data to the disk. - disk logical block access: the ability to read and write to a disk block number, without needing to know the physical location of the block. - file system: the ability to read and write files at a specified offset, block, or byte.
With old disks, only physical block access was available. With modern disks, the controller provides a higher-level interface that maps physical regions on disk (cylinders, sectors, ect.) to logical block numbers from $\lbrack 0, n \rbrack$, (ie a contiguous range of blocks to the OS).
Performance Issues
The HDD's performance is affected by its mechanically moving parts. Limiting the amount of seeking and defragmenting help, but only to an extent.