Module 5.1 Operating Systems

(HDDs) and (NVM) devices

The bulk of secondary storage for modern computers

spin platters of magnetically-coated material under moving read-write heads, their drives roatate at 60 to 250 times per sc

Positioning time (random-access time)

ime to move the disk arm to the desired cylinder (seek time) and time for the desired sector to rotate under the disk head (rotational latency)

Head crash

results from disk head making contact with the disk surface

Hard Disk Drives

Platters range from .85” to 14” (historically)

• Commonly 3.5”, 2.5”, and 1.8”

Range from 30GB to 3TB per drive

Hard Disk Performance

• Access Latency = Average access time = average seek time + average latency (milliseconds)

  • For fastest disk 3ms + 2ms = 5ms

  • For slow disk 9ms + 5.56ms = 14.56ms

The first commercial disk

1956IBM RAMDAC computer included the IBM Model 350 disk storage system

Non-volatile Memory Drives

If disk-drive like, then called solid-state disks (SSDs)

Non-volatile Memory Devices

Read and written in “page” increments (think sector) but can’t overwrite in place

NAND Flash Controller Algorithms

With no overwrite, pages end up with a mix of valid and invalid data

flash translation layer (FTL) table

To track which logical blocks are valid, the controller maintains a

garbage collection

frees invalid page space

overprovisioning

working space for GC

wear-leveling 

Each cell has a lifespan, so _____ needed to write equally to all cells

DRAM

used as a mass-storage device

RAM drives

present as raw block devices, common file system formatted

Disk drives

addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer

logical blocks

Low-level formatting creates _____ on physical media

Disk Attachment

Host-attached storage accessed through I/O ports talking to I/O busses

NVM express (NVMe)

a new fast interface for NVM, connects directly to the PCI bus

controllers (or host-bus adapters, HBAs)

Data transfers on a bus carried out by special electronic processors

Disk drives

are addressed as large 1-dimensional arrays of logical blocks

logical block

the smallest unit of transfer

Shortest Seek Time First

selects the request with the minimum to seek time from the current head position

SSTF scheduling

form of SJF scheduling; may cause starvation of some requests

Scan

The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues

elevator algorithm

SCAN algorithm Sometimes called?

C-Scan

The head moves from one end of the disk to the other, servicing requests as it goes

C-Scan

Provides a more uniform wait time than SCAN

True

True or false

SCAN and C-SCAN perform better for systems that place a heavy load on the disk

deadline scheduler

To avoid starvation Linux implemented a?

deadline scheduler

• Maintains separate read and write queues, gives read priority

• Implements four queues: 2 x read and 2 x write

NVM Scheduling

No disk heads or rotational latency but still room for optimization

Error Detection and Correction

The fundamental aspect of many parts of computing (memory, networking, storage)

Error detection

determines if there a problem has occurred (for example a bit flipping)

Checksum

Parity is one form of?

checksum

uses modular arithmetic to compute, store, compare values of fixed-length words

cyclic redundancy check (CRC)

uses a hash function to detect multiple-bit errors

Error-correction code (ECC)

not only detects but can correct some errors

Low-level formatting, or physical formatting

Dividing a disk into sectors that the disk controller can read and write

Root partition

contains the OS, other partitions can hold other Oses, other file systems, or be raw

boot loader

set of blocks that contain enough code to know how to load the kernel from the file system