HDD vs SSD: Modeling Physical Storage Latency

Mechanical vs Flash Latency

HDD random reads pay seek plus rotational delay before a 4KB sector arrives. SSD reads are electrical with tens of microseconds latency but very different write semantics: NAND must erase before reprogramming, driving write amplification and wear leveling.

Latency Components

Model HDD random read as seek + average rotation + transfer. Model SSD read as fixed microseconds; model SSD random write as read-modify-erase-write on affected pages.

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Sequential throughput can saturate interface on both media
SSD FTL hides physical layout; logical LBA still maps to pages
Databases cache hot pages in RAM regardless of media
Understanding the gap explains why LSM trees favor sequential writes

Working Through the Exercise

Keep the relevant documentation open while you implement. When your output disagrees with the reference, trace one failing case by hand before changing random lines.

Trace first: walk one failing input step by step
Read the manual: skim the official doc for the mechanism named in this task
Compare baselines: save measurements from the prior step so speedups are honest

Why for this exercise

You will compute latency and IOPS tables for HDD and SSD constants provided in the starter code. This exercise asks you to print a comparison that motivates buffering and sequential layout.

Write a C program that models HDD and SSD latency for different access patterns.

Requirements:

Define HDD constants: seek_avg_ms=8.5, rotation_rpm=7200, sequential_mb_per_s=150
Define SSD constants: read_us=50, write_us=300, erase_ms=2, page_size_kb=4, block_size_kb=256
For an HDD, calculate latency for:
- Random 4KB read: seek + rotation + transfer
- Sequential 1MB read: first random + sequential transfer of remainder
- Random 4KB write: same as read (HDD writes are symmetric)
For an SSD, calculate latency for:
- Random 4KB read: read time
- Random 4KB write: read old page + erase block + write new page (simplified model)
- Sequential 1MB write: 1MB / page_size writes, but some can be batched
Calculate IOPS for random 4KB reads on both devices
Print a comparison table: Device | Random Read Latency | Random Write Latency | Sequential Read 1MB | IOPS (4KB random)
Include a note about why databases on HDDs need caching and sequential layouts

Mechanical vs Flash Latency

Latency Components

Model HDD random read as seek + average rotation + transfer. Model SSD read as fixed microseconds; model SSD random write as read-modify-erase-write on affected pages.

c
Loading…

Sequential throughput can saturate interface on both media
SSD FTL hides physical layout; logical LBA still maps to pages
Databases cache hot pages in RAM regardless of media
Understanding the gap explains why LSM trees favor sequential writes

Working Through the Exercise

Keep the relevant documentation open while you implement. When your output disagrees with the reference, trace one failing case by hand before changing random lines.

Trace first: walk one failing input step by step
Read the manual: skim the official doc for the mechanism named in this task
Compare baselines: save measurements from the prior step so speedups are honest

Why for this exercise

You will compute latency and IOPS tables for HDD and SSD constants provided in the starter code. This exercise asks you to print a comparison that motivates buffering and sequential layout.

Write a C program that models HDD and SSD latency for different access patterns.

Requirements:

Define HDD constants: seek_avg_ms=8.5, rotation_rpm=7200, sequential_mb_per_s=150
Define SSD constants: read_us=50, write_us=300, erase_ms=2, page_size_kb=4, block_size_kb=256
For an HDD, calculate latency for:
- Random 4KB read: seek + rotation + transfer
- Sequential 1MB read: first random + sequential transfer of remainder
- Random 4KB write: same as read (HDD writes are symmetric)
For an SSD, calculate latency for:
- Random 4KB read: read time
- Random 4KB write: read old page + erase block + write new page (simplified model)
- Sequential 1MB write: 1MB / page_size writes, but some can be batched
Calculate IOPS for random 4KB reads on both devices
Print a comparison table: Device | Random Read Latency | Random Write Latency | Sequential Read 1MB | IOPS (4KB random)
Include a note about why databases on HDDs need caching and sequential layouts

Topic Overview

Mechanical vs Flash Latency

Latency Components

Working Through the Exercise

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source

HDD vs SSD: Modeling Physical Storage Latency

Topic Overview

Mechanical vs Flash Latency

Latency Components

Working Through the Exercise

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source