Storage Systems
We study secondary storage technologies: Hard Disk Drives (HDD), Solid State Drives (SSD), and RAID configurations. The focus is on performance metrics (access time, throughput, IOPS) and how RAID provides redundancy and performance.
Learning Objectives
Key Concepts
Hard Disk Drives
An HDD stores data on magnetic platters that spin at high speed. Access time has three components:
- -Seek time: time to move the read/write head to the correct track
- -Rotational latency: time for the desired sector to rotate under the head
- -Transfer time: time to read/write the data once positioned
- -
Average seek time: typically 3-15 ms
- -
Average rotational latency = 0.5 / RPM × 60s (half rotation)
- -
Transfer rate depends on data density and rotation speed
- -
Sequential access is much faster than random access on HDD
Solid State Drives & RAID
SSDs use NAND flash memory with no moving parts — much faster random access than HDDs. RAID (Redundant Array of Independent Disks) combines multiple drives for performance and/or reliability:
- -RAID 0: striping (performance, no redundancy)
- -RAID 1: mirroring (redundancy, 50% capacity)
- -RAID 5: striping with distributed parity (N-1 capacity, tolerates 1 failure)
- -RAID 6: like RAID 5 with double parity (tolerates 2 failures)
- -RAID 10: mirror + stripe (good performance + redundancy)
- -
SSD read latency: ~25-100 μs vs HDD: ~5-15 ms (100-500× faster for random reads)
- -
SSD write: limited by erase-before-write and wear leveling (limited P/E cycles)
- -
RAID 0: max performance, any disk failure = total data loss
- -
RAID 5: good balance of performance, capacity, and redundancy
- -
RAID 10: best performance + redundancy but 50% capacity overhead