HDD and SSD
HDD
- Hard Disk Drive
- Non-volatile storage that records data magnetically on rotating platters
- Read/write heads move across the platters to access data
- Moving parts add seek and rotational latency, especially for random access
- Common form factors include 3.5-inch desktop drives and 2.5-inch laptop drives
HDD interfaces
- PATA (Parallel ATA - Advanced Technology Attachment)
- commonly called IDE (Integrated Drive Electronics)
- Older parallel interface for HDDs and optical drives
- SATA
- Serial ATA
- Common interface for consumer HDDs
- SATA 3.0 has a link rate of 6 Gbps
- Hot plugging is supported when the drive, controller, firmware, and operating system are configured for it
- AHCI
- Advanced Host Controller Interface
- Common host-controller programming interface for SATA
- Parallel SCSI (Small Computer System Interface)
- Older interface used in workstations and enterprise storage
- SAS (Serial Attached SCSI)
- Serial interface commonly used for enterprise HDDs
SSD
- Solid State Drive
- Non-volatile storage, usually built from NAND flash memory
- Has no moving parts, so access latency is lower than on an HDD
- Flash is read and programmed in pages, but erased in larger blocks
- The controller manages logical block mapping, error correction, garbage collection, and wear leveling
- Flash cells have limited write endurance; wear leveling spreads writes across the device
SSD Interfaces
- Form factors and connectors
- 2.5-inch drives
- commonly use SATA
- enterprise models may use SAS
- M.2 (form factor and connector)
- may use SATA or PCIe with NVMe protocol
- U.2 (form factor and connector)
- commonly used for enterprise PCIe/NVMe SSDs
- Data interfaces and protocols
- SATA
- Common for 2.5-inch SSDs and some M.2 SSDs
- Commonly uses AHCI
- PCIe with NVMe
- PCIe provides the connection; NVMe is the host interface and command protocol designed for non-volatile storage
- Used by many modern high-performance SSDs