Storage Devices – Hard Disks and SSDs

HDD

• Form Factors
  • 5 in desktop hard drive
    • Spinning platter
  • 5 in traditional laptop hard drive
    • Spinning platter
  • 8 in hard drive
    • In original iPod
    • Phased out due to SSD
• Consumer HDD
  • SATA
    • Serial
    • Card-edge connector
  • PATA
    • Parallel
    • Precursor to SATA
    • Has pin connector
    • Obsolete
• IDE
  • Integrated Drive Electronics
  • AKA PATA
• SCSI
  • Small computer system interface
    • Scuzzy
  • Servers and high-end equipment
  • Built to be robust and less prone to errors
  • Several revisions
    • SCSI 2
      • 50-pin interface
    • SCSI 3
      • 68 pin interface
    • SCSI 3 variant
      • 80 pin variant
• PATA/IDE Interface
  • 40 position connector, 39 pin interface
  • Ribbon cable
  • IDE Drive jumper options
    • Master
    • Slave
    • Cable select
  • Standard power connector
• Current Technology
  • SATA
    • Serial Advanced Technology Attachment
  • SAS ' Serial attached SCSI
• Where does the data reside?
  • Disk Platter
    • Round, flat, magnetic or ceramic disks in a hard drive that holds the actual data
    • Made of two materials
      • Substrate material
      • Magnetic media coating
• Track
  • Concentric circles where all the information is stored
    • Modern disks contain 10k+ tracks
      • Tracks are .115m or less
    • Each platter within a hard drive will have the same amount of tracks
  • Track numbering starts from 0 and starts from the outside of the platter
• Definition
  • Cylinder
    • Group of tracks in a hard drive that can be accessed at the same time by the read/write head
      • Typically called a cylinder and not track number
• Disk Drive Terminology
  • Putting it all together
    • Number of cylinders (track), head (sides), and sectors (CHS) uniquely identify physical geometry of the storage units on a disk
      • CHS also yields disk capacity
        • Capacity = cylinders x heads x sectors x sector size
• Sectors
  • Physical unit of hard drive data storage
    • A bucket to store parts of a file into
    • Typically 512 bytes and some bytes for internal info
    • 1 sector = 4096 (4K) bytes for new disks
  • Info
    • ID information
      • Sector number and status
    • Synchronization fields
      • Helps controller guide read process
    • ECC
      • Error correcting code and ensures data integrity
  • Modern hard drives will have a pool of reserved sectors to pull from when one fails
    • Spare Sectoring
  • When bad sectors are identified, they cannot be easily seen by any operating system
    • Typically hardware devices such as the PC-3000 are required

SSD

• Solid state drives
  • Increased in capacity
  • Cheaper now and most devices use SSD
• File Fragmentation
  • On HDD
  • Overtime with use, files get fragmented
    • No longer stored in a continuous fashion
• Advantages of SSH
  • No tracks or disks
    • Stored in cells
  • Constant seek time for even random access
  • No file fragmentation
  • No moving parts
    • No mechanical errors
    • Fast startup time
    • No motor noise
  • Light in weight
  • Low power
• SSD Problem areas
  • Overwriting is complicated and expensive
    • Read-erase-modify-write cycle
  • Cells have a limited lifetime/cycle
    • Wear leveling must be used
  • Data recovery issues
  • Data carving issues
  • Power faults can cause data loss or a bricked device
• SSD Internals
  • Flash based
    • NAND Flash
  • Controller
• Data storage
  • Written in blocks and pages
    • Data moves so it maintains a map
    • Mapping to LBA is maintained
    • Blocks are broken down into pages
      • Pages store sections
    • 4069 bytes per sector
• Erasing and Write Amplification
  • Write is fast
    • Rewrite/overwrite involves additional erasing step
    • Entire blocks (not pages or sections) must be erased
      • Block is smallest erasable unit
• Optimization Strategies
  • Adding a page
  • Page is holding new data when something is marked as stale
• Garbage Collection
  • Allocate a new unused block
  • Copy new contents to the block
  • Mark
• TRIM Command
  • File deletion on a normal HDD
    • File is marked as deleted in the file system's database
    • HDD has no clue which sectors are freed up
  • SSDs need to know this info
    • Modern OS have support for this command
• Over-provisioning
  • Individual memory cells have limited read/write cycles
    • Few thousand cycles
  • Like hard disks
    • Extra capacity not accessible by normal users
  • Unlike hard disks
    • 10-25% more storage than stated capacity
    • Overprovisioned areas are continuously used