Monday, November 10, 2008

Conventional hard drive vs. Solid state drive: Which should you buy?

As more tablet PCs come with optional Solid State Drives, I've wondered and been asked if there is a significant advantage that justifies the current cost delta between SSDs and convential hard drives. Wikipedia provides a great answer which I've summarized below.


  • Faster start-up, as no spin-up is required
  • Typically, fast random access for reading, as there is no read/write head to move
  • Extremely low read latency times, as SSD seek-times are orders of magnitude lower than the best current hard disk drives
  • In applications where hard disk seeks are the limiting factor, this results in faster boot and application launch times
  • Extremely fast write
  • No noise: a lack of moving parts makes SSDs completely silent
  • For low-capacity flash SSDs, low power consumption and heat production when in active use, although high-end SSDs and DRAM-based SSDs may have significantly higher power requirements
  • High mechanical reliability, as the lack of moving parts almost eliminates the risk of mechanical failure
  • Ability to endure extreme shock, high altitude, vibration and extremes of temperature: once again because there are no moving parts. This makes SSDs useful for laptops, mobile computers, and devices that operate in extreme conditions.
  • Larger range of operating temperatures. Typical hard drives have an operating range of 5-55 degrees C. Most flash drives can operate at 70 degrees, and some industrial grade drives can operate over an even larger temperature range.
  • Relatively deterministic read performance: unlike hard disk drives, performance of SSDs is almost constant and deterministic across the entire storage. This is because the seek time is almost constant and does not depend on the physical location of the data, and so, file fragmentation has almost no impact on read performance.
  • For low-capacity SSDs, lower weight and size: although size and weight per unit storage are still better for traditional hard drives. When failures occur, they tend to occur either 'on write', or 'on erase', rather than 'on read'. With traditional HDDs, failure tends to occur 'on read'. If the drive detects failure on write, data can be written to a new cell without data loss occuring. If a drive fails on read, then data is usually lost permanently.


  • Cost – as of mid-2008, SSD prices are still considerably more costly per gigabyte than are comparable conventional hard drives: consumer grade drives are typically USD 2 to 3.50 per GB for flash drives and over USD 80 per GB for RAM-based compared to less than USD 0.15 per gigabyte for hard drives.
  • Capacity – currently far lower than that of conventional hard drives
  • DRAM based SSDs have a higher vulnerability to abrupt power loss.
  • Limited write (erase) cycles – flash-memory cells will often wear out after 1,000 to 10,000 write cycles for MLC, and up to 100,000 write cycles for SLC
  • Slower write speeds – as erase blocks on flash-based SSDs generally are quite large (e.g. 0.5 - 1 megabyte), they are far slower than conventional disks for random writes and therefore vulnerable to write fragmentation, and in some cases for sequential writes. SSDs based on DRAM do not suffer from this problem.
  • Lower storage density – hard disks can store more data per unit volume than DRAM or flash SSDs, except for very low capacity/small devices.
  • Higher power consumption – at idle or under low workloads laptop battery runtimes decrease when using an SSD over a 7200 RPM 2.5" laptop hard drive, flash drives also take more power per gigabyte. RAM based SSD require more power than hard disks, both operating and when turned off.

1 comment:

Cristina said...

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