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Hybrid SSD/HDD and SSD are 2 Different Things Print
March 2012
Solid State Disk (SSD) is a product category name of the integrated circuit version of hard disk drive (HDD).  HDD was invented around 1950 using magnetism to store data on a rotating disk.  Current products are still based on the same architecture although the storage density and capacity have greatly improved over the years.  As of March 2012, HDD with a SATA interface has reached 4TB of recordable size and 500GB is considered entry level.  SSD appeared about 50 years later.  It does not involve any rotation parts and therefore the initial applications were for satisfying computers in motion or adverse environments when HDD would fail almost immediately. SSD technology has improved over the last 10 years but is not ready to replace or take over HDD yet as of 2012.  Its per GB price is about 8 times that of HDD.   As such, SSD applications are still limited to adverse environments, small size (such as 60GB), and as cache for RAID (Redundant Array of Inexpensive Disks).

This article looks at 2 SSD product releases from vendors for general use in the first quarter of 2012: Hybrid SSD from Seagate and MLC SSD based on SandForce.  Both were in their second generation.  (Note: MLC is Multi Level Cell.  It is a cheaper category of SSD.  The more expensive and faster category is SLC for Single Level Cell.) Let us rewind the clock by 18 months to revisit their initial releases.  It was roughly 2010 Quarter 4. 

Seagate SSHD (solid state HDD) is based on a standard HDD but with additional cache.  A standard HDD has RAM type of cache for years not counted.  Cache is a buffer between the usage side of data and the storage side.  The usage side can be denoted by SATA interface which has a maximum capacity of 6Gbps for the current standard of SATA.  The storage side is the rotating disk which is efficient for recording or reading sequentially (such as for movies and music) but not randomly (other than for large file transfer).  The cache improves the efficiency of storage for the rotating disk.  (Note: RAM stands for Random Access memory and is used for the main memory of PC.  NAND is NOT-AND gated memory and is used for USB drive mostly.  RAM is volatile and the stored content must be energised to stay.  NAND is not volatile and the stored content will stay irrespective of power on or off.)

Seagate increased the RAM cache from 16MB to 32MB for the 500GB Momentus XT model and added an extra 4GB of NAND cache for its first generation of SSHD.  Selling price was doubled, but speed improvement was marginal.  Now comes the 2nd generation.  There are several changes.  The storage size has gone up to 750GB. The NAND cache has gone up to 8GB.  The interface with the host PC has upgraded to SATA 6Gbps.  Part of the SSD cache is partitioned off for boot files.  As a result, Windows 7 booted up in 14 seconds after BIOS POST and this compares very well with 28 seconds for the non-Hybrid version of Seagate Momentus XT.  (Note: all performance figures are not exact and are averaged from various sources for illustration only).  Unfortunately, SSHD performed badly in a Random Read test.  Based on an Anandtech publication, Intel SSD achieved 49MB/s for 4KB blocks whereas SSHD achieved 0.32MB/s.  The difference is huge!

This brings our attention back to pure SSD.  SSD has 2 major sub-systems, namely the storage made up of NAND, and the controller which also interfaces with the host PC.  Intel had the technology lead among a pack of SSD vendor companies until SandForce announced their 1200 series controller.  SandForce continued to announce 2200 and lately 2800 series controllers in recent months.  Major improvements are on several fronts. 

Security of storage is first.  SF encrypts storage on AES 128 bit standard minimum at the drive level not needing any CPU resources. 

Reliability of storage is second.  NAND has an un-recoverable bit error rate of 0.1%.  For a SSD with 32 dies of 4GB for 1286GB, the SSD failure rate from known causes would be 3.2% which is pretty bad.  SF writes one data to multiple dies like the concept of RAID for HDD and called their technology RAISE for Redundant Array of Independent Silicon Elements.  The total failure rate has reduced 100 times! 

Speed of data transfer is third.  SSD is much faster than HDD as electronics is faster than mechanical.  How fast is fast?  A benchmark comparing SF2800 SSD with Seagate SSHD showed a difference of up to 5 times for peak sequential reading and writing.

Cost is fourth.  By staying with version 1 of NAND (ONFI is an open NAND forum defining the interface version), SF can slash the price of SSD by about 20% when compared to SSD using version 2 NAND.  There is one sacrifice and it is the speed for multimedia files.  SF uses the term incompressible data to describe multimedia files.  In the worst scenario on incompressible data, sequential read is 206MB/s and random read (4KB) is 18MB/s.  Otherwise, SF attains 540MB/s sequential read and write, and 60MB/s for random read and write.   (Note: these figures are for illustrating the difference with SSHD only as benchmarking is more complex than expressible with a few numbers)

Please see the spec of the 2 implementations as of March 2012: S510 is based on ONFIv1 and S511 is based on v2.  S510 and S511 are commercial product codes.

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