By Camuel Gilyadov, on October 13th, 2010

Debunking common misconceptions in SSD, particularly for analytics

1. SSD is NOT synonymous for flash memory.

First of all let’s settle on terms. SSD is best described as a concept of using semiconductor memory as disk. There is two common cases: DRAM-as-disk and flash-as-disk. And flash-memory is a semiconductor technology pretty similar to DRAM, just with slightly different set of trade-offs made.

Today there are little options to use flash memory in analytics beyond SSD. Nevertheless, it should not suggest that SSD is synonymous for flash memory. Flash memory can be used in products beyond SSD, and SSD can use non-flash technology, for example DRAM.

So the question is: do we have any option of using flash-memory in other form rather than flash-as-disk?

FusionIO is the only one and was always bold in claiming that their product are not-SSD but a totally new category product, called ioMemory. Usually I dismiss such claims automatically in subconscious as a common-practice of  term-obfuscation. However, in the case of FusionIO I found it to be a a rare exception and technically true. On hardware level there is no disk-related overhead in FusionIO solution and in my opinion FusionIO are closest to the flash-on-motherboard vision among all the rest of SSD manufacturers. That said, FusionIO succumbed to implementing a disk-oriented storage layer in software because unavailability of any other standards covering  flash-as-flash concept.

You can find a more in-depth coverage of New-Dynasty SSD versus Legacy SSD issue in recent article of Zsolt Kerekes on StorageSearch.com. Albeit I’m not 100% agree with his categorization.

2. SSD DOESN’T provide more throughput than HDD.

The bragging about performance density of SSD could safely be dismissed. There is no problem in stacking up HDDs together. As many as 48 of them can be put in single server 4U box providing aggregate throughput of 4GB/sec for fraction of SSD price. Same goes to power, vibration, noise and etc… The extent to which this properties are superior to disk is uninteresting and not justifying the associated premium in cost.

Further, for any amount of  money, HDD can provide significantly more IO throughput , than SSD of any of today vendor. On any workload: read, write or combined.  Not only this, but it will do so with an order of magnitude more capacity for your big data as additional bonus. However, a few nuances are to be considered:

  • If  data is accessed in random small chunks (let’s say 16KB chunks), then SSD will provide significantly more throughput (factor of x100 may be) than disk will do. Reading in chunks at least 1MB will put HDD as a winner in the throughput game again.
  • The flash memory itself, has great potential to provide an order of magnitude more throughput than disks. Mechanical “gramophone” technology of disks cannot compete in agility with the electrons. However, this potential throughput is hopelessly being left unexploited by the nowadays SSD controller. How bad it is? Pretty bad, SSD controllers pass on less than 10% of potential throughput. The reasons include: flash-management complexity, cost-constraints leading to small embedded DRAM buffers and computationally-weak controllers,  and the main reason being that there is no standards for 100 faster disk, neither legacy software could potentially keep with higher multi-gigabyte throughputs, so SSD vendors don’t bother and are obsessed with the laughable idea of bankrupting HDD manufacturers calling the technology disruptive which it is not by definition. So we have a much more expensive disk replacement that is only barely more performant, throughput-wise, than vanilla low-cost HDD array.

3. Array of SSDs DOESN’T provide larger number of useful IOPS than arrays of disks.

While it is true that one SSD can match disk array easily in IOPS, it should not suggest that array of SSD will provide larger number of useful IOPS. The reason is prosaic, array of disks provides an abundance of IOPS, many times more than enough for any analytic application. So any additional IOPS are not needed and astronomical number of IOPS in SSD arrays is a solution looking for a problem in analytics industry.

4. SSD are NOT disruptive to disks.

Well if it is true it is not according to Clayton Christiansen definition of “disruptiveness”.  As far as I remember Christiansen defines “disruptiveness” as technology A being disruptive to technology B when all following holds true:

  • A is worse than technology B in quality and features
  • A is cheaper than technology B
  • A is affordable to a large number of new users to whom technology B is appealing but too costly.

SSD-to-disk pair is clearly not true for any condition above so I’m puzzled how one can call it disruptive to disks?

Again. I’m not claiming that SSD or flash-memory is not disruptive to any technology I just claiming that SSD are not disruptive to HDD. In fact, I think flash-memory IS disruptive to DRAM. All three conditions above hold for flash-to-DRAM pair. Also a pair of directly attached SSDs are highly disruptive to SAN.

—————

Make no mistake I’m a true believer in flash-memory as a game-changer for analytics just not in the form of disk replacement. I’ll explore in my upcoming posts the ideas where flash memory can make a change. I know I totally missed any quantification proofs for all the claims above but…. well…. let’s leave it for comment section.

Also one of best coverage of flash-memory for analytics (and not coming from a flash vendor) is of Curt Monash on DBMS2 blog: http://www.dbms2.com/2010/01/31/flash-pcmsolid-state-memory-disk/


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