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Challenges with SSD Caching: What Challenges?

What You Should Know About Array-Based SSD Solutions

George Crump from Storage Switzerland recently wrote an article, arguing that it is better to deploy SSDs in a storage array instead of as a server cache.  The article was sponsored by Pure Storage which happens to sell SSD based storage arrays.

George identifies challenges with SSD caching and tiering solutions. While some of the topics he raised in the article are valid, he does not present all sides of these topics.  As a result, the article contains positions that overstate the magnitude of the problems and may lead the reader to the wrong conclusion. I would like to present some additional information on the topics presented in George’s article for the reader to consider when researching SSDs for increasing enterprise IT performance.  

Let’ s look at the topics one-by-one:

Data Analysis “Problem”
George argues that because determining what high-priority data to store in cache is CPU intensive and slow, 100% of data should simply be kept on expensive SSD arrays.  None of the IT managers I have talked to think storing 100% of data on SSD is practical.  Typically a small percentage of data is accessed frequently and hence needs to be in high cost, high speed storage so storing 100% in SSD is wasteful and unpractical.  Further, CPU speed increases have outpaced HDD I/O speed increases for years now.  For I/O bound applications, using CPU cycles to reduce I/O would yield an overall system improvement. For example, VeloBit requires less than 5% CPU load and in return, increases application performance by up to 10x.

The "Miss" Problem
George correctly states that when there is a cache miss, data has to be fetched from slow HDD. His solution (or is it Pure Storage’s solution?) of course is not to cache any data at all but to store all data in an SSD array. The problem with this idea is it has been shown that deploying SSD in the server produces better performance than when SSDs are deployed in the array. Network latency issues will always limit array-based solutions. So in an apples-to-apples comparison (where the costs of the two solutions are equal), hybrid server SSD/array HDD solutions will beat all SSD array solutions. There is no argument here, Fusion IO has proven it; EMC and other large storage vendors have validated it.

With respect to SSD server side cache size topic: Velobit SSD Caching algorithm compresses cached data at line speed, not only effectively increasing the cache size and decreasing miss percentage but also reducing SSD wear with less write data.

The Data Movement Problem
This is an easy one to address. The problem does not exist if SSD is deployed as cache: primary storage remains unchanged

The Write Problem
George is correct that there are concerns with life expectancy of flash-based SSD with respect to limited write cycles. However, if an SSD cache solution like Content Locality Caching is used, data that does not need to be cached is never written to SSD. This reduces the SSD wear problem associated with data tiering.

What You Should Know About Array-Based SSD Solutions

High Cost Of SSD: The biggest issue for customers deploying SSD as a primary store is cost. According to Forrester, SSDs can be up to 10 times more expensive than hard drives. At this cost premium, few customers can afford to use an SSD for all of their data. Placing all of your data on an SSD, as in the “all or nothing” SSD array model, is not a viable option. Performance: As mentioned above, because of the network lags, SSD arrays may offer less performance for the buck than server-side SSD.

Disruption: Deploying an all-SSD array increases disruption, as data must be moved from the array you have in place to the new array. This forklift upgrade is expensive as well. Capital Commitment and Changes to Operations: Finally, purchasing a new SSD array requires a very large commitment — both in capital expense and in changes to operations — in comparison to the “pay as you grow” approach of server-side SSD and transparent, plug-and-play SSD caching solutions like Velobit.

More Stories By Peter Velikin

Peter Velikin has 12 years of experience creating new markets and commercializing products in multiple high tech industries. Prior to VeloBit, he was VP Marketing at Zmags, a SaaS-based digital content platform for e-commerce and mobile devices, where he managed all aspects of marketing, product management, and business development. Prior to that, Peter was Director of Product and Market Strategy at PTC, responsible for PTC’s publishing, content management, and services solutions. Prior to PTC, Peter was at EMC Corporation, where he held roles in product management, business development, and engineering program management.

Peter has an MS in Electrical Engineering from Boston University and an MBA from Harvard Business School.

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