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Linux Containers Authors: Elizabeth White, Liz McMillan, Pat Romanski, Ian Goldsmith, Yeshim Deniz

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Don’t Buy RAID cards for SSD Caching

“Where have you been Dog?”

The IT Dog here again, back from summer sabbatical. Well, it was more like an “election” sabbatical since I spent the last six months working on the campaign to elect Barkington T. Howl, III as President of United Dogs of America (UDA). “Bark,” as his frat brothers call him, turned out to be a little too high-brow and out of touch with the rank-and-file UDA constituency. He lost in a landslide. Anyway, I am now back at my day job, talking about SSDs in the IT marketplace. Did you miss me?

Back to Our Regularly Scheduled Blog Topic

So what is the first thing to write about since coming back? How about looking at a trend in the RAID marketplace – offering RAID controller cards sold with proprietary SSD caching software designed to boost performance over traditional RAID offerings.

SSD Caching RAID Controller Cards

Redundant Array of Independent Disks or RAIDs, as they are affectionately known, have been around for many years. They are a vital component in many IT installations offering data redundancy and performance improvements over standard disk array. With the introduction of SSDs and SSD caching, many RAID controller card manufactures have updated their product offering to include the ability to run SSD caching algorithms on the RAID controller card itself. Examples of this in the market place include LSI MegaRAID controller card with CacheCade SSD caching software and Adaptec Series 7 controller cards with maxCache SSD caching software. The basic idea here is to buy the controller card from a particular vendor and use the SSD caching software they offer that runs only on their controller card.

Show Me The Money

Ok, so I understand the idea. Let’s see if we can figure out if this is the right way to do SSD caching. I am going to talk about LSI’s solution, not because “LSI” is easier to type than “Adaptec”, but because there happen to be some independent test results published on the web by Demartek for LSI MegaRAID with CacheCade. I am going to try and decipher just what the results are telling me. You can click on the link to read the entire report which documents test set-up etc. I am just going to discuss one chart presented to see what the fuss is about.

Figure 1 below is from page 8 of the report. It shows throughput in Megabits per second for a 90 minute web server test for the baseline system with no SSD caching and for the same system with SSD caching using one or two Intel X25-E 32GB SLC SSDs. The chart shows the baseline system without SSD caching maxed out at about 58 Mbps and that using one SSD and SSD caching, the performance improved to approximately 211 Mbps. Pretty nice. 3.6X improvement. And, with 2 SSDs for caching the throughput improved to 416 Mbps. 7.1X improvement! Excellent.

But let me dig into this a little more. The first thing I am trying to understand is just how they could get only 58 Mbps out of the baseline system. Remember now this is MegaBITS per second, not MegaBYTES per second. I am not a RAID controller guru, but I would have expected the baseline performance to be greater than 7.25 Mbytes/s. A quick internet search of RAID performance led me to this ZDNet page which listed a test RAID performance in MegaBYTES/s ranging from 64 to 257. So the baseline figure for the LSI test is suspect and therefore 3.6X or 7.1X improvement of really bad performance is not that impressive. But, if we ignore ___X improvement and just look at the data – 211 Mbits/sec (that’s 26 Mbytes/sec to you and me) is nothing to be too excited about.

Figure 1. LSI MegaRAID with CaceCade Demartek Throughput. Source: http://www.demartek.com/Reports_Free/Demartek_LSI_CacheCade_Performance_...

Velobit SSD Caching Software Results

So, I was trying to come up with an apples-to-apples comparison for the RAID SSD caching test data shown above but unfortunately I don’t have data which exactly duplicates the LSI test. However, Demartek performed system testing using Velobit HyperCache SSD caching software under different conditions which can be used to provide some general comparison observations.

Figure 2. below is taken from page 12 the Velobit Demartek report. The chart title is confusing: “Average MBPS – Linux without RD2”. When translated to English, the title means: Average MegaBYTES/s of a Linux based system running the vdbench workload test. The ‘without RD2’ part of the title means that there is a second chart in the report with a test called ‘RD2’ (mostly read operations test) whose results (more than 3200 Mbytes/s) cause the other test results shown in the first graph on P12 to be compressed and difficult to read. So the RD2 test results are removed for this graph (see the report) to zoom in on RD1, RD3 and RD4 results. Well, that was a lot to explain for a chart title, sorry about that.

Anyway, several takeaways (don’t you just hate that word) from this data:
1. The baseline performance for each test ranged from approx. 20-35 Mbytes/s (Mbytes!). Much higher than the baseline for the LSI tests (7.25 Mbytes/s).
2. The Velobit Hypercache performance ranged from 200 to 275 Mbytes/s (that is 1600 - 2200 Mbits/s to keep the same units as the LSI results)
3. These tests results also show testing results of FlashSoft SSD caching software because FlashSoft was available at Demartek for comparison purposes. By the way, FlashSoft outperforms the LSI CacheCade software significantly also.


Figure 2. Velobit HyperCache SSD Caching Software Demartek Test Results. Source: http://www.velobit.com/Portals/106427/docs/demartek_velobit_ssd_caching_...

Conclusion

This was a long and winding road to try to make a simple point. RAID systems and SSD caching are two independent components within your IT system. If you have to go with a RAID solution for redundancy reasons, don’t get lured into the illusion that you can solve two problems with one product: you do need to have your RAID solution also be your SSD caching solution. It may seem easy to combine RAID and SSD caching for you, but if you try to solve two independent ‘problems’ with one product, you may not be getting the best performance for either problem. SSD caching enabled RAID controller cards:

  • tend to be more expensive than standard RAID controller cards
  • limit your SSD caching software solutions to software compatible with that card (vendor lock-in)
  • do not perform as well as other SSD caching options


The benefit of using RAID based SSD caching is the software runs on the RAID card and does not consume any server CPU/memory resources. However, the benefits don’t seem to be worth the cost and performance hit you take by using this solution.

Read the original blog entry...

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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