Tag: Hardware

Greenplum and Teradata: Simliar Architecture, Different Strategies

Hardware systems, servers and network fabric, provide the foundation upon which all shared-nothing database management systems rest. Hardware systems are a major contributor to the overall price/performance and total cost of ownership for a data warehouse platform. This blog considers the hardware strategies of EMC/Greenplum: applying the idea of using common, off-the-shelf (COTS) components to build a competitive foundation; and Teradata: developing a proprietary hardware system by tightly integrating components.

Strategies

The Teradata hardware strategy is simple to describe. They expend R&D dollars to couple low-level technologies into a tightly integrated system. Their servers are custom-designed within a set of guidelines that allows both the LINUX and Microsoft Windows operating systems to execute there. Their network fabric is highly proprietary, using cycles within the fabric to offload sort/merge data processing from the server CPU.

In other words, Teradata believes that the time and effort required to engineer an integrated proprietary offering will improve the performance of their offering enough to offset the cost.

EMC and Greenplum have taken a different approach. They have elected a strategy that leverages off-the-shelf servers offered by hardware vendors like Dell or HP, and network switches from vendors like Brocade, Arista, and Cisco. They have elected to expend few dollars on hardware design and development and to leverage the R&D investments made by these other vendors. In other words, Greenplum believes that the advantages in price and performance provided by using off-the-shelf hardware provides a sustainable advantage.

Price

The lower costs associated with Greenplum’s strategy clearly provide an advantage. Greenplum does not have to expend to design and manufacture custom hardware. The manufacturing costs may not be significant, but the staff costs required by the Teradata strategy must affect the price. Clearly the Greenplum strategy provides an advantage on the price side.

Performance

The Teradata strategy has to be about performance… so lets speculate:

  • How much of a performance increase might their integration provide on the server-side?
  • How much of a performance increase might their integration provide on the network side?

In the days before there was a microprocessor based enterprise server market, Teradata could gain substantially here. Microprocessors were built for personal computing and not designed for the high-availability and high-performance requirements in an enterprise. Teradata had much to gain from building rather than buying server.

But today, there is little to gain from a highly customized design. The requirement to run standard LINUX and Windows operating systems limit their ability to innovate and the resulting servers have to be very similar to those built for off-the-shelf enterprise servers. There is little or no performance advantage here.

On the network side, there once was a distinct advantage to Teradata’s ByNet. It was both faster than available off-the shelf switches and it offloaded cycles from the under-powered CPU. Today, however, there are plenty of cheap, fast switches… so the speed advantage has disappeared. Worse still, the introduction of multi-core CPUs have eliminated the advantage of the in-the-switch sort/merge that makes ByNet unique. CPU is inexpensive these days.

The bottom line: it is unclear if the Teradata hardware strategy affords them a performance advantage.

Cost of Ownership

An argument could be made that supporting COTS hardware is inherently less expensive than supporting a Teradata cluster. But there is a more substantial savings that is clear.

Every 2-3 years, as newer Teradata technology obsoletes your currently installed cluster the value of the current hardware goes to zero and the cost of ownership goes up significantly. The costs of this are especially high when you are required to add several nodes to accommodate growth as Teradata refreshes their technology. You may have to buy servers that are already obsolete.

With Greenplum, your current cluster is built from general-purpose servers that are re-purposed with ease. In fact, since the nodes in a Greenplum cluster are usually high-end servers, customers often cycle new technology into their data warehouse and cycle the old servers out into their server farm. The result is a higher performance warehouse and full use of all of the server technology.

A Final Word

The words “proprietary hardware” are sometimes thrown around as an insult. But Teradata’s proprietary approach is based on the belief that a tightly integrated configuration adds benefit to offset the costs. Greenplum believes that today the enterprise server and the network switch vendors have matured their products to the point where off-the-shelf technology can match or exceed the performance of custom hardware… at a significantly reduced cost. You may have an opinion or you may wait to see how the benchmarks, the proof-of-concepts, and the market decide… but its interesting to understand the differing approaches.

Stop Tuning and Scan…

After years of tuning data warehouses, queries, data loads, and BI applications, I give up. In the long run it is not really possible anyway… and better still… no longer necessary. A better approach is to build your database and your hardware infrastructure to scan fast and smart. So here’s a blog on why it’s impossible to tune a warehouse… and on why it’s no longer necessary.

My argument against tuning is easy to grasp. By definition a data warehouse serves many constituencies: Marketing and Finance and Customer Support and Distribution; and these business units will each access the data from their unique perspective following a unique path through the warehouse. A designer cannot lay out the data effectively to support each access path… cannot index every column, cannot map more than one zone, cannot replicate the data again and again with aggregates and materialized views, cannot cache the entire warehouse. Even if you get it right changing business requirements will fracture your approach; or worse, the design will not support new queries and constrain your business.

Many readers will be skeptical at this point… suggesting that the software and hardware to eliminate tuning does not exist. So let’s build a model and test the state of the art.

Let us imagine and model a 25TB data warehouse with a 20TB fact table that holds 25 months of daily facts partitioned by day. The fact table is 100 columns wide and we will model two queries that reference 20 of the columns… One that touches every row and one that is date constrained and touches only 14 days of data.

Here are some hardware specs. A server with a single I/O controller can read about 1.5GB/second into the database. With two controllers can read around 2.7GB/sec. Note that these are not the theoretical limits of the hardware but real measurements taken from the current hardware on the market: Dell, HP, and SUN/Oracle.

Now let’s deploy our imaginary warehouse on a strong state of the market multi-core server with, to be conservative, a single controller. This server would scan our fact table in around 222 minutes. Partition elimination would allow the date constrained query to complete in just over 4 minutes. Note that these imaginary queries ignore the effort to join and/or aggregate data. Later I’ll have more to say on this…

If we deploy our warehouse on a shared-nothing cluster with 20 nodes the aggregate I/O bandwidth increases to 30GB/sec and the execution times for our two queries improves to 11 minutes and 12 seconds, respectively. This is the power of parallel I/O.

Now we have to factor in compression. Typical row-based compression yields approximately a 2.5x result… columnar compression varies wildly… But let’s assume 25X in our model. There is a cost to be paid to decompress the data… But since it is paid by everyone and CPU is a relatively inexpensive commodity, we’ll ignore it in our model.

For 2.5X row-based compression our big query now completes in 4.4 minutes and the smaller query completes in 4.8 seconds.

The model is a little more complicated when we throw in columnar compression so let’s consider two columnar models. For an implementation such as Exadata we get the benefit of columnar compression but not the benefit of columnar projection. 25X hybrid columnar compression will execute our two scans in 26 seconds and .5 seconds. Now we are talking! A more complete columnar implementation will only touch the columns required by our query, 20% of the data, providing another 5X improvement. This drops our scan queries to 5.2 seconds and .1 second, respectively. Smoking fast. Note that the more simple columnar compression approach will provide the same fast response when every column is touched and the more complex approach will slow down in that case… so you can make the trade off in your shop as required.

Let me remind you again… This is a full scan of 20TB with no tricks: no indices, no pre-aggregation, no materialized views, no cache and no flash, no pre-sorted zone maps. All that is required is a parallel implementation with partitioning, compression, and a columnar table type… and this implementation works. It is robust.

A note on joins… It is more difficult to model joins… and I’ll attempt a simple model in another post. But you can see that this fast scan approach has solved the costly part of the problem using parallel processing… and you can imagine that a shared-nothing massively parallel approach to joins may hold the key.

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