The Benefits of Hybrid Arrays over All-Flash Arrays - EnterpriseStorageForum.com

The Benefits of Hybrid Arrays over All-Flash Arrays

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All-flash arrays offer unbeatable storage performance, but a comparison of the benefits of hybrid array vs flash arrays show that hybrid storage solutions should not be written off on performance grounds alone.

Hybrid vs SSD Performance Comparison

To make an SSD vs hybrid array comparison, it's necessary to understand the underlying storage media:

  • HDD - The traditional spinning hard disk drive (HDD) has been the staple storage medium for years thanks to its ability to store vast amounts of data at a very low cost per GB. The major drawback to HDDs is that the performance they offer is low: typically around 75 – 200 IOPS, with a latency of the order of 10ms. The traditional approach to increasing performance is to "increase the number of spindles" -  add more HDDs to an array – and to "short stroke" the disks by using only a small portion of their capacity This results in significantly fewer head movements when accessing random data, and therefore reduced latency, at the cost of much reduced capacity utilization.
  • SSD - Solid state drives (SSDs) containing flash memory provide another solution to the performance problem. SSDs use different technologies, and therefore some offer more performance than others, but any SSD comparison will show unambiguously that  SSDs are faster than HDDs, offering many thousands of IOPS and sub-millisecond latency.

But, inevitably, there are a number of catches. In terms of cost per GB they are many times more expensive, and except at the high end they offer much lower storage density.

  • SSHD - One attempt to combine the benefits of both storage mediums has been the introduction of the solid state hybrid drive (SSHD), a device which combines an HDD with a small amount of flash storage to use as a high speed cache. These still offer a low cost per GB and high storage density, while offering better performance for workloads where a small flash cache is sufficient. They are a little more expensive than conventional drives, but when performance is important this makes hybrid drives worth it. But in a straight hybrid SSD v SSD performance comparison, SSDs are always the winners.

Hybrid and All-Flash Arrays

In an enterprise environment the benefits of these storage mediums can be brought to entire storage arrays to increase performance over the traditional HDD array.


All-flash arrays containing SSDs or other forms of flash storage offer large volumes of storage with high, predictable performance characteristics, while hybrid storage solutions offer more moderate and less predictable performance, at a far lower cost.

The difference between SSHDs and hybrid storage solutions in the form of arrays is that the latter tend not to use SSHDs to provide the benefits of hybridity. Instead they rely on traditional HDDs for the majority of the storage capacity, but add a small quantity of flash storage either to act as a cache for the array as a whole, or to provide a limited tier of predictable high performance storage for applications that require it.

So to summarize the benefits of hybrid v flash arrays:

Hybrid storage solutions

  • Far lower up-front capital cost per GB of storage required than all-flash arrays. This makes them suitable for applications with high storage requirements such as backups and recovery and pure file storage.
  • Provide a performance boost in low transaction environments where a small layer of high speed flash storage can make a big difference.
  • Storage density is not significantly lower than an HDD array.

BUT:

  • Performance can be unpredictable and can deteriorate significantly when the storage resources are heavily utilized.
  • Where too may application compete for limited flash cache resources this may actually reduce the performance of some applications. This means that hybrid SSD v SSD performance comparisons cannot always be relied upon.

All-flash storage arrays

  • Provide higher performance than hybrid storage solutions for high transaction environments and data analytics applications, because of the difference between SSD v hybrid performance.
  • Storage resources can be highly utilized without affecting performance.

BUT:

  • Up front capital cost per GB is much higher than for a hybrid array.
  • Storage density for some systems is lower so they may not be suitable when data center space is limited.

Hybrid Drive vs SSD Performance/Price Comparison

The financial argument in favor of replacing a traditional storage array with a hybrid array is very strong where enterprises have high capacity storage requirements but storage performance is an issue.

That's because a hybrid array with a small proportion (perhaps 5%) of flash storage can offer double the IOPS of an HDD array in many circumstances, while reducing typical latency from 10ms to 4-5ms. That means a hybrid array offering twice the storage performance (in terms of IOPS and latency) of a traditional HDD array may cost just 15% -20% more.

But unless the highest possible storage performance characteristics are required (perhaps for a very high transaction environment) the financial argument for all-flash arrays is more difficult when looking at the up-front costs of the two types of arrays.  

To understand why, it's necessary to take a look at the price/performance characteristics of all-flash in more detail.

  • All flash arrays offer blistering performance  - hundreds of thousands of even millions of IOPS with sub-millisecond latency.
  • All-flash arrays are very costly: a high-capacity HDD array may cost as little as $0.02 per Gigabyte, while an all-flash array costs around $0.35c per Gigabyte. The cost of hybrid storage solutions comes in somewhere between the two.

Current Storage Infrastructure Considerations

Both hybrid and all-flash arrays offer improved storage system performance when compared to traditional HDD arrays, and all-flash arrays in particular can remove any performance bottlenecks that organizations may be experiencing from their storage systems.

But, inevitably, when the storage bottleneck is removed a new bottleneck will appear, sooner or later, elsewhere in the organization's infrastructure.

Likely places include:

  • the storage data network.
  • other storage infrastructure hardware.
  •  the application software which consumes the data.

If that latter is the case then many organizations see that as a positive development, according to Scott Sinclair, an analyst at Enterprise Strategy Group. "They have the attitude that data is so important that  they want it as fast as possible. If the application is the bottleneck because they implement an all-flash array then they can focus on application development and analytics, without the need to worry any more about the infrastructure," he says.


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