Tuning Flash in a Hybrid Storage Array
One of the most common architectural choices for deploying flash has been to emulate hard disk drivess (HDDs) on a storage array. Although that improves the overall storage performance of the array, treating flash as if it were simply a faster hard disk does not take full advantage of its essential features and capabilities. Let’s consider why.
A traditional scale-up storage array, such as an EMC VNX mid-tier storage array, is really an enclosed system with servers, networking and storage all interwoven together. The storage controller (actually dual controllers) that sits in front of the physical storage provides a lot of CPU firepower, along with sophisticated software that includes a storage-oriented customized operating system (OS).
Storage software can perform many tasks such as snapshot management, but its primary task is to maximize the performance of storage-related functions in the array (such as IOPS). Vendors have spent many years and millions of dollars to provide the value-added tuning needed, either through hardware upgrades, such as faster processors or internal networking connections, or better software, such as algorithms to manage the controller cache. Introducing flash storage into an array created a major wrinkle, as all those hardware and software resources were not tuned to take advantage of the significant difference between a chip-based flash and the electromagnetic mechanical (spinning) disk HDDs.
With its launch of the newest VNX models, EMC redesigned hardware and software for VNX with flash as the foundational component. One of VNX’s major new features is Multicore Code Path Optimization (MCx), which helps the system take better advantage of the multithreading capabilities of Intel’s Xeon 5600 (Sandy Bridge) multicore CPUs that optimize how flash storage is accessed without the mechanical constraints--such as the movement of read-write heads--of HDDs. MCx is new software that evenly distributes all VNX data services across all cores in a system.
[IBM's new FlashCache Storage Accelerator uses software to boost flash performance. Read how it could signal a trend for the industry in "Speeding Up Flash Storage Via Software."]
EMC says buyers of new VNX models can enjoy significantly improved price/performance over current models: either the same performance for one-third the cost or up to four times faster performance at a modest premium.
EMC VNX: Lessons Learned
EMC executives said the company's VNX development efforts led it to draw several conclusions about flash storage:
• A little flash is generally enough. Although there are exceptions, including specialized business use cases that require much higher performance, EMC used 5% as a rule of thumb for how much flash should be used in a hybrid VNX array. (In fact, the actual average is 4.6%.)
This implies that the totality of general business data tends to have very little “hot” data that truly needs high performance; if one understands the rapid aging process of business information and that most data is fixed content that is rarely, if ever, accessed, this conclusion seems reasonable. EMC can show that this small amount of flash is cost effective from a system perspective, rather than according to price per gigabyte perspective; most storage remains on cost-effective, capacity-focused hard disks.
• Everyone benefits from using flash. Well, perhaps not everybody, but 60% of VNX customers now use flash, and EMC believes this number could go to 85%. Of the remaining 15%, how many do not really have any hot data, and how many are simply resistant to change? EMC believes the current acceptance level of flash (even non-optimized) validates its focus on the technology.
• Optimizing for flash proved difficult. Achieving what EMC has in tuning new VNX systems has not been easy; changing millions of lines of code and getting them to work reliably and robustly was a daunting task. The general availability of new VNX systems means that EMC’s reputation is on the line, so systems must work as advertised.
Enterprise storage is in a major transition, and flash is a main driver that has roiled this market. Trying to make sense of all the different products is challenging. With the new products in its VNX mid-tier product line, EMC has dropped a very big shoe. Even in traditional scale-up, mid-tier storage arrays, EMC feels that everyone can benefit from a little bit of flash in an array that is still mainly composed of traditional hard disks.
Now, one size certainly does not fit all. There are use cases that benefit from other storage architectures. But how all these architectures line up to meet customers’ evolving needs will continue to be an active topic for discussion. That said, with its new VNX offerings, EMC is making the case that taking small sips of flash in traditional scale-up, mid-tier storage arrays can quench the needs of many organizations.
EMC is a client of David Hill and the Mesabi Group.
Find out about the pros and cons of flash storage in Network Computing contributing editor Howard Marks' session "SSDs In The Data Center"at the Interop New York.