Hard disk vs. flash storage: The fight of the century?
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The pre-eminence of flash as the performance storage of choice in today's data center is undisputed, whether it...
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comes in the form of an SSD or vendor-specific custom flash module, and is located in the server, all-flash array or hybrid array.
Flash data storage performs up to three orders of magnitude faster than HDD storage, and has up to two orders of magnitude better reliability based on unrecoverable bit error rate (UBER). That is why flash makes sense for mission-critical and performance workloads for the vast majority of IT managers.
But what about flash-based storage for primary but non-mission-critical workloads? Using the technology for all virtual and physical machines is becoming increasingly frequent in data centers across the world. Yet there is a pervasive perception by many IT organizations that flash data storage is just too expensive for those cases. But is that an accurate assessment? Based on objective reality, it's not. Here's why.
Those who claim that HDD storage systems using 15K/10K rpm 2.5-inch small form factor drives still cost less than flash SSDs base that claim strictly on purchase price, which is commonly represented as dollars per raw gigabytes. But what about the cost of storage software services, power, cooling, rack space, floor space, maintenance, reliability, availability and serviceability?
Adding in all these factors delivers a more realistic total cost picture represented as dollars per effective usable GB. Failing to include these factors is analogous to saying that the cost of acquiring a new car is only tied to the purchase price and excludes the cost of gasoline, oil, maintenance, tires and insurance.
When you compare flash data storage and HDD storage based on dollars per effective usable GB, the picture changes considerably.
Storage services software
Storage services software is usually bundled with flash-based storage, while it tends to be a separate licensed and maintenance line item for HDD storage. Data reduction technologies also do a better job on flash storage than HDD storage. This may feel a bit counterintuitive since those techs are compute-intensive; however, the metadata used in all of them is kept on the storage medium.
Flash storage latency is 1,000x lower than HDD storage. That means data reduction technologies in flash storage can operate inline at maximum optimization without causing noticeable differences in application response time.
Because data reduction adds latency, HDD latency can be onerous. To ensure applications do not time out and cause the application to crash, data reduction is not tuned for maximum optimization.
IT organizations using both types of storage have yielded an average result of three times better data reduction on flash data storage. Flash commonly averages a 6:1 data reduction rate over a variety of data -- some data reduces much more than others -- while the HDD storage rate averages closer to 2:1. That additional, effective usable capacity means flash storage systems require less raw storage capacity upfront. In HDDs, additional raw storage increases the storage services software licenses that are commonly tied to raw capacity.
Power and cooling
Flash storage consumes considerably less power and cooling resources because it has no moving parts. HDD storage consists of electro-mechanical platters that spin at a very high speed and generate tremendous friction. The average power and cooling consumption for flash-based storage ranges from 50% to 90% less per GB.
Rack and floor space
NAND flash has been gaining density at a much faster rate than HDDs. This means flash 2.5-inch SSDs and custom flash modules have considerably more capacity than equivalent 15K/10K rpm HDDs. A flash 2.5-inch SSD ranges up to approximately 4 TB raw. The largest 2.5-inch 15K/10K rpm HDD is 1.2 TB raw, and these are disappearing from the market.
The more common capacity available today is 600 GB raw. Exceptionally high-write-optimized enterprise 2.5-inch SSDs come in at 1.6 TB raw. If you add in data reduction, the effective usable capacity differences between flash data storage and HDD storage become more glaring. For example, that 1.6 TB raw capacity SSD becomes 7.2 TB effective usable [1.6TB * (1-.25) * 6]. The 600 GB raw capacity HDD becomes 900 GB effective usable [600GB * (1-.25) * 2].
This makes 1.6 TB flash storage eight times denser than the 600 GB HDD. The 4 TB SSDs and 8 TB CFMs are 20 times and 40 times denser, which translates into fewer racks, cabinets and floor tiles. Data center real estate is not cheap. It is usually allocated as fixed overhead assigned per rack unit, cabinet and floor tile. This reduction in storage real estate for flash storage systems is financially significant and will vary significantly between users.
One way to increase an HDD storage system's IOPS is to add more drives. This has led to overprovisioning HDD storage systems with far more capacity than is called for by the increase in data growth. That adds to the cost in several ways.
Maintenance on storage hardware is commonly a percentage of the purchase price. Flash data storage requires less maintenance than HDD storage.
Reliability, availability and serviceability
As previously mentioned, drive-level, enterprise-grade flash storage has an UBER of 10-18; that is 100 times better than the best HDD UBER of 10-16.
HDDs treat an unrecoverable bit error as a failed drive. Flash drives do not. The flash drive controller treats an unrecoverable bit error as a bad cell or program erase (P/E) block and marks it as unavailable.
Rarely do enterprise flash drives completely fail. This is because those drives are overprovisioned. Overprovisioning on the drive allows individual NAND cells or P/E blocks to fail and not reduce the total available capacity. There is no equivalent in HDD storage.
That higher availability means RAID is not a necessity for flash drives. Flash drives do not often require rebuilding. It is more effective to utilize snapshots to rebuild lost data or efficient services such as erasure codes.
Flash storage also delivers a more consistent quality of service, which is rated by the number of nines in the availability percentage. In the case of flash storage drives, it is often as high as three 9s or 99.9%. HDDs are the very definition of inconsistency, and have a difficult time providing any 9s.
Storage is an essential part of the total IT infrastructure underlying applications. Faster, more performance-consistent storage leads to faster time-to-market with development projects, Web rollouts, e-commerce and more.
In most cases, flash data storage comes in with either less than or the same dollars per effective usable GB price than 15K/10K rpm HDDs when compared in total. This is true for all-flash arrays, hybrid arrays and server-side flash. When the revenue side of the equation is taken into account, flash storage is cheap at twice the price.
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