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Hybrid storage vs. All-flash arrays for enterprise storage

Posted: 19 Mar 2015 ?? ?Print Version ?Bookmark and Share

Keywords:NetApp? hybrid storage? flash? AFA? SSD?

Flash or SSDs have become the leading storage of choice of next-gen applications. With performance as the top benchmark, there is still the never-ending quest of finding the 'faster' technology. Nonetheless, is it better to add an all-flash array (AFA) to an existing SAN, or just upgrade the existing arrays by adding SSDs?

This was an easier decision when AFAs were expensive and had just a few terabytes of capacity. They were economically infeasible as primary storage and found their first market as caching devices in front of the SAN. At the same time, apps still weren't optimised for high-speed, low-latency IO.

The result was a new class of hybrid arrays, with a few SSDs and a lot of hard disk drives. Data in these appliances can be segregated by setting up LUNS and then using auto-tiering software to migrate data back and forth. Cost and the capabilities of the array limited the SSD storage to perhaps four or eight drives, typically in a RAID 1 configuration.

In many ways, hybrid arrays have the characteristics of bolting a V12 engine into a low-end sedan. Enterprise SAS SSDs can deliver 500K or more IOPS. Most RAID controllers struggle above one million IOPS, and older arrays can't sustain anywhere close to that number. Another issue is the response latency through traditional RAID controllers and fibre-channel networks. This was good enough for a bunch of hard drives (that manage around 150 IOPS each), but inadequate for SSD speeds.

Array vendors had to decide whether to speed up the controllers to cope with SSD or to look at alternatives. Second-generation all-flash arrays made the decision essentially moot. These units took advantage of faster, denser flash devices and got much bigger and faster.

At the same time, features such as auto-tiering connected these units more effectively to the traditional hard-drive arrays, largely reducing the drudgery associated with storage management. With all-flash arrays adding a compression option for the hard drive arrays, there are savings to offset acquisition cost of the AFA, mainly in not having to expand HDD capacity to cope with storage growth for at least a couple of years. Taken together, these factors make the story for the all-flash array very strong against the hybrid approach.

With the playing field tilting so strongly towards the all-flash array, one would expect the vendor side to provide all-flash appliances. It seems the industry was somewhat in denial, and only EMC and IBM among the traditional large storage vendors have a true AFA. Dell, HP and HDS offer souped-up hybrid arrays with some optimisation for flash. Typically, these are marketed as all-flash arrays since they can be loaded up with just SSD, but this is more spin than reality.

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