Instead of introduction: a fair summary at TheRegister.
This, and many other writings on the topic reiterate the pros and cons, restate the importance of considering your use case and your workload, mention the cost per IOPS and cost per TB. The latter, by the way, ranges between $70/TB (low-end SATA) to thousands and tens of thousands (high-end all-flash array).
To be fair, per IOPS price for those high-end arrays still beats 7.2K RPM SATA by about 20 to 100 times.
In the meantime, tiering in the last couple years went out of vogue, looks like. In part and specifically this (coming out of vogue) relates to automated within-LUN (sub-LUN or intra-LUN) and within-target hot <=> cold tiering by storage array transparently for the users and applications. Case in point: Compellent’s Data Progression, 3PAR’s Dynamic Optimization of the previous decade.
And many others. We don’t hear about this kind of tiering, the “automated data migration: much anymore.
The reason, one might say, is that SSDs become better and cheaper, denser and more reliable at the rate set by Moore’s law back in 1965. The real reason, I say, is different: hybrid is tough. Designing, building and delivering storage software to optimize IO datapath across hybrid array – is a rocket science.
Let’s make a mental experiment though (a leap of faith, really) and assume there’s a piece of such storage software, something like this when looking from 30,000 ft:
Wouldn’t we want then to combine excellent read performance (whereby active data timely and on-demand migrates to SSDs) and total capacity, while at the same time using the same flash tier for user writes, to complete them at flash latencies?
Of course we would.
The fact that an optimal and stable hybrid storage array(*) is not done yet does not mean that it cannot be done. Yes – the corresponding software is racy and hard to debug (**). More importantly, it is tough to test across exploding number of use cases. But – it is doable. And once it is done and demonstrated, it might as well change the perspective and the common story line..
Hybrid storage arrays or all-flash?
(*) Term “hybrid” is used in many different contexts. On the drive level, there are hybrid SSDs, HDDs and even SSHDs. At the opposite extreme storage administrators build hybrid solutions combining all-flash arrays (the proverbial “primary tier”) and archival Cloud services. Hybrid storage array, hybrid volume (LUN) – is somewhere in the middle..
(**) Disclaimer: some of the related work is being done at Nexenta, and will be part of the next-gen release of the storage appliance..