Dell EMC spills beans on plans for storage-class memory in PowerMax

Let them eat cache

Dell EMC wants us to understand that it's going to provide PowerMax with its idea of storage-class memory (SCM), and that means more than a fast cache.

SCM is a storage tier, meaning it's non-volatile, faster than basic flash but both slower and cheaper than DRAM. It has, compared to flash, near-DRAM speed and can be addressed by software directly at byte-level with memory (load, store) semantics.

An example of SCM is Intel's 3D Xpoint Optane technology, which is available as a DC P4800X SSD or DC Persistent Memory NVDIMM. The Optane SSD's average read latency is 10,000 nanoseconds (10μs); the Optane NVDIMM is 350ns; DRAM is less than 100ns and an SAS SSD is about 75,000ns. An Optane NVDIMM is about 28 times faster than an Optane SSD and 214 times faster than a SAS SSD.

HPE added Optane caching (Memory-Driven Flash) to its 3PAR arrays last month, and said it lowers latency up to 2X and is up to 50 per cent faster than all-flash arrays, such as Dell EMC's PowerMax, with NVMe solid state drives.

That red rag sparked a response from Round Rock and Dell EMC execs briefed The Register on what was happening with their high-end PowerMax array and SCM, which it has previously said would be supported.

SCM encryption and dual-porting

SCM will be a storage tier in PowerMax, between DRAM and flash, and directly supported by the PowerMax OS. The array will use a low-latency NVMe-oF link to connect to accessing servers and their data access will be faster as a result of SCM tier hits.

Inside the array Optane SSDs will be mounted in the DAEs (Disk or Drive Array Enclosures), which are hooked up to the directors using PCIe. These directors can provide data-at-rest encryption, which is not an Optane drive-level option.

The Optane SSDs will be dual-ported; Intel's DC P4800X is a single-port drive so an Intel drive update is needed. Dual-porting is regarded as a requirement for enterprise-class storage by Dell EMC, providing resiliency against single port failures.

Each port will be used by PowerMax and each will have its own queue, meaning more IO processing. The PowerMax software can use separate queues for reads and writes or small block requests and large ones.

It can also use different directors for each port. This can make RAID groups more efficient, with two directors working on drive rebuilds instead of just one. At the 7.2K disk drive level a two-director rebuild takes about 2.5 hours, three times faster than the 7-8 hours needed by one director. We can expect the same relationship with Optane drive rebuilds.

SRDF (Symmetrix Remote Data Facility) offers another replication-based layer of protection.

Data placement for the SCM will be driven by the PowerMAX's machine learning algorithms.

Optane DIMM support

Dell EMC is also looking at PowerMax supporting Optane DC Persistent Memory (Xpoint DIMMs) and that will be another and faster tier of storage-class memory than the Optane SSDs. PowerMax supports a memory today, with power-protected NVDIMMs.

When PowerMax gets CPUs that support Optane DIMMs, and those DIMMs are widely available, then the PowerMax OS is ready to make use of them.

We can envisage PowerMax having fast and (relatively) slow SCM tiers formed from Optane DIMMs and SSDs, then slower NVMe flash SSDs and slower still bulk-capacity disk drives as the base in-array tier. It may still have non-volatile DRAM DIMMs but they may become effectively redundant when Optane DIMMs arrive in PowerMAX.

Caitlin Gordon, VP Marketing for Dell EMC storage, tells us this is how NVMe, at both drive and fabric level, and SCM should be implemented: "Most SCM implementations today are cache. We are using it as a persistent tier."

We could hope for an SCM'd PowerMax to have halved latency and 50 per cent faster IO than existing PowerMax, maybe more on both parameters.

Mid-range SCM support

PowerMax SCM support will arrive in 2019. Dell EMC isn't willing to be any more precise about the time scale. Not surprising as Intel's dual-port Optane timescale is a dependency. Neither is it willing to talk about when Unity, SC or XtremIO might get SCM.

HPE has said that its Nimble arrays will get SCM support in 2019; presumably caching-based like 3PAR.

Our read of this is that a Dell EMC new mid-range system is coming, to be called PowerSomething, with an upgrade path from the existing mid-range systems. SCM support might be added to that single system rather than to the existing mid-range. ®

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