Does Intel’s Optane DC Persistent Memory decrease TCO for SAP?
When this new type of persistent memory DIMM (PMEM) was announced by Intel about a year ago, improving restart times was the most important factor cited by Intel and vendors of systems that utilize Intel Cascade Lake processors. Some of my previous blog posts have discussed the performance issues of PMEM and despite numerous searches, I can find no data presented by Intel or any other vendor to suggest that any improvement has occurred since this technology was made generally available. Over time, and perhaps as more customers realized that faster restarts at the cost of slower operational performance might not be very compelling, the message started to morph into saving money.
Regarding TCO specifically for SAP Suite on HANA (SoH) and S/4HANA, let’s start with the basic assertion, i.e. PMEM is less expensive than DRAM. This is documented by pricing which shows a 128GB PMEM DIMM costs approximately 60% of the cost of a 128 DRAM DIMM[i] on one site and 40%[ii] on another site. This discrepancy may result when one vendor shows effective prices and another list prices with the list price example showing a higher cost savings with PMEM.
I was interested to see what would happen with actual SAP instances. For comparison, let us start with a conventional DRAM memory system and assume that after using appropriate sizing tools, we have determined that an SoH or S/4HANA system requires a total of 6TB of memory to support 3TB of data with 3TB dedicated to system and HANA working memory. I chose 6TB because this fits perfects on most Intel systems using 4 processors and 48@128GB memory DIMMs. This config also has the added bonus of no waste at all and maximized performance since parallelism is optimized when every memory channel is used.
By comparison, we need to figure out how much memory is required if we utilize PMEM. The SAP note on persistent memory[iii] describes ratios of DRAM to PMEM ranging from 2:1 to 1:4. For SoH and S/4HANA, the advice given is to run QuickSizer, /SDF/HDB_SIZING or ZNEWHDB_SIZE depending on where you are starting from. I asked 3 different customers, one small, one medium and one very large, to provide me with the output of their sizing reports based on existing ECC systems. I have included two key sections for the midsized customer:
The Persistent Memory FAQ[iv] says: “Persistent memory can be used for the main storage of column store table that is typically the dominating factor of data space consumption in SAP HANA environments. Other areas like delta storage, caches, intermediate result sets or row store remain solely in dynamic RAM (DRAM). Disk LOBs (SAP Note 2220627) are also not part of the persistent memory.” If you add up the numbers above using this rule, you may notice that this means that when using persistent memory, the amount of data housed in PMEM vs. DRAM does not fit with any of the ratios mentioned earlier. Looking at the sizing reports that I obtained, the amount of PMEM vs. DRAM was more in the range of 1:1.5[v].
Now, let’s apply the very best ratio of the three reports, i.e. the very large customer, to our 6TB example above and we see that we need 6TB x .433 = 2.53TB PMEM and 6TB x .567 = 3.47TB DRAM. Assuming 128GB DIMMs, this translates to 20.2 PMEM DIMMs and 27.8 DRAM DIMMs which rounded up comes to 21 and 28 DIMMs, i.e. 49 DIMMs total. Clearly, this is one more than the max number (48) in a 4-socket system. In addition, SAP note 2786237 states that a configuration must have: “Homogeneous symmetrical assembly of DRAM and PMEM DIMMs with maximum utilization of all memory channels per processor”, so the minimum configuration would be 28 of each type of DIMM for a total of 56 DIMM slots.
To the best of my knowledge, no Cascade Lake system supports this number of DIMM slots. Several vendors support 64 DIMM slots on a 6 or 8-socket system. Those that do not would require a 96 DIMM slots configuration. At 64 DIMM slots, this configuration would waste the difference between the HANA memory requirement and the system configuration requirement, i.e. 540GB of DRAM and 1,508GB of PMEM would be wasted. At 96 DIMM slots, the waste would be 2,588GB and 3,556GB respectively. With either a 64 DIMM slot or 96 DIMM slot configuration, instead of a relatively affordable 4-socket system, a significantly more expensive 6 or 8-socket system would be required.
I chose to use the best pricing that I could find for DIMM prices assuming that other vendors would be able to match these prices. I then applied that pricing to those vendors that can utilize 64 DIMM slots on a 6 or 8-socket configuration. After a simple calculation[vi], the cost of just the memory of the DRAM+PMEM system came out $7,648 higher than the DRAM only system. And remember, this is before adding in any additional costs for more processors and a system which can support more processors.
Of course, 256GB DRAM DIMMs could be used, reducing the DRAM DIMM count to 15, but this raises a thorny issue; No appliance has been certified by SAP[vii] with 256GB DRAM DIMMs. Even if we ignored that issue and went out on a limb using TDI V5 relaxed rules, the significantly higher cost of 256GB DRAM DIMMs over 128GB DRAM DIMMs[viii] plus the need to round up to 24 DIMM slots would result in a configuration that was still substantially higher cost than the DRAM only configuration.
Any way that you cut it, the use of PMEM in a realistic SoH or S/4HANA configuration results in a higher cost of acquisition than a DRAM only configuration. In other words, as shown in the previous blog posts, performance takes a major hit when using PMEM for HANA, it does not save any money and actually costs more and the only potential gain comes from faster restarts.
SAPonPower 
Excellent blog Alfred. Thank you for sharing.
I really appreciate the way you can cut through the marketing and get to the real world Alfred. I will make a point of sharing this with my colleagues.