How Effective is Your Adaptive Flash Cache?

Have you ever wondered whether you should enable Adaptive Flash Cache on your HPE 3PAR?

Adaptive Optimization (AO) is HPE 3PARs automatic tiering solution. It provides the user with several performance and capacity related parameters for influencing the behavior of the automatic tiering. I covered this in detail in this whitepaper about HPE 3PAR AO. One of the findings from that study was that in this particular customer’s environment there were too many I/Os on the 450 GB 10K RPM drives and there were not enough I/Os on the SSDs. The result was that the 450 GB 10K RPM drives were running at nearly 100% busy all the time. My suggestion was to enable Adaptive Flash Cache (AFC)  by allocating some of the under-utilized SSD capacity. AFC supplements DRAM with NAND flash devices to cache small (<64 KB) frequently accessed read blocks and ultimately to improve read response time.

The workloads, in this case, were VMWare, and an online database system. It should be noted this blog was written for a specific workload with specific AO policies and will not apply as a general guideline.

Here are the key points of this customer’s example:

• 450 GB 10K RPM drives had too much workload
• SSDs didn’t have enough workload
• Original data was captured on March 9^th, 2015 – No AFC
• New comparative data was captured on December 7^th, 2015 – AFC enabled with 1,024 GB of SSD

Figure 1 shows the overall Back-end Drive Rate. The overall rate was 3.61% lower on Monday, December 7^th, 2015, as compared with Monday, March 9^th, 2015, but close enough for this comparison.

Figure 1: Back-end Drive Rate

 

As you can see from the distribution of workloads in figure 2 on the 450 GB 10K RPM drives for the dates of March 9^th compared to December 7^th workloads, there was a 9.18% decrease in I/Os on the 450 GB 10K RPM drives. This was not as drastic as we had hoped, but does consist of a measurable improvement.

 

Figure 2: Drive I/O Rate for 450 GB 10K RPM drives

 

Figure 3 shows the distribution of workload on the 900 GB 10K RPM drives for the dates of March 9^th, as compared with December 7^th. There was a 34.1% decrease in I/Os on the 900 GB 10K RPM drives. We were not trying to reduce the load on the 900 GB drives, so this was an unexpected find. Apparently, a healthy chunk of the I/Os on the 900 GB drives was well suited for the AFC.

 

Figure 3: Drive I/O Rate for 900 GB 10K RPM drives

 

When comparing the SSDs, you can see in figure 4 that the workload on the SSDs increased dramatically: 90.85% to be exact. This included SSD space for AO policy as well as AFC usage.

 

Figure 4: SSD Drive I/O Rate

 

In summary, the HP AFC feature was effective at moving a significant amount of the workload previously on spinning drives to the SSDs. In my opinion, there are still far too many I/Os on the 450 GB drives and this could potentially be an issue later as the workload grows. If you would like to understand the effectiveness of your AO policies or your AFC, contact us.