The issue described is frequent read cache misses on an HPE Nimble Storage hybrid array (CS3000). In the Nimble Cache Accelerated Sequential Layout (CASL) architecture, flash storage (SSD) is used as a read cache to accelerate performance for frequently accessed "hot" data. Frequent cache misses indicate that the active working set of data is larger than the available flash cache. To resolve this, the size of the flash cache must be increased to accommodate more of the working set, thereby improving the cache hit rate and reducing read latency from the slower disk (HDD) tier. Increasing the flash-to-disk ratio is the direct method to expand this cache.

A. Upgrade the setup with additional expansion shelf.

This action adds more HDD capacity, not flash cache. It would likely decrease the flash-to-disk ratio, potentially worsening the cache miss problem for a larger data set.

B. Upgrade the controllers to Alletra 5050 models.

While a controller upgrade provides more CPU and memory, it does not inherently increase the flash cache size, which is the direct cause of read cache misses.

C. Increase the pinned cache size.

Pinning guarantees cache residency for specific volumes but reduces the dynamic cache available for all other workloads, which could exacerbate the cache miss issue for unpinned data.

1. HPE Nimble Storage Architecture White Paper (a00001843enw):

Section: Read Cache, Page 8: "The read cache is populated on demand. When a random read is requested and the data is not already in cache, it is retrieved from disk and simultaneously written to the read cache and served to the host... The size of the flash cache required is a function of the application’s active data set size." This directly supports that a cache sized appropriately for the active data set is required to avoid misses. Increasing the flash component (Option D) achieves this.

2. HPE Nimble Storage - Sizing Storage for VDI Workloads Technical White Paper (4AA5-7601ENW):

Section: Sizing Cache, Page 6: "For Nimble Storage hybrid arrays, cache is the most important factor for VDI performance... If the working set does not fit in cache, read requests will be served from spinning disk, which will result in higher latency." This document, while specific to VDI, explains the universal principle for Nimble hybrid arrays: insufficient cache for the working set leads to misses and high latency. The solution is to increase cache, which aligns with increasing the flash-to-disk ratio.