Dell Technologies recommends the following L3 cache best practices:
- Use a small number (ideally no more than two) of large capacity SSDs rather than multiple small SSDs.
- Use the appropriate SSD capacity that will fit your working dataset. The isi_cache_stats utility can help to determine the appropriate capacity on existing clusters. A good guideline is to size L3 cache SSD capacity per node according to the following formula:
L2 capacity + L3 capacity >= 150% of working set size
- While L3 cache can potentially use up to a 2:1 ratio of hard drives to SSDs per node, use at most two to three SSDs per node for L3 cache.
- Repeated random read workloads typically benefit most from L3 cache through latency improvements.
- Although not recommended, both L3 cache and Global Namespace Acceleration (GNA) are supported within the same cluster.
- The same procedure is used for replacing failed L3 cache SSDs as for other storage drives. However, L3 cache SSDs do not require FlexProtect or AutoBalance to run post replacement, so it is typically a faster process.
- For a legacy node pool using a SmartPools metadata-write strategy, do not convert to L3 cache unless:
- The SSDs are seriously underutilized.
- The SSDs in the pool are oversubscribed and spilling over to hard disk.
- Your primary concern is SSD longevity.