You can enable GNA only if 20 percent or more of the nodes in the cluster contain at least one SSD and 1.5 percent or more of total cluster storage is SSD-based. For best results, before enabling GNA, make sure that at least 2.0 percent of total cluster storage is SSD-based.
Even when enabled, GNA becomes inactive if the ratio of SSDs to HDDs falls below the 1.5 percent threshold, or if the percentage of nodes containing at least one SSD falls below 20 percent. GNA is reactivated when those requirements are met again. While GNA is inactive in such cases, existing SSD mirrors are readable, but newly written metadata does not get the extra SSD mirror.