Cost-Effective Log Storage for Long-Term Retention

Quick Answer

Azure Blob Storage's access tiers exist precisely to let cost track actual usage patterns, so the right tier for this scenario depends entirely on how often the data gets touched, which the question tells you directly: rarely, after the first 30 days. The cool tier is built for infrequently accessed but still occasionally needed data; it charges less for storage than the default hot tier, in exchange for charging somewhat more per read, a good trade when reads are occasional rather than routine. The archive tier goes further in the same direction: it offers the lowest possible storage cost of the three, but the data is not immediately readable, since it has to be rehydrated, a process that takes time, before it can be accessed again. That makes archive best suited to data that is kept mainly to satisfy retention requirements and is almost never actually opened, while cool suits data that is still queried occasionally, just not often. Because this scenario describes only occasional analysis, cool or archive can both reasonably apply depending on exactly how infrequent that analysis is. The general principle to carry forward: when a scenario describes data accessed rarely after some initial period, look past the default hot tier and match the tier's access-cost-versus-storage-cost trade-off to how infrequently that data actually needs to be read.