Most of your data is accessed once and then never again. Storing it on fast, expensive storage forever is just burning money. Hot, Warm, Cold The canonical model is three tiers based on access frequency. Hot storage (SSD-backed, high IOPS) handles recent data that’s accessed constantly. Warm storage (standard HDD or S3 Standard-IA) holds data accessed occasionally. Cold storage (archival, like Glacier) holds data that might never be touched again but legally must be retained.

You save a 200 MB file. One word changed. Re-uploading 200 MB to sync that change is absurd. Delta sync is how you avoid it. The Core Idea Split the file into blocks. On an update, compare the new version’s blocks against the stored version’s blocks. Transfer only the blocks that changed. Rsync pioneered this. It computes a fast rolling checksum for each block on the remote side, sends those checksums to the client, the client finds which local blocks match and which don’t, and transmits only the mismatches.

Two users upload the same 50 MB file. Naive storage keeps two copies. Content-addressable storage keeps one. What “Content-Addressable” Means Instead of locating data by where it lives (a path, a filename), you locate it by what it is. Hash the content, use the hash as the key. Same content, same hash, same storage location. SHA-256 a file and store the result as its address. The practical consequence: deduplication becomes automatic.

A user hits Ctrl+Z forty times and expects to land exactly where they were yesterday. That is not just undo. That is a complete audit trail of every edit, stored efficiently, queryable at any point in time. The naive approach: store a full copy of the document after every change. Works for ten users. Collapses at ten thousand. Deltas, Not Copies Instead of storing full document state after every edit, store only what changed: the operation (insert 3 chars at position 12, delete 5 chars at position 20).