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Design Google Drive (File Storage & Sync) at Microsoft

The problem at Microsoft

Functional: upload/download files, sync across devices, share folders, support offline edits, large file support.
Non-functional: reliable sync, large file support, low-latency listing, eventual conflict resolution.
Scale: ~1B users, ~1TB new files/day, files up to multiple GB.
Consistency: metadata (file tree, versions) is strongly consistent; chunk replication is eventually consistent.

Out of scope (covered in detail below)

Real-time collaborative editing (Google Docs), enterprise admin, OCR/search, and file locking — deferred and discussed in the dedicated section.

At Microsoft, this problem is framed around growth mindset, collaborative, product-minded. Expect Microsoft-specific scale numbers and a focus on production readiness and the company's engineering bar.

Capacity estimation

Resource	Estimate	Notes
New files/day	~1 TB	Plus revisions
Chunk size	~4 MB	Balance dedup vs metadata
Metadata	~tens of TB	File tree, versions, ACLs
Block storage	~petabytes	Dedup saves a lot here

Architecture overview

Chunked storage + sync

+--------+  upload chunks  +-----------------+  store  +----------------+
| Client |---------------->| Metadata Service|-------->| Block Storage  |
+--------+                  +-----------------+         | (content-addr) |
                                |   ^                +----------------+
                                |   |  chunk refs
                                v   |
                        +---------------+   change events   +--------+
                        | Notification  |------------------>| Client |
                        | Service       |                   +--------+
                        +---------------+

Files become content-addressed chunks; a notification service pushes change events to clients.

Step-by-step request flow

A file is split into fixed-size chunks (e.g., 4 MB); each chunk is content-hashed (SHA-256).
The client asks the metadata service which chunks it already has (dedup); only new chunks are uploaded to block storage.
The metadata service records the file's chunk list and a new version; block storage stores chunks by hash (identical chunks across users share storage).
The notification service pushes a change event to the user's other devices, which fetch only the missing chunks.
On download/sync, the client reconstructs the file from its chunk list, fetching any missing chunks from block storage.

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