RDB Snapshots

## What a snapshot is An RDB snapshot is a point-in-time binary image of the entire dataset — every key, its value in the internal encoding, and its expiration — written to the file named by `dbfilename` inside `dir` (by default `dump.rdb`). It is compact, optionally LZF-compressed, and carries a CRC64 checksum so a truncated or corrupted file is detected on load. Redis has two commands to produce one on demand, plus automatic save points that use the background form. ## `SAVE`: synchronous, on the main thread Redis processes client commands on a single thread. `SAVE` performs the serialization inline on that thread. Until the last byte is written, the server executes nothing else: clients block, the accept queue backs up, health checks time out, and any monitoring that speaks Redis reports the instance as down. On a 10 GB dataset that is not a hiccup, it is an outage of tens of seconds or more. The upside of that same property is determinism: when `SAVE` returns `OK`, the file on disk is complete and no further writes have been applied. That makes it occasionally useful in a maintenance script on an instance that is already drained — for instance, before deliberately killing a process on a box you are decommissioning. Redis also performs a blocking save on `SHUTDOWN` when save points are configured, for exactly this reason. ## `BGSAVE`: fork a child `BGSAVE` returns immediately with `Background saving started`. Under the hood Redis calls `fork()`. The child process gets a logically identical copy of the parent's memory, implemented by the kernel as **copy-on-write**: nothing is duplicated up front, page tables are marked read-only, and a page is only copied when one side writes to it. The child then walks that frozen view and writes the RDB to a temporary file; on success it renames the temp file over `dump.rdb`, which is atomic on the same filesystem, so a crash mid-save leaves the *previous* snapshot intact. The parent, meanwhile, keeps serving. The one unavoidable pause is `fork()` itself, which must copy the page tables — cost proportional to how much memory the process maps, commonly on the order of ten-ish milliseconds per gigabyte on bare metal and noticeably worse on some virtualized instances. `INFO stats` reports the last one as `latest_fork_usec`, and it is one of the first things to check when a user reports a periodic latency spike. The less obvious cost is memory: because the parent keeps taking writes, every page it modifies during the save gets copied, so resident memory grows during the snapshot in proportion to the write rate and the save duration. ## Concurrency and status Only one child of this kind runs at a time. If a `BGSAVE` is already running, another `BGSAVE` is refused; if an AOF rewrite is running, Redis will typically defer a save point rather than fork twice. Useful fields from `INFO persistence`: - `rdb_bgsave_in_progress` — 1 while a child is writing - `rdb_last_bgsave_status` — `ok` or `err` - `rdb_last_save_time` — Unix time of the last successful save (also returned by `LASTSAVE`) - `rdb_changes_since_last_save` — how many writes are currently unprotected A scripted snapshot usually reads `LASTSAVE`, issues `BGSAVE`, and polls `LASTSAVE` until it advances — that is the non-blocking equivalent of waiting for `SAVE` to return. ## Failure behavior worth knowing If the background save fails — full disk, permissions, fork denied for lack of memory — `rdb_last_bgsave_status` becomes `err`, and with the default `stop-writes-on-bgsave-error yes` Redis begins **rejecting write commands** with a `MISCONF` error. This surprises people badly: the instance is up, reads work, writes fail. It is deliberate, on the reasoning that a server configured to persist should tell you loudly when it no longer can. On a pure cache, where persistence is a nicety, setting it to `no` is defensible; on a store of record, fix the disk instead. ## Summary line for an interview Same output, different execution: `SAVE` blocks the single command thread for the whole write; `BGSAVE` forks and writes from a copy-on-write child, paying only the fork pause plus copy-on-write memory. Automatic save points use `BGSAVE`; `SAVE` belongs in maintenance scripts on quiet instances.