BGSAVEStart a snapshot save in the background.
| Syntax | BGSAVE |
| Group | server |
| Arity | 1 |
| Since | 1.0.0 |
| Flags | write, admin |
| Complexity | O(S) fan-out; O(N/S) copy-on-write freeze per shard on the reactor, then serialise off-thread |
| Redis compatibility | differs: replies +OK rather than Redis's status line, and writes one dump per shard |