BGSAVEStart a snapshot save in the background.
| 语法 | BGSAVE |
| 分组 | server |
| 参数个数 | 1 |
| 起始版本 | 1.0.0 |
| 标志 | write, admin |
| 复杂度 | O(S) fan-out; O(N/S) copy-on-write freeze per shard on the reactor, then serialise off-thread |
| Redis 兼容性 | differs: replies +OK rather than Redis's status line, and writes one dump per shard |