persistence

In , persistence refers to the characteristic of that outlives the that created it. Without this capability, would only exist in , and would be lost when this RAM loses power, such as a computer shutdown
(for example, x = 0, when the process of doing x=0 ends, the state of x = 0 is still there, somewhere in the disk, and won’t be lost or become x = 1 when the RAM is down.)
Why DB data is put in memory, please refer to redis, because it is fast  to read and write if the data is in memory.
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