postgresql - 日志序列号在 postgres 中真的是单调的吗？

Question

阅读 postgreSQL 文档时:

WAL records are appended to the WAL logs as each new record is written. The insert position is described by a Log Sequence Number (LSN) that is a byte offset into the logs, increasing monotonically with each new record. LSN values are returned as the datatype pg_lsn. Values can be compared to calculate the volume of WAL data that separates them, so they are used to measure the progress of replication and recovery.

我们可以相信这个序列号是严格单调的吗？是否存在将此偏移量重置为先前位置的时间点(由于 WAL 存档或其他类型的操作)？

Answer 1

它是严格单调的，但LSN 指向某个段文件中的物理偏移量。

Isn't there a point in time where this offset resetting to a previous position (due to WAL archive, or another type of operations ) ?

有这么一个时间点但是在另一个segment文件中。

仅供引用，这里有一个来自 PG 代码的宏，它从 LSN ( https://github.com/postgres/postgres/blob/master/src/include/access/xlog_internal.h ) 中提取段号:

/*
 * Compute a segment number from an XLogRecPtr.
 ........
 */
#define XLByteToSeg(xlrp, logSegNo, wal_segsz_bytes) \
    logSegNo = (xlrp) / (wal_segsz_bytes)