![]() ![]() rsync of the pg_xlog folder to the would-be replica.rsync of the database to the would-be replica.What I'm doing, taking inspiration from the instructions on the wiki and documentation: How to install PostgreSQL 7/6 and phpPgAdmin in CentOS 5/6.I'm running PostgreSQL 9.4, trying to get the replication going. Install and configure phppgAdmin on UbuntuĬonfigure a Streaming Replication cluster in PostgreSQL Install PostgreSQL 6.3 on yum in CentOS 9.3Ĭonfiguration and installation of LAPP (Linux + Apache + PostgreSQL + PHP) Environment in Ubuntu First, reset the current xlog location to an xlog that is less than the maximum value.įirst, an error is reported, the instance is terminated, and the xlog is cyclic, starting from the minimum value. Next we can use an experiment to verify the above process:ġ. The following is an illegal xlog record pointor defined in the source code. The xlog record pointor is invalid starting from "0". Therefore, when the size is 16 MB, The Minimum File number is "0001ffffffffff00000001" instead of "0001ffffffff00000000 ". In addition, when the PostgreSQL database is initialized, the first position of xlog starts at the logical offset of XLOG_SEG_SIZE. Therefore, the maximum xlog file name should be "00000001ffffff000000ff", rather than the theoretical maximum "00000001FFFFFFFFFFFFFFFF" mentioned above ". The number of segments corresponding to an xlog is 2 ^ 32/2 ^ 24 = 2 ^ 8 = 256. The source code of Xlog segment is defined as follows:Īssume that the segment size of xlog is 16 Mb. The 32-bit high represents the serial number of the xlog, and the 32-bit low represents the segment number contained in the xlog. As mentioned above, the second part of the log file xlogsegno can be split into two 32-bit segments. What is the relationship between this and the log file name? This is enough.Īt this point, many people are in a hurry. Is that actually the case? We can calculate:Īssume that the number of logs generated in a day is 10 TB, that is, 2 ^ 40 power. Many people think that it is easy to use up 64-bit. PG uses a 64-bit address to indicate the location of the log. ![]() In PostgreSQL, logs are expressed by memory offset addresses. The following is a detailed description.įirst, let's understand the memory addressing representation of the log stream. In fact, the file number cannot reach this value. This is the theoretical maximum value of xlogsgno, that is, "ffffffffffffff ". This is actually the maximum value that a 64-bit system can represent. The second part is "0000000000000002", which is xlogsegno, expressed in 64 bits. This is somewhat similar to the concept of Incarnation in Oracle. Normally, shutting down and starting the database does not change this value. After each restoration, it will increase, the main purpose is to distinguish the recovered database from the previous database,įor example, Point In-Time Recovery. It is used to identify the database status. ![]() The first part is "00000001", which is the TimeLineId, which is commonly referred to as TLI. In addition, the xlog file name can be divided into two parts, taking "000000010000000000000002" as an example, and divided into the following two parts: The configure parameter "- with-wal-segsize = 64" is required during compilation. Currently, PG's default xlog segment size is 16 M, which can only be set during source code compilation. In addition, you may find that the PostgreSQL log file name is very long and the size is 16 Mb. Logs are flushed out by continuous IO, while data files are generally random IO. The reason for writing logs first is that writing logs to a disk is faster than writing data to a disk. If I write the redo log to the disk prior to the data, the transaction can always be restored, although the time may be longer. That is, any transaction that has been committed must be persistent. Why is there WAL logs? This is the same as mainstream databases such as Oracle and MySQL to ensure transaction persistence. ![]() PostgreSQL xlog, that is, WAL (Write Ahead Log), is located In the pg_xlog directory and is the Log on which PostgreSQL fully or PITR (Point In-Time Recovery) depends. PostgreSQL xlog file name specification source code parsing ![]()
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