9.2. Transaction Log and WAL Segment Files

Logically, PostgreSQL writes XLOG records into a virtual file with an 8-byte address space (16 Exabytes).

The transaction log capacity is effectively unlimited. While 8 bytes of address space is vast, managing a single file of this size is impossible. Therefore, PostgreSQL divides the transaction log into smaller files, typically 16 megabytes each. Each file is known as a WAL segment. See Figure 9.6.

The WAL segment filename is a 24-digit hexadecimal number. The naming rule is as follows:

$$ \begin{align} \text{WAL segment file name} = \text{timelineId} + (\text{uint32}) \frac{\text{LSN}-1}{16\text{M}*256} + ( \text{uint32})\left(\frac{\text{LSN}-1}{16\text{M}}\right) % 256 \end{align} $$

The first WAL segment file is 000000010000000000000001. When XLOG records fill the first file, PostgreSQL provides the second file: 000000010000000000000002. Files are used in ascending order. After 0000000100000000000000FF is full, the file provided is 000000010000000100000000. In this way, the middle 8-digit number increases by one whenever the last 2 digits carry over.

Similarly, after 0000000100000001000000FF is full, PostgreSQL provides 000000010000000200000000, and so on.

testdb=# SELECT pg_xlogfile_name('1/00002D3E');  # In versions 10 or later, "SELECT pg_walfile_name('1/00002D3E');"
     pg_xlogfile_name
--------------------------
 000000010000000100000000
(1 row)