The PostgreSQL Write-Ahead Log (WAL) is a recording location within the Postgres cluster, capturing all modifications made to the cluster’s data files before being written to the heap. During crash recovery, the WAL contains sufficient data for Postgres to restore its state to the point of the last committed transaction.
There may arise circumstances where it becomes necessary to determine the numerical difference between the WAL files. For instance, when recovering from a significant delay or while configuring replication on a sizable database, the recovery process can be time-consuming as new WAL files are replayed. Initially, when setting up replication, the server may not permit login access. In such cases, calculating the disparity in the number of WAL files can provide an estimation of the recovery time, allowing for an assessment of the lag.
Another practical application for calculating the difference between WAL files is in the context of the archiver process. Determining the variance between WAL files makes it possible to estimate the number of remaining files that are yet to be archived.
To calculate the difference between two WAL files, let’s understand the WAL file name format.
The name format for PostgreSQL Write-Ahead Logs (WAL) files is TTTTTTTTXXXXXXXXYYYYYYYY, a 24-character hexadecimal representation of the LSN (Log Sequence Number) associated with the WAL record. The LSN is a unique identifier for each WAL record.
In format TTTTTTTTXXXXXXXXYYYYYYYY, ‘T’ is the timeline, ‘X’ is the high 32-bits from the LSN(Segment number), and ‘Y’ is the low 32-bits of the LSN.
For example, a WAL file name might look like this: “0000000100001234000000AB”.
Here’s a breakdown of the components in the example:
– “00000001”: This represents the timeline ID. It is usually 1 for the default timeline.
– “00012340”: This represents the WAL file number, indicating the sequential order of the WAL file within the timeline.
– “000000AB”: This is the hexadecimal representation of the segment file number.
We can determine the numerical difference between two WAL files with the below sql:
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SELECT ABS(('x' || SUBSTRING(current_wal, 1, 8))::bit(32)::int - ('x' || SUBSTRING(old_wal, 1, 8))::bit(32)::int) +<br> ABS(('x' || SUBSTRING(current_wal, 9, 8))::bit(32)::int*256 - ('x' || SUBSTRING(old_wal, 9, 8))::bit(32)::int*256) +<br> ABS(('x' || SUBSTRING(current_wal, 17))::bit(32)::int - ('x' || SUBSTRING(old_wal, 17))::bit(32)::int)<br>FROM (select '000000330000006900000047' current_wal, '0000003200000069000000AB' old_wal) as wal_segs; |
Let’s create a PostgreSQL function that facilitates the calculation of the numerical difference between two WAL files, making it more convenient to use to determine the variance between them.
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CREATE OR REPLACE FUNCTION public.get_walfile_diff(current_wal text, old_wal text)<br>RETURNS numeric<br>LANGUAGE plpgsql<br>AS $function$<br>DECLARE<br>v_wal_diff numeric;<br>BEGIN<br>IF length(current_wal) != 24 OR length(old_wal) != 24 THEN<br>RAISE EXCEPTION 'Invalid wal file length';<br>END IF;<br>v_wal_diff := (<br>ABS(('x' || SUBSTRING(current_wal, 1, 8))::bit(32)::int - ('x' || SUBSTRING(old_wal, 1, 8))::bit(32)::int) +<br>ABS(('x' || SUBSTRING(current_wal, 9, 8))::bit(32)::int*256 - ('x' || SUBSTRING(old_wal, 9, 8))::bit(32)::int*256) +<br>ABS(('x' || SUBSTRING(current_wal, 17))::bit(32)::int - ('x' || SUBSTRING(old_wal, 17))::bit(32)::int)<br>);<br>RETURN v_wal_diff;<br>END;<br>$function$; |
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postgres=# SELECT public.get_walfile_diff('000000330000006900000047','000000330000006900000048');<br> get_walfile_diff<br>------------------<br> 1<br>(1 row)<br>postgres=# SELECT public.get_walfile_diff('000000330000006900000047','0000003300000069000000AB');<br> get_walfile_diff<br>------------------<br> 100<br>(1 row) |
Overall, being able to calculate the numerical difference between WAL files contributes to effective management and understanding of Postgres database recovery/archiver processes.
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