From: | Luca Ferrari <fluca1978(at)gmail(dot)com> |
---|---|
To: | pgsql-general <pgsql-general(at)postgresql(dot)org> |
Subject: | question about zeroes in the wal file names |
Date: | 2019-08-18 14:17:03 |
Message-ID: | CAKoxK+67oS+PSygdmRHbn-GSjE_TY+t6Gzk6CRzcw-HVGswzOA@mail.gmail.com |
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Lists: | pgsql-general |
I'm just curious to better understand the naming convention behind wal
files, because I've seen on a system of mine that the wals created
were:
000000050000020E000000FF
000000050000020F00000000
while I was expecting 20E0x100. So I digged into the code and I've
seen, from the XLogFileName macro, that the last part is built as the
reminder of the number of segments per wal file:
#define XLogFileName(fname, tli, logSegNo, wal_segsz_bytes) \
snprintf(fname, MAXFNAMELEN, "%08X%08X%08X", tli, \
(uint32) ((logSegNo) / XLogSegmentsPerXLogId(wal_segsz_bytes)), \
(uint32) ((logSegNo) % XLogSegmentsPerXLogId(wal_segsz_bytes)))
and with the default wal size of 16 MB that gives a remainder of 256
(FF + 1). Assuming I haven't missed anything, this means that there
are 6 zeroes that will never change in the last 8 chars of the wal
filename. Is therefore this only done to handle PostgreSQL WAL sizes
of 4 GB each?
Thanks,
Luca
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