Improve comments in slru.{c,h} about segment name format

slru.h described incorrectly how SLRU segment names are formatted
depending on the segment number and if long or short segment names are
used.  This commit closes the gap with a better description, fitting
with the reality.

Reported-by: Noah Misch
Author: Aleksander Alekseev
Discussion: https://postgr.es/m/20240626002747[email protected]
Backpatch-through: 17

diff --git a/src/backend/access/transam/slru.c b/src/backend/access/transam/slru.c
index 77b05cc0a746db79b5f8968880bb673b83ecbb4c..248aa1a6553b6106013b4f4391e65a9914900d5b 100644 (file)
--- a/src/backend/access/transam/slru.c
+++ b/src/backend/access/transam/slru.c
@@ -72,6 +72,21 @@
 #include "storage/shmem.h"
 #include "utils/guc_hooks.h"
 
+/*
+ * Converts segment number to the filename of the segment.
+ *
+ * "path" should point to a buffer at least MAXPGPATH characters long.
+ *
+ * If ctl->long_segment_names is true, segno can be in the range [0, 2^60-1].
+ * The resulting file name is made of 15 characters, e.g. dir/123456789ABCDEF.
+ *
+ * If ctl->long_segment_names is false, segno can be in the range [0, 2^24-1].
+ * The resulting file name is made of 4 to 6 characters, as of:
+ *
+ *  dir/1234   for [0, 2^16-1]
+ *  dir/12345  for [2^16, 2^20-1]
+ *  dir/123456 for [2^20, 2^24-1]
+ */
 static inline int
 SlruFileName(SlruCtl ctl, char *path, int64 segno)
 {

diff --git a/src/include/access/slru.h b/src/include/access/slru.h
index 8a8d1918733f45b26d72bb95882fd67bdbafcd59..97e612cd100b983d7ada268a96698dc4840de76a 100644 (file)
--- a/src/include/access/slru.h
+++ b/src/include/access/slru.h
@@ -134,10 +134,9 @@ typedef struct SlruCtlData
    bits16      bank_mask;
 
    /*
-    * If true, use long segment filenames formed from lower 48 bits of the
-    * segment number, e.g. pg_xact/000000001234. Otherwise, use short
-    * filenames formed from lower 16 bits of the segment number e.g.
-    * pg_xact/1234.
+    * If true, use long segment file names.  Otherwise, use short file names.
+    *
+    * For details about the file name format, see SlruFileName().
     */
    bool        long_segment_names;