Improve comment for struct BufferDesc.

Add a note that per-buffer I/O condition variables currently live
outside the BufferDesc struct.  Follow-up for commit d8725104.

Reported-by: Julien Rouhaud 
Discussion: https://postgr.es/m/20210311031118.hucytmrgwlktjxgq%40nol

diff --git a/src/include/storage/buf_internals.h b/src/include/storage/buf_internals.h
index 9cbf1fc3fad0248674389f502859daaa70bc9152..33fcaf5c9a871272e0e231d564fb7fdac5e64f48 100644 (file)
--- a/src/include/storage/buf_internals.h
+++ b/src/include/storage/buf_internals.h
@@ -174,6 +174,10 @@ typedef struct buftag
  * Be careful to avoid increasing the size of the struct when adding or
  * reordering members.  Keeping it below 64 bytes (the most common CPU
  * cache line size) is fairly important for performance.
+ *
+ * Per-buffer I/O condition variables are currently kept outside this struct in
+ * a separate array.  They could be moved in here and still fit within that
+ * limit on common systems, but for now that is not done.
  */
 typedef struct BufferDesc
 {