[翻译]PG15新特性-加速WAL日志归档

$ ls -alrth pg_wal/
drwx------ 3 postgres postgres  68 Dec  7 05:47 .
drwx------ 21 postgres postgres 32 Dec 21 03:54 ..
-rw------- 1 postgres postgres 16M Dec 21 04:38 0000000200000008000000E4
-rw------- 1 postgres postgres 16M Dec 21 04:38 0000000200000008000000E5
-rw------- 1 postgres postgres 16M Dec 21 04:38 0000000200000008000000E6
-rw------- 1 postgres postgres 16M Dec 21 04:38 0000000200000008000000E7
drwx------ 2 postgres postgres   6 Dec 21 04:38 archive_status
-rw------- 1 postgres postgres 16M Dec 21 04:38 0000000200000008000000E8
-bash-4.2$ ls -alrth pg_wal/archive_status/
total 5.0K
drwx------ 3 postgres postgres 68 Dec 7 05:47 ..
-rw------- 1 postgres postgres 0 Dec 21 04:38 0000000200000008000000E4.ready
-rw------- 1 postgres postgres 0 Dec 21 04:38 0000000200000008000000E5.ready
-rw------- 1 postgres postgres 0 Dec 21 04:38 0000000200000008000000E6.ready
-rw------- 1 postgres postgres 0 Dec 21 04:38 0000000200000008000000E7.ready
drwx------ 2 postgres postgres 6 Dec 21 04:38 .

postgres=# SELECT pg_walfile_name(pg_current_wal_lsn());
  pg_walfile_name    
--------------------------
0000000200000008000000E8
(1 row)

$ ls -alrth pg_wal/archive_status/
total 5.0K
drwx------ 3 postgres postgres 68 Dec 7 05:47 ..
-rw------- 1 postgres postgres 0 Dec 21 04:38 0000000200000008000000E4.done
-rw------- 1 postgres postgres 0 Dec 21 04:38 0000000200000008000000E5.done
-rw------- 1 postgres postgres 0 Dec 21 04:38 0000000200000008000000E6.done
-rw------- 1 postgres postgres 0 Dec 21 04:38 0000000200000008000000E7.done

/*
* pgarch_readyXlog
*
* Return name of the oldest xlog file that has not yet been archived.
* No notification is set that file archiving is now in progress, so
* this would need to be extended if multiple concurrent archival
* tasks were created. If a failure occurs, we will completely
* re-copy the file at the next available opportunity.
*
* It is important that we return the oldest, so that we archive xlogs
* in order that they were written, for two reasons:
* 1) to maintain the sequential chain of xlogs required for recovery
* 2) because the oldest ones will sooner become candidates for
* recycling at time of checkpoint
*
* NOTE: the "oldest" comparison will consider any .history file to be older
* than any other file except another .history file.  Segments on a timeline
* with a smaller ID will be older than all segments on a timeline with a
* larger ID; the net result being that past timelines are given higher
* priority for archiving.  This seems okay, or at least not obviously worth
* changing.
*/

/*
* Maximum number of .ready files to gather per directory scan.
*/
#define NUM_FILES_PER_DIRECTORY_SCAN 64