On Tue, Aug 02, 2022 at 07:37:27AM -0700, Noah Misch wrote:
> On Tue, Aug 02, 2022 at 10:14:22AM -0400, David Steele wrote:
> > On 7/31/22 02:17, Noah Misch wrote:
> > >On Tue, Jul 26, 2022 at 07:21:29AM -0400, David Steele wrote:
> > >>On 6/19/21 16:39, Noah Misch wrote:
> > >>>On Tue, Feb 02, 2021 at 07:14:16AM -0800, Noah Misch wrote:
> > >>>>Recycling and preallocation are wasteful during archive recovery,
> > >>>>because
> > >>>>KeepFileRestoredFromArchive() unlinks every entry in its path. I
> > >>>>propose to
> > >>>>fix the race by adding an XLogCtl flag indicating which regime
> > >>>>currently owns
> > >>>>the right to add long-term pg_wal directory entries. In the archive
> > >>>>recovery
> > >>>>regime, the checkpointer will not preallocate and will unlink old
> > >>>>segments
> > >>>>instead of recycling them (like wal_recycle=off). XLogFileInit() will
> > >>>>fail.
> > >>>
> > >>>Here's the implementation. Patches 1-4 suffice to stop the user-visible
> > >>>ERROR. Patch 5 avoids a spurious LOG-level message and wasted filesystem
> > >>>writes, and it provides some future-proofing.
> > >>>
> > >>>I was tempted to (but did not) just remove preallocation. Creating one
> > >>>file
> > >>>per checkpoint seems tiny relative to the max_wal_size=1GB default, so I
> > >>>expect it's hard to isolate any benefit. Under the old
> > >>>checkpoint_segments=3
> > >>>default, a preallocated segment covered a respectable third of the next
> > >>>checkpoint. Before commit 63653f7 (2002), preallocation created more
> > >>>files.
> > >>
> > >>This also seems like it would fix the link issues we are seeing in [1].
> > >>
> > >>I wonder if that would make it worth a back patch?
> > >
> > >Perhaps. It's sad to have multiple people deep-diving into something
> > >fixed on
> > >HEAD. On the other hand, I'm not eager to spend risk-of-backpatch points
> > >on
> > >this. One alternative would be adding an errhint like "This is known to
> > >happen occasionally during archive recovery, where it is harmless." That
> > >has
> > >an unpolished look, but it's low-risk and may avoid deep-dive efforts.
> >
> > I think in this case a HINT might be sufficient to at least keep people from
> > wasting time tracking down a problem that has already been fixed.
Here's a patch to add that HINT. I figure it's better to do this before next
week's minor releases. In the absence of objections, I'll push this around
2022-08-05 14:00 UTC.
> > However, there is another issue [1] that might argue for a back patch if
> > this patch (as I believe) would fix the issue.
>
> > [1]
> > https://www.postgresql.org/message-id/CAHJZqBDxWfcd53jm0bFttuqpK3jV2YKWx%3D4W7KxNB4zzt%2B%2BqFg%40mail.gmail.com
>
> That makes sense. Each iteration of the restartpoint recycle loop has a 1/N
> chance of failing. Recovery adds >N files between restartpoints. Hence, the
> WAL directory grows without bound. Is that roughly the theory in mind?
On further reflection, I don't expect it to happen that way. Each failure
message is LOG-level, so the remaining recycles still happen. (The race
condition can yield an ERROR under PreallocXlogFiles(), but recycling is
already done at that point.)
Author: Noah Misch <n...@leadboat.com>
Commit: Noah Misch <n...@leadboat.com>
Add HINT for restartpoint race with KeepFileRestoredFromArchive().
The five commits ending at cc2c7d65fc27e877c9f407587b0b92d46cd6dd16
closed this race condition for v15+. For v14 through v10, add a HINT to
discourage studying the cosmetic problem.
Reviewed by FIXME.
Discussion: https://postgr.es/m/20220731061747.ga3692...@rfd.leadboat.com
diff --git a/src/backend/access/transam/xlog.c
b/src/backend/access/transam/xlog.c
index 5cdd01f..9c845b7 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -3480,7 +3480,10 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent,
bool use_lock)
if (fd < 0)
ereport(ERROR,
(errcode_for_file_access(),
- errmsg("could not open file \"%s\": %m",
path)));
+ errmsg("could not open file \"%s\": %m", path),
+ (AmCheckpointerProcess() ?
+ errhint("This is known to fail occasionally
during archive recovery, where it is harmless.") :
+ 0)));
elog(DEBUG2, "done creating and filling new WAL file");
diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index 6688dee..e76daff 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -834,7 +834,10 @@ durable_rename_excl(const char *oldfile, const char
*newfile, int elevel)
ereport(elevel,
(errcode_for_file_access(),
errmsg("could not link file \"%s\" to \"%s\":
%m",
- oldfile, newfile)));
+ oldfile, newfile),
+ (AmCheckpointerProcess() ?
+ errhint("This is known to fail occasionally
during archive recovery, where it is harmless.") :
+ 0)));
return -1;
}
unlink(oldfile);
@@ -844,7 +847,10 @@ durable_rename_excl(const char *oldfile, const char
*newfile, int elevel)
ereport(elevel,
(errcode_for_file_access(),
errmsg("could not rename file \"%s\" to
\"%s\": %m",
- oldfile, newfile)));
+ oldfile, newfile),
+ (AmCheckpointerProcess() ?
+ errhint("This is known to fail occasionally
during archive recovery, where it is harmless.") :
+ 0)));
return -1;
}
#endif
