Re: [HACKERS] Restricting maximum keep segments by repslots

Hello,

I'll add this to CF2017-09.

At Mon, 06 Mar 2017 18:20:06 +0900 (Tokyo Standard Time), Kyotaro HORIGUCHI  wrote in <20170306.182006.172683338.horiguchi.kyotaro@lab.ntt.co.jp>
> Thank you for the comment.
> 
> At Fri, 3 Mar 2017 14:47:20 -0500, Peter Eisentraut  wrote in 
> > On 3/1/17 19:54, Kyotaro HORIGUCHI wrote:
> > >> Please measure it in size, not in number of segments.
> > > It was difficult to dicide which is reaaonable but I named it
> > > after wal_keep_segments because it has the similar effect.
> > > 
> > > In bytes(or LSN)
> > >  max_wal_size
> > >  min_wal_size
> > >  wal_write_flush_after
> > > 
> > > In segments
> > >  wal_keep_segments
> > 
> > We have been moving away from measuring in segments.  For example,
> > checkpoint_segments was replaced by max_wal_size.
> > 
> > Also, with the proposed patch that allows changing the segment size more
> > easily, this will become more important.  (I wonder if that will require
> > wal_keep_segments to change somehow.)
> 
> Agreed. It is 'max_slot_wal_keep_size' in the new version.
> 
> wal_keep_segments might should be removed someday.

- Following to min/max_wal_size, the variable was renamed to "max_slot_wal_keep_size_mb" and used as ConvertToXSegs(x)"

- Stopped warning when checkpoint doesn't flush segments required by slots even if max_slot_wal_keep_size have worked.

- Avoided subraction that may be negative.

regards,

*** a/src/backend/access/transam/xlog.c
--- b/src/backend/access/transam/xlog.c
***************
*** 105,110 **** int			wal_level = WAL_LEVEL_MINIMAL;
--- 105,111 ---- int			CommitDelay = 0;	/* precommit delay in microseconds */ int			CommitSiblings = 5; /* # concurrent xacts needed to sleep */ int			wal_retrieve_retry_interval = 5000;
+ int			max_slot_wal_keep_size_mb = 0;  #ifdef WAL_DEBUG bool		XLOG_DEBUG = false;
***************
*** 9353,9361 **** KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
--- 9354,9385 ---- 	if (max_replication_slots > 0 && keep != InvalidXLogRecPtr) 	{ 		XLogSegNo	slotSegNo;
+ 		int			slotlimitsegs = ConvertToXSegs(max_slot_wal_keep_size_mb);  		XLByteToSeg(keep, slotSegNo); 
+ 		/*
+ 		 * ignore slots if too many wal segments are kept.
+ 		 * max_slot_wal_keep_size is just accumulated on wal_keep_segments.
+ 		 */
+ 		if (max_slot_wal_keep_size_mb > 0 && slotSegNo + slotlimitsegs < segno)
+ 		{
+ 			segno = segno - slotlimitsegs; /* must be positive */
+ 
+ 			/*
+ 			 * warn only if the checkpoint flushes the required segment.
+ 			 * we assume here that *logSegNo is calculated keep location.
+ 			 */
+ 			if (slotSegNo < *logSegNo)
+ 				ereport(WARNING,
+ 					(errmsg ("restart LSN of replication slots is ignored by checkpoint"),
+ 					 errdetail("Some replication slots have lost required WAL segnents to continue by up to %ld segments.",
+ 					   (segno < *logSegNo ? segno : *logSegNo) - slotSegNo)));
+ 
+ 			/* emergency vent */
+ 			slotSegNo = segno;
+ 		}
+  		if (slotSegNo <= 0) 			segno = 1; 		else if (slotSegNo < segno)
*** a/src/backend/utils/misc/guc.c
--- b/src/backend/utils/misc/guc.c
***************
*** 2366,2371 **** static struct config_int ConfigureNamesInt[] =
--- 2366,2382 ---- 	},  	{
+ 		{"max_slot_wal_keep_size", PGC_SIGHUP, REPLICATION_SENDING,
+ 			gettext_noop("Sets the maximum size of extra WALs kept by replication slots."),
+ 		 NULL,
+ 		 GUC_UNIT_MB
+ 		},
+ 		&max_slot_wal_keep_size_mb,
+ 		0, 0, INT_MAX,
+ 		NULL, NULL, NULL
+ 	},
+ 
+ 	{ 		{"wal_sender_timeout", PGC_SIGHUP, REPLICATION_SENDING, 			gettext_noop("Sets the maximum time to wait for WAL replication."), 			NULL,
*** a/src/backend/utils/misc/postgresql.conf.sample
--- b/src/backend/utils/misc/postgresql.conf.sample
***************
*** 235,240 ****
--- 235,241 ---- #max_wal_senders = 10		# max number of walsender processes 				# (change requires restart) #wal_keep_segments = 0		# in logfile segments, 16MB each; 0 disables
+ #max_slot_wal_keep_size = 0	# measured in bytes; 0 disables #wal_sender_timeout = 60s	# in milliseconds; 0 disables  #max_replication_slots = 10	# max number of replication slots
*** a/src/include/access/xlog.h
--- b/src/include/access/xlog.h
***************
*** 97,102 **** extern bool reachedConsistency;
--- 97,103 ---- extern int	min_wal_size_mb; extern int	max_wal_size_mb; extern int	wal_keep_segments;
+ extern int	max_slot_wal_keep_size_mb; extern int	XLOGbuffers; extern int	XLogArchiveTimeout; extern int	wal_retrieve_retry_interval;