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Morph pg_replication_slots.min_safe_lsn to safe_wal_size
The previous definition of the column was almost universally disliked, so provide this updated definition which is more useful for monitoring purposes: a large positive value is good, while zero or a negative value means danger. This should be operationally more convenient. Backpatch to 13, where the new column to pg_replication_slots (and the feature it represents) were added. Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com> Author: Álvaro Herrera <alvherre@alvh.no-ip.org> Reported-by: Fujii Masao <masao.fujii@oss.nttdata.com> Discussion: https://postgr.es/m/9ddfbf8c-2f67-904d-44ed-cf8bc5916228@oss.nttdata.com
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9 files changed

+60
-34
lines changed

9 files changed

+60
-34
lines changed

‎doc/src/sgml/catalogs.sgml

Lines changed: 5 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -11275,10 +11275,13 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
1127511275

1127611276
<row>
1127711277
<entry role="catalog_table_entry"><para role="column_definition">
11278-
<structfield>min_safe_lsn</structfield> <type>pg_lsn</type>
11278+
<structfield>safe_wal_size</structfield> <type>int8</type>
1127911279
</para>
1128011280
<para>
11281-
The minimum LSN currently available for walsenders.
11281+
The number of bytes that can be written to WAL such that this slot
11282+
is not in danger of getting in state "lost". It is NULL for lost
11283+
slots, as well as if <varname>max_slot_wal_keep_size</varname>
11284+
is <literal>-1</literal>.
1128211285
</para></entry>
1128311286
</row>
1128411287
</tbody>

‎src/backend/access/transam/xlog.c

Lines changed: 2 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -764,8 +764,7 @@ static ControlFileData *ControlFile = NULL;
764764
* Convert values of GUCs measured in megabytes to equiv. segment count.
765765
* Rounds down.
766766
*/
767-
#define ConvertToXSegs(x, segsize) \
768-
((x) / ((segsize) / (1024 * 1024)))
767+
#define ConvertToXSegs(x, segsize) XLogMBVarToSegs((x), (segsize))
769768

770769
/* The number of bytes in a WAL segment usable for WAL data. */
771770
static int UsableBytesInSegment;
@@ -9513,8 +9512,7 @@ GetWALAvailability(XLogRecPtr targetLSN)
95139512
XLogSegNo targetSeg; /* segid of targetLSN */
95149513
XLogSegNo oldestSeg; /* actual oldest segid */
95159514
XLogSegNo oldestSegMaxWalSize; /* oldest segid kept by max_wal_size */
9516-
XLogSegNo oldestSlotSeg = InvalidXLogRecPtr; /* oldest segid kept by
9517-
* slot */
9515+
XLogSegNo oldestSlotSeg; /* oldest segid kept by slot */
95189516
uint64 keepSegs;
95199517

95209518
/*

‎src/backend/catalog/system_views.sql

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -879,7 +879,7 @@ CREATE VIEW pg_replication_slots AS
879879
L.restart_lsn,
880880
L.confirmed_flush_lsn,
881881
L.wal_status,
882-
L.min_safe_lsn
882+
L.safe_wal_size
883883
FROM pg_get_replication_slots() AS L
884884
LEFT JOIN pg_database D ON (L.datoid = D.oid);
885885

‎src/backend/replication/slotfuncs.c

Lines changed: 30 additions & 10 deletions
Original file line numberDiff line numberDiff line change
@@ -242,6 +242,7 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
242242
Tuplestorestate *tupstore;
243243
MemoryContext per_query_ctx;
244244
MemoryContext oldcontext;
245+
XLogRecPtr currlsn;
245246
int slotno;
246247

247248
/* check to see if caller supports us returning a tuplestore */
@@ -274,6 +275,8 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
274275

275276
MemoryContextSwitchTo(oldcontext);
276277

278+
currlsn = GetXLogWriteRecPtr();
279+
277280
LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
278281
for (slotno = 0; slotno < max_replication_slots; slotno++)
279282
{
@@ -282,7 +285,6 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
282285
Datum values[PG_GET_REPLICATION_SLOTS_COLS];
283286
bool nulls[PG_GET_REPLICATION_SLOTS_COLS];
284287
WALAvailability walstate;
285-
XLogSegNo last_removed_seg;
286288
int i;
287289

288290
if (!slot->in_use)
@@ -380,36 +382,54 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
380382
* we looked. If checkpointer signalled the process to
381383
* termination, then it's definitely lost; but if a process is
382384
* still alive, then "unreserved" seems more appropriate.
385+
*
386+
* If we do change it, save the state for safe_wal_size below.
383387
*/
384388
if (!XLogRecPtrIsInvalid(slot_contents.data.restart_lsn))
385389
{
386390
int pid;
387391

388392
SpinLockAcquire(&slot->mutex);
389393
pid = slot->active_pid;
394+
slot_contents.data.restart_lsn = slot->data.restart_lsn;
390395
SpinLockRelease(&slot->mutex);
391396
if (pid != 0)
392397
{
393398
values[i++] = CStringGetTextDatum("unreserved");
399+
walstate = WALAVAIL_UNRESERVED;
394400
break;
395401
}
396402
}
397403
values[i++] = CStringGetTextDatum("lost");
398404
break;
399405
}
400406

401-
if (max_slot_wal_keep_size_mb >= 0 &&
402-
(walstate == WALAVAIL_RESERVED || walstate == WALAVAIL_EXTENDED) &&
403-
((last_removed_seg = XLogGetLastRemovedSegno()) != 0))
407+
/*
408+
* safe_wal_size is only computed for slots that have not been lost,
409+
* and only if there's a configured maximum size.
410+
*/
411+
if (walstate == WALAVAIL_REMOVED || max_slot_wal_keep_size_mb < 0)
412+
nulls[i++] = true;
413+
else
404414
{
405-
XLogRecPtr min_safe_lsn;
415+
XLogSegNo targetSeg;
416+
XLogSegNo keepSegs;
417+
XLogSegNo failSeg;
418+
XLogRecPtr failLSN;
406419

407-
XLogSegNoOffsetToRecPtr(last_removed_seg + 1, 0,
408-
wal_segment_size, min_safe_lsn);
409-
values[i++] = Int64GetDatum(min_safe_lsn);
420+
XLByteToSeg(slot_contents.data.restart_lsn, targetSeg, wal_segment_size);
421+
422+
/* determine how many segments slots can be kept by slots ... */
423+
keepSegs = XLogMBVarToSegs(max_slot_wal_keep_size_mb, wal_segment_size);
424+
/* ... and override by wal_keep_segments as needed */
425+
keepSegs = Max(keepSegs, wal_keep_segments);
426+
427+
/* if currpos reaches failLSN, we lose our segment */
428+
failSeg = targetSeg + keepSegs + 1;
429+
XLogSegNoOffsetToRecPtr(failSeg, 0, wal_segment_size, failLSN);
430+
431+
values[i++] = Int64GetDatum(failLSN - currlsn);
410432
}
411-
else
412-
nulls[i++] = true;
413433

414434
Assert(i == PG_GET_REPLICATION_SLOTS_COLS);
415435

‎src/include/access/xlog_internal.h

Lines changed: 7 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -121,6 +121,13 @@ typedef XLogLongPageHeaderData *XLogLongPageHeader;
121121
#define XLByteToPrevSeg(xlrp, logSegNo, wal_segsz_bytes) \
122122
logSegNo = ((xlrp) - 1) / (wal_segsz_bytes)
123123

124+
/*
125+
* Convert values of GUCs measured in megabytes to equiv. segment count.
126+
* Rounds down.
127+
*/
128+
#define XLogMBVarToSegs(mbvar, wal_segsz_bytes) \
129+
((mbvar) / ((wal_segsz_bytes) / (1024 * 1024)))
130+
124131
/*
125132
* Is an XLogRecPtr within a particular XLOG segment?
126133
*

‎src/include/catalog/catversion.h

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -53,6 +53,6 @@
5353
*/
5454

5555
/* yyyymmddN */
56-
#define CATALOG_VERSION_NO 202007061
56+
#define CATALOG_VERSION_NO 202007071
5757

5858
#endif

‎src/include/catalog/pg_proc.dat

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -10075,9 +10075,9 @@
1007510075
proname => 'pg_get_replication_slots', prorows => '10', proisstrict => 'f',
1007610076
proretset => 't', provolatile => 's', prorettype => 'record',
1007710077
proargtypes => '',
10078-
proallargtypes => '{name,name,text,oid,bool,bool,int4,xid,xid,pg_lsn,pg_lsn,text,pg_lsn}',
10078+
proallargtypes => '{name,name,text,oid,bool,bool,int4,xid,xid,pg_lsn,pg_lsn,text,int8}',
1007910079
proargmodes => '{o,o,o,o,o,o,o,o,o,o,o,o,o}',
10080-
proargnames => '{slot_name,plugin,slot_type,datoid,temporary,active,active_pid,xmin,catalog_xmin,restart_lsn,confirmed_flush_lsn,wal_status,min_safe_lsn}',
10080+
proargnames => '{slot_name,plugin,slot_type,datoid,temporary,active,active_pid,xmin,catalog_xmin,restart_lsn,confirmed_flush_lsn,wal_status,safe_wal_size}',
1008110081
prosrc => 'pg_get_replication_slots' },
1008210082
{ oid => '3786', descr => 'set up a logical replication slot',
1008310083
proname => 'pg_create_logical_replication_slot', provolatile => 'v',

‎src/test/recovery/t/019_replslot_limit.pl

Lines changed: 10 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -28,7 +28,7 @@
2828

2929
# The slot state and remain should be null before the first connection
3030
my $result = $node_master->safe_psql('postgres',
31-
"SELECT restart_lsn IS NULL, wal_status is NULL, min_safe_lsn is NULL FROM pg_replication_slots WHERE slot_name = 'rep1'"
31+
"SELECT restart_lsn IS NULL, wal_status is NULL, safe_wal_size is NULL FROM pg_replication_slots WHERE slot_name = 'rep1'"
3232
);
3333
is($result, "t|t|t", 'check the state of non-reserved slot is "unknown"');
3434

@@ -52,9 +52,9 @@
5252
# Stop standby
5353
$node_standby->stop;
5454

55-
# Preparation done, the slot is the state "normal" now
55+
# Preparation done, the slot is the state "reserved" now
5656
$result = $node_master->safe_psql('postgres',
57-
"SELECT wal_status, min_safe_lsn is NULL FROM pg_replication_slots WHERE slot_name = 'rep1'"
57+
"SELECT wal_status, safe_wal_size IS NULL FROM pg_replication_slots WHERE slot_name = 'rep1'"
5858
);
5959
is($result, "reserved|t", 'check the catching-up state');
6060

@@ -64,7 +64,7 @@
6464

6565
# The slot is always "safe" when fitting max_wal_size
6666
$result = $node_master->safe_psql('postgres',
67-
"SELECT wal_status, min_safe_lsn is NULL FROM pg_replication_slots WHERE slot_name = 'rep1'"
67+
"SELECT wal_status, safe_wal_size IS NULL FROM pg_replication_slots WHERE slot_name = 'rep1'"
6868
);
6969
is($result, "reserved|t",
7070
'check that it is safe if WAL fits in max_wal_size');
@@ -74,7 +74,7 @@
7474

7575
# The slot is always "safe" when max_slot_wal_keep_size is not set
7676
$result = $node_master->safe_psql('postgres',
77-
"SELECT wal_status, min_safe_lsn is NULL FROM pg_replication_slots WHERE slot_name = 'rep1'"
77+
"SELECT wal_status, safe_wal_size IS NULL FROM pg_replication_slots WHERE slot_name = 'rep1'"
7878
);
7979
is($result, "reserved|t", 'check that slot is working');
8080

@@ -94,9 +94,7 @@
9494
));
9595
$node_master->reload;
9696

97-
# The slot is in safe state. The distance from the min_safe_lsn should
98-
# be as almost (max_slot_wal_keep_size - 1) times large as the segment
99-
# size
97+
# The slot is in safe state.
10098

10199
$result = $node_master->safe_psql('postgres',
102100
"SELECT wal_status FROM pg_replication_slots WHERE slot_name = 'rep1'");
@@ -110,7 +108,7 @@
110108
$result = $node_master->safe_psql('postgres',
111109
"SELECT wal_status FROM pg_replication_slots WHERE slot_name = 'rep1'");
112110
is($result, "reserved",
113-
'check that min_safe_lsn gets close to the current LSN');
111+
'check that safe_wal_size gets close to the current LSN');
114112

115113
# The standby can reconnect to master
116114
$node_standby->start;
@@ -152,9 +150,9 @@
152150
# Advance WAL again without checkpoint; remain goes to 0.
153151
advance_wal($node_master, 1);
154152

155-
# Slot gets into 'unreserved' state
153+
# Slot gets into 'unreserved' state and safe_wal_size is negative
156154
$result = $node_master->safe_psql('postgres',
157-
"SELECT wal_status, min_safe_lsn is NULL FROM pg_replication_slots WHERE slot_name = 'rep1'"
155+
"SELECT wal_status, safe_wal_size <= 0 FROM pg_replication_slots WHERE slot_name = 'rep1'"
158156
);
159157
is($result, "unreserved|t",
160158
'check that the slot state changes to "unreserved"');
@@ -186,7 +184,7 @@
186184

187185
# This slot should be broken
188186
$result = $node_master->safe_psql('postgres',
189-
"SELECT slot_name, active, restart_lsn IS NULL, wal_status, min_safe_lsn FROM pg_replication_slots WHERE slot_name = 'rep1'"
187+
"SELECT slot_name, active, restart_lsn IS NULL, wal_status, safe_wal_size FROM pg_replication_slots WHERE slot_name = 'rep1'"
190188
);
191189
is($result, "rep1|f|t|lost|",
192190
'check that the slot became inactive and the state "lost" persists');

‎src/test/regress/expected/rules.out

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -1464,8 +1464,8 @@ pg_replication_slots| SELECT l.slot_name,
14641464
l.restart_lsn,
14651465
l.confirmed_flush_lsn,
14661466
l.wal_status,
1467-
l.min_safe_lsn
1468-
FROM (pg_get_replication_slots() l(slot_name, plugin, slot_type, datoid, temporary, active, active_pid, xmin, catalog_xmin, restart_lsn, confirmed_flush_lsn, wal_status, min_safe_lsn)
1467+
l.safe_wal_size
1468+
FROM (pg_get_replication_slots() l(slot_name, plugin, slot_type, datoid, temporary, active, active_pid, xmin, catalog_xmin, restart_lsn, confirmed_flush_lsn, wal_status, safe_wal_size)
14691469
LEFT JOIN pg_database d ON ((l.datoid = d.oid)));
14701470
pg_roles| SELECT pg_authid.rolname,
14711471
pg_authid.rolsuper,

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