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Commitd8cd0c6

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Remove the restriction that the relmap must be 512 bytes.
Instead of relying on the ability to atomically overwrite theentire relmap file in one shot, write a new one and durablyrename it into place. Removing the struct padding and thecalculation showing why the map is exactly 512 bytes, and changethe maximum number of entries to a nearby round number.Patch by me, reviewed by Andres Freund and Dilip Kumar.Discussion:http://postgr.es/m/CA+TgmoZq5%3DLWDK7kHaUbmWXxcaTuw_QwafgG9dr-BaPym_U8WQ%40mail.gmail.comDiscussion:http://postgr.es/m/CAFiTN-ttOXLX75k_WzRo9ar=VvxFhrHi+rJxns997F+yvkm==A@mail.gmail.com
1 parente530be2 commitd8cd0c6

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4 files changed

+58
-46
lines changed

4 files changed

+58
-46
lines changed

‎doc/src/sgml/monitoring.sgml

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -1409,8 +1409,8 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
14091409
<entry>Waiting for a read of the relation map file.</entry>
14101410
</row>
14111411
<row>
1412-
<entry><literal>RelationMapSync</literal></entry>
1413-
<entry>Waiting forthe relation map file to reach durable storage.</entry>
1412+
<entry><literal>RelationMapReplace</literal></entry>
1413+
<entry>Waiting fordurable replacement of a relation map file.</entry>
14141414
</row>
14151415
<row>
14161416
<entry><literal>RelationMapWrite</literal></entry>

‎src/backend/utils/activity/wait_event.c

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -633,8 +633,8 @@ pgstat_get_wait_io(WaitEventIO w)
633633
caseWAIT_EVENT_RELATION_MAP_READ:
634634
event_name="RelationMapRead";
635635
break;
636-
caseWAIT_EVENT_RELATION_MAP_SYNC:
637-
event_name="RelationMapSync";
636+
caseWAIT_EVENT_RELATION_MAP_REPLACE:
637+
event_name="RelationMapReplace";
638638
break;
639639
caseWAIT_EVENT_RELATION_MAP_WRITE:
640640
event_name="RelationMapWrite";

‎src/backend/utils/cache/relmapper.c

Lines changed: 53 additions & 41 deletions
Original file line numberDiff line numberDiff line change
@@ -60,21 +60,26 @@
6060
/*
6161
* The map file is critical data: we have no automatic method for recovering
6262
* from loss or corruption of it. We use a CRC so that we can detect
63-
* corruption. To minimize the risk of failed updates, the map file should
64-
* be kept to no more than one standard-size disk sector (ie 512 bytes),
65-
* and we use overwrite-in-place rather than playing renaming games.
66-
* The struct layout below is designed to occupy exactly 512 bytes, which
67-
* might make filesystem updates a bit more efficient.
63+
* corruption. Since the file might be more than one standard-size disk
64+
* sector in size, we cannot rely on overwrite-in-place. Instead, we generate
65+
* a new file and rename it into place, atomically replacing the original file.
6866
*
6967
* Entries in the mappings[] array are in no particular order. We could
7068
* speed searching by insisting on OID order, but it really shouldn't be
7169
* worth the trouble given the intended size of the mapping sets.
7270
*/
7371
#defineRELMAPPER_FILENAME"pg_filenode.map"
72+
#defineRELMAPPER_TEMP_FILENAME"pg_filenode.map.tmp"
7473

7574
#defineRELMAPPER_FILEMAGIC0x592717/* version ID value */
7675

77-
#defineMAX_MAPPINGS62/* 62 * 8 + 16 = 512 */
76+
/*
77+
* There's no need for this constant to have any particular value, and we
78+
* can raise it as necessary if we end up with more mapped relations. For
79+
* now, we just pick a round number that is modestly larger than the expected
80+
* number of mappings.
81+
*/
82+
#defineMAX_MAPPINGS64
7883

7984
typedefstructRelMapping
8085
{
@@ -88,7 +93,6 @@ typedef struct RelMapFile
8893
int32num_mappings;/* number of valid RelMapping entries */
8994
RelMappingmappings[MAX_MAPPINGS];
9095
pg_crc32ccrc;/* CRC of all above */
91-
int32pad;/* to make the struct size be 512 exactly */
9296
}RelMapFile;
9397

9498
/*
@@ -877,6 +881,7 @@ write_relmap_file(RelMapFile *newmap, bool write_wal, bool send_sinval,
877881
{
878882
intfd;
879883
charmapfilename[MAXPGPATH];
884+
charmaptempfilename[MAXPGPATH];
880885

881886
/*
882887
* Fill in the overhead fields and update CRC.
@@ -890,17 +895,47 @@ write_relmap_file(RelMapFile *newmap, bool write_wal, bool send_sinval,
890895
FIN_CRC32C(newmap->crc);
891896

892897
/*
893-
*Open the targetfile. We prefer todo this before entering the
894-
*critical section, so that an open() failure need not force PANIC.
898+
*Construct filenames -- a temporaryfile that we'll create towrite the
899+
*data initially, and then the permanent name to which we will rename it.
895900
*/
896901
snprintf(mapfilename,sizeof(mapfilename),"%s/%s",
897902
dbpath,RELMAPPER_FILENAME);
898-
fd=OpenTransientFile(mapfilename,O_WRONLY |O_CREAT |PG_BINARY);
903+
snprintf(maptempfilename,sizeof(maptempfilename),"%s/%s",
904+
dbpath,RELMAPPER_TEMP_FILENAME);
905+
906+
/*
907+
* Open a temporary file. If a file already exists with this name, it must
908+
* be left over from a previous crash, so we can overwrite it. Concurrent
909+
* calls to this function are not allowed.
910+
*/
911+
fd=OpenTransientFile(maptempfilename,
912+
O_WRONLY |O_CREAT |O_TRUNC |PG_BINARY);
899913
if (fd<0)
900914
ereport(ERROR,
901915
(errcode_for_file_access(),
902916
errmsg("could not open file \"%s\": %m",
903-
mapfilename)));
917+
maptempfilename)));
918+
919+
/* Write new data to the file. */
920+
pgstat_report_wait_start(WAIT_EVENT_RELATION_MAP_WRITE);
921+
if (write(fd,newmap,sizeof(RelMapFile))!=sizeof(RelMapFile))
922+
{
923+
/* if write didn't set errno, assume problem is no disk space */
924+
if (errno==0)
925+
errno=ENOSPC;
926+
ereport(ERROR,
927+
(errcode_for_file_access(),
928+
errmsg("could not write file \"%s\": %m",
929+
maptempfilename)));
930+
}
931+
pgstat_report_wait_end();
932+
933+
/* And close the file. */
934+
if (CloseTransientFile(fd)!=0)
935+
ereport(ERROR,
936+
(errcode_for_file_access(),
937+
errmsg("could not close file \"%s\": %m",
938+
maptempfilename)));
904939

905940
if (write_wal)
906941
{
@@ -924,40 +959,17 @@ write_relmap_file(RelMapFile *newmap, bool write_wal, bool send_sinval,
924959
XLogFlush(lsn);
925960
}
926961

927-
errno=0;
928-
pgstat_report_wait_start(WAIT_EVENT_RELATION_MAP_WRITE);
929-
if (write(fd,newmap,sizeof(RelMapFile))!=sizeof(RelMapFile))
930-
{
931-
/* if write didn't set errno, assume problem is no disk space */
932-
if (errno==0)
933-
errno=ENOSPC;
934-
ereport(ERROR,
935-
(errcode_for_file_access(),
936-
errmsg("could not write file \"%s\": %m",
937-
mapfilename)));
938-
}
939-
pgstat_report_wait_end();
940-
941962
/*
942-
* We choose to fsync the data to disk before considering the task done.
943-
* It would be possible to relax this if it turns out to be a performance
944-
* issue, but it would complicate checkpointing --- see notes for
945-
* CheckPointRelationMap.
963+
* durable_rename() does all the hard work of making sure that we rename
964+
* the temporary file into place in a crash-safe manner.
965+
*
966+
* NB: Although we instruct durable_rename() to use ERROR, we will often
967+
* be in a critical section at this point; if so, ERROR will become PANIC.
946968
*/
947-
pgstat_report_wait_start(WAIT_EVENT_RELATION_MAP_SYNC);
948-
if (pg_fsync(fd)!=0)
949-
ereport(data_sync_elevel(ERROR),
950-
(errcode_for_file_access(),
951-
errmsg("could not fsync file \"%s\": %m",
952-
mapfilename)));
969+
pgstat_report_wait_start(WAIT_EVENT_RELATION_MAP_REPLACE);
970+
durable_rename(maptempfilename,mapfilename,ERROR);
953971
pgstat_report_wait_end();
954972

955-
if (CloseTransientFile(fd)!=0)
956-
ereport(ERROR,
957-
(errcode_for_file_access(),
958-
errmsg("could not close file \"%s\": %m",
959-
mapfilename)));
960-
961973
/*
962974
* Now that the file is safely on disk, send sinval message to let other
963975
* backends know to re-read it. We must do this inside the critical

‎src/include/utils/wait_event.h

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -194,7 +194,7 @@ typedef enum
194194
WAIT_EVENT_LOGICAL_REWRITE_TRUNCATE,
195195
WAIT_EVENT_LOGICAL_REWRITE_WRITE,
196196
WAIT_EVENT_RELATION_MAP_READ,
197-
WAIT_EVENT_RELATION_MAP_SYNC,
197+
WAIT_EVENT_RELATION_MAP_REPLACE,
198198
WAIT_EVENT_RELATION_MAP_WRITE,
199199
WAIT_EVENT_REORDER_BUFFER_READ,
200200
WAIT_EVENT_REORDER_BUFFER_WRITE,

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