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Commit995ba28

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Rearrange mdsync() looping logic to avoid the problem that a sufficiently
fast flow of new fsync requests can prevent mdsync() from ever completing.This was an unforeseen consequence of a patch added in Mar 2006 to preventthe fsync request queue from overflowing. Problem identified by HeikkiLinnakangas and independently by ITAGAKI Takahiro; fix based on ideas fromTakahiro-san, Heikki, and Tom.Back-patch as far as 8.1 because a previous back-patch introduced the probleminto 8.1 ...
1 parentebb6bae commit995ba28

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1 file changed

+178
-91
lines changed
  • src/backend/storage/smgr

1 file changed

+178
-91
lines changed

‎src/backend/storage/smgr/md.c‎

Lines changed: 178 additions & 91 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/storage/smgr/md.c,v 1.127 2007/01/17 16:25:01 tgl Exp $
11+
* $PostgreSQL: pgsql/src/backend/storage/smgr/md.c,v 1.128 2007/04/12 17:10:55 tgl Exp $
1212
*
1313
*-------------------------------------------------------------------------
1414
*/
@@ -122,14 +122,19 @@ typedef struct
122122
BlockNumbersegno;/* which segment */
123123
}PendingOperationTag;
124124

125+
typedefuint16CycleCtr;/* can be any convenient integer size */
126+
125127
typedefstruct
126128
{
127129
PendingOperationTagtag;/* hash table key (must be first!) */
128-
intfailures;/* number of failed attempts to fsync */
130+
boolcanceled;/* T => request canceled, not yet removed */
131+
CycleCtrcycle_ctr;/* mdsync_cycle_ctr when request was made */
129132
}PendingOperationEntry;
130133

131134
staticHTAB*pendingOpsTable=NULL;
132135

136+
staticCycleCtrmdsync_cycle_ctr=0;
137+
133138

134139
typedefenum/* behavior for mdopen & _mdfd_getseg */
135140
{
@@ -856,70 +861,125 @@ mdimmedsync(SMgrRelation reln)
856861

857862
/*
858863
*mdsync() -- Sync previous writes to stable storage.
859-
*
860-
* This is only called during checkpoints, and checkpoints should only
861-
* occur in processes that have created a pendingOpsTable.
862864
*/
863865
void
864866
mdsync(void)
865867
{
866-
boolneed_retry;
868+
staticboolmdsync_in_progress= false;
869+
870+
HASH_SEQ_STATUShstat;
871+
PendingOperationEntry*entry;
872+
intabsorb_counter;
867873

874+
/*
875+
* This is only called during checkpoints, and checkpoints should only
876+
* occur in processes that have created a pendingOpsTable.
877+
*/
868878
if (!pendingOpsTable)
869879
elog(ERROR,"cannot sync without a pendingOpsTable");
870880

871881
/*
872-
* The fsync table could contain requests to fsync relations that have
873-
* been deleted (unlinked) by the time we get to them. Rather than
874-
* just hoping an ENOENT (or EACCES on Windows) error can be ignored,
875-
* what we will do is retry the whole process after absorbing fsync
876-
* request messages again. Since mdunlink() queues a "revoke" message
877-
* before actually unlinking, the fsync request is guaranteed to be gone
878-
* the second time if it really was this case. DROP DATABASE likewise
879-
* has to tell us to forget fsync requests before it starts deletions.
882+
* If we are in the bgwriter, the sync had better include all fsync
883+
* requests that were queued by backends before the checkpoint REDO
884+
* point was determined. We go that a little better by accepting all
885+
* requests queued up to the point where we start fsync'ing.
880886
*/
881-
do {
882-
HASH_SEQ_STATUShstat;
883-
PendingOperationEntry*entry;
884-
intabsorb_counter;
887+
AbsorbFsyncRequests();
888+
889+
/*
890+
* To avoid excess fsync'ing (in the worst case, maybe a never-terminating
891+
* checkpoint), we want to ignore fsync requests that are entered into the
892+
* hashtable after this point --- they should be processed next time,
893+
* instead. We use mdsync_cycle_ctr to tell old entries apart from new
894+
* ones: new ones will have cycle_ctr equal to the incremented value of
895+
* mdsync_cycle_ctr.
896+
*
897+
* In normal circumstances, all entries present in the table at this
898+
* point will have cycle_ctr exactly equal to the current (about to be old)
899+
* value of mdsync_cycle_ctr. However, if we fail partway through the
900+
* fsync'ing loop, then older values of cycle_ctr might remain when we
901+
* come back here to try again. Repeated checkpoint failures would
902+
* eventually wrap the counter around to the point where an old entry
903+
* might appear new, causing us to skip it, possibly allowing a checkpoint
904+
* to succeed that should not have. To forestall wraparound, any time
905+
* the previous mdsync() failed to complete, run through the table and
906+
* forcibly set cycle_ctr = mdsync_cycle_ctr.
907+
*
908+
* Think not to merge this loop with the main loop, as the problem is
909+
* exactly that that loop may fail before having visited all the entries.
910+
* From a performance point of view it doesn't matter anyway, as this
911+
* path will never be taken in a system that's functioning normally.
912+
*/
913+
if (mdsync_in_progress)
914+
{
915+
/* prior try failed, so update any stale cycle_ctr values */
916+
hash_seq_init(&hstat,pendingOpsTable);
917+
while ((entry= (PendingOperationEntry*)hash_seq_search(&hstat))!=NULL)
918+
{
919+
entry->cycle_ctr=mdsync_cycle_ctr;
920+
}
921+
}
885922

886-
need_retry= false;
923+
/* Advance counter so that new hashtable entries are distinguishable */
924+
mdsync_cycle_ctr++;
887925

926+
/* Set flag to detect failure if we don't reach the end of the loop */
927+
mdsync_in_progress= true;
928+
929+
/* Now scan the hashtable for fsync requests to process */
930+
absorb_counter=FSYNCS_PER_ABSORB;
931+
hash_seq_init(&hstat,pendingOpsTable);
932+
while ((entry= (PendingOperationEntry*)hash_seq_search(&hstat))!=NULL)
933+
{
888934
/*
889-
* If we are in the bgwriter, the sync had better include all fsync
890-
* requests that were queued by backends before the checkpoint REDO
891-
* point was determined. We go that a little better by accepting all
892-
* requests queued up to the point where we start fsync'ing.
935+
* If the entry is new then don't process it this time. Note that
936+
* "continue" bypasses the hash-remove call at the bottom of the loop.
893937
*/
894-
AbsorbFsyncRequests();
938+
if (entry->cycle_ctr==mdsync_cycle_ctr)
939+
continue;
895940

896-
absorb_counter=FSYNCS_PER_ABSORB;
897-
hash_seq_init(&hstat,pendingOpsTable);
898-
while ((entry= (PendingOperationEntry*)hash_seq_search(&hstat))!=NULL)
941+
/* Else assert we haven't missed it */
942+
Assert((CycleCtr) (entry->cycle_ctr+1)==mdsync_cycle_ctr);
943+
944+
/*
945+
* If fsync is off then we don't have to bother opening the file
946+
* at all. (We delay checking until this point so that changing
947+
* fsync on the fly behaves sensibly.) Also, if the entry is
948+
* marked canceled, fall through to delete it.
949+
*/
950+
if (enableFsync&& !entry->canceled)
899951
{
952+
intfailures;
953+
900954
/*
901-
* If fsync is off then we don't have to bother opening the file
902-
* at all. (We delay checking until this point so that changing
903-
* fsync on the fly behaves sensibly.)
955+
* If in bgwriter, we want to absorb pending requests every so
956+
* often to prevent overflow of the fsync request queue. It is
957+
* unspecified whether newly-added entries will be visited by
958+
* hash_seq_search, but we don't care since we don't need to
959+
* process them anyway.
904960
*/
905-
if (enableFsync)
961+
if (--absorb_counter <=0)
962+
{
963+
AbsorbFsyncRequests();
964+
absorb_counter=FSYNCS_PER_ABSORB;
965+
}
966+
967+
/*
968+
* The fsync table could contain requests to fsync segments that
969+
* have been deleted (unlinked) by the time we get to them.
970+
* Rather than just hoping an ENOENT (or EACCES on Windows) error
971+
* can be ignored, what we do on error is absorb pending requests
972+
* and then retry. Since mdunlink() queues a "revoke" message
973+
* before actually unlinking, the fsync request is guaranteed to
974+
* be marked canceled after the absorb if it really was this case.
975+
* DROP DATABASE likewise has to tell us to forget fsync requests
976+
* before it starts deletions.
977+
*/
978+
for (failures=0; ;failures++)/* loop exits at "break" */
906979
{
907980
SMgrRelationreln;
908981
MdfdVec*seg;
909982

910-
/*
911-
* If in bgwriter, we want to absorb pending requests every so
912-
* often to prevent overflow of the fsync request queue. This
913-
* could result in deleting the current entry out from under
914-
* our hashtable scan, so the procedure is to fall out of the
915-
* scan and start over from the top of the function.
916-
*/
917-
if (--absorb_counter <=0)
918-
{
919-
need_retry= true;
920-
break;
921-
}
922-
923983
/*
924984
* Find or create an smgr hash entry for this relation. This
925985
* may seem a bit unclean -- md calling smgr? But it's really
@@ -940,50 +1000,64 @@ mdsync(void)
9401000
/*
9411001
* It is possible that the relation has been dropped or
9421002
* truncated since the fsync request was entered. Therefore,
943-
* allow ENOENT, but only if we didn't failoncealready on
1003+
* allow ENOENT, but only if we didn't fail already on
9441004
* this file. This applies both during _mdfd_getseg() and
9451005
* during FileSync, since fd.c might have closed the file
9461006
* behind our back.
9471007
*/
9481008
seg=_mdfd_getseg(reln,
9491009
entry->tag.segno* ((BlockNumber)RELSEG_SIZE),
9501010
false,EXTENSION_RETURN_NULL);
951-
if (seg==NULL||
952-
FileSync(seg->mdfd_vfd)<0)
953-
{
954-
/*
955-
* XXX is there any point in allowing more than one try?
956-
* Don't see one at the moment, but easy to change the
957-
* test here if so.
958-
*/
959-
if (!FILE_POSSIBLY_DELETED(errno)||
960-
++(entry->failures)>1)
961-
ereport(ERROR,
962-
(errcode_for_file_access(),
963-
errmsg("could not fsync segment %u of relation %u/%u/%u: %m",
964-
entry->tag.segno,
965-
entry->tag.rnode.spcNode,
966-
entry->tag.rnode.dbNode,
967-
entry->tag.rnode.relNode)));
968-
else
969-
ereport(DEBUG1,
970-
(errcode_for_file_access(),
971-
errmsg("could not fsync segment %u of relation %u/%u/%u, but retrying: %m",
972-
entry->tag.segno,
973-
entry->tag.rnode.spcNode,
974-
entry->tag.rnode.dbNode,
975-
entry->tag.rnode.relNode)));
976-
need_retry= true;
977-
continue;/* don't delete the hashtable entry */
978-
}
979-
}
1011+
if (seg!=NULL&&
1012+
FileSync(seg->mdfd_vfd) >=0)
1013+
break;/* success; break out of retry loop */
9801014

981-
/* Okay, delete this entry */
982-
if (hash_search(pendingOpsTable,&entry->tag,
983-
HASH_REMOVE,NULL)==NULL)
984-
elog(ERROR,"pendingOpsTable corrupted");
1015+
/*
1016+
* XXX is there any point in allowing more than one retry?
1017+
* Don't see one at the moment, but easy to change the
1018+
* test here if so.
1019+
*/
1020+
if (!FILE_POSSIBLY_DELETED(errno)||
1021+
failures>0)
1022+
ereport(ERROR,
1023+
(errcode_for_file_access(),
1024+
errmsg("could not fsync segment %u of relation %u/%u/%u: %m",
1025+
entry->tag.segno,
1026+
entry->tag.rnode.spcNode,
1027+
entry->tag.rnode.dbNode,
1028+
entry->tag.rnode.relNode)));
1029+
else
1030+
ereport(DEBUG1,
1031+
(errcode_for_file_access(),
1032+
errmsg("could not fsync segment %u of relation %u/%u/%u, but retrying: %m",
1033+
entry->tag.segno,
1034+
entry->tag.rnode.spcNode,
1035+
entry->tag.rnode.dbNode,
1036+
entry->tag.rnode.relNode)));
1037+
1038+
/*
1039+
* Absorb incoming requests and check to see if canceled.
1040+
*/
1041+
AbsorbFsyncRequests();
1042+
absorb_counter=FSYNCS_PER_ABSORB;/* might as well... */
1043+
1044+
if (entry->canceled)
1045+
break;
1046+
}/* end retry loop */
9851047
}
986-
}while (need_retry);
1048+
1049+
/*
1050+
* If we get here, either we fsync'd successfully, or we don't have
1051+
* to because enableFsync is off, or the entry is (now) marked
1052+
* canceled. Okay to delete it.
1053+
*/
1054+
if (hash_search(pendingOpsTable,&entry->tag,
1055+
HASH_REMOVE,NULL)==NULL)
1056+
elog(ERROR,"pendingOpsTable corrupted");
1057+
}/* end loop over hashtable entries */
1058+
1059+
/* Flag successful completion of mdsync */
1060+
mdsync_in_progress= false;
9871061
}
9881062

9891063
/*
@@ -1027,8 +1101,8 @@ register_dirty_segment(SMgrRelation reln, MdfdVec *seg)
10271101
*
10281102
* The range of possible segment numbers is way less than the range of
10291103
* BlockNumber, so we can reserve high values of segno for special purposes.
1030-
* We define two: FORGET_RELATION_FSYNC means todrop pending fsyncs for
1031-
* a relation, and FORGET_DATABASE_FSYNC means todrop pending fsyncs for
1104+
* We define two: FORGET_RELATION_FSYNC means tocancel pending fsyncs for
1105+
* a relation, and FORGET_DATABASE_FSYNC means tocancel pending fsyncs for
10321106
* a whole database. (These are a tad slow because the hash table has to be
10331107
* searched linearly, but it doesn't seem worth rethinking the table structure
10341108
* for them.)
@@ -1049,10 +1123,8 @@ RememberFsyncRequest(RelFileNode rnode, BlockNumber segno)
10491123
{
10501124
if (RelFileNodeEquals(entry->tag.rnode,rnode))
10511125
{
1052-
/* Okay, delete this entry */
1053-
if (hash_search(pendingOpsTable,&entry->tag,
1054-
HASH_REMOVE,NULL)==NULL)
1055-
elog(ERROR,"pendingOpsTable corrupted");
1126+
/* Okay, cancel this entry */
1127+
entry->canceled= true;
10561128
}
10571129
}
10581130
}
@@ -1067,10 +1139,8 @@ RememberFsyncRequest(RelFileNode rnode, BlockNumber segno)
10671139
{
10681140
if (entry->tag.rnode.dbNode==rnode.dbNode)
10691141
{
1070-
/* Okay, delete this entry */
1071-
if (hash_search(pendingOpsTable,&entry->tag,
1072-
HASH_REMOVE,NULL)==NULL)
1073-
elog(ERROR,"pendingOpsTable corrupted");
1142+
/* Okay, cancel this entry */
1143+
entry->canceled= true;
10741144
}
10751145
}
10761146
}
@@ -1090,8 +1160,25 @@ RememberFsyncRequest(RelFileNode rnode, BlockNumber segno)
10901160
&key,
10911161
HASH_ENTER,
10921162
&found);
1093-
if (!found)/* new entry, so initialize it */
1094-
entry->failures=0;
1163+
/* if new or previously canceled entry, initialize it */
1164+
if (!found||entry->canceled)
1165+
{
1166+
entry->canceled= false;
1167+
entry->cycle_ctr=mdsync_cycle_ctr;
1168+
}
1169+
/*
1170+
* NB: it's intentional that we don't change cycle_ctr if the entry
1171+
* already exists. The fsync request must be treated as old, even
1172+
* though the new request will be satisfied too by any subsequent
1173+
* fsync.
1174+
*
1175+
* However, if the entry is present but is marked canceled, we should
1176+
* act just as though it wasn't there. The only case where this could
1177+
* happen would be if a file had been deleted, we received but did not
1178+
* yet act on the cancel request, and the same relfilenode was then
1179+
* assigned to a new file. We mustn't lose the new request, but
1180+
* it should be considered new not old.
1181+
*/
10951182
}
10961183
}
10971184

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