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Support an optional asynchronous commit mode, in which we don't flush WAL

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before reporting a transaction committed.  Data consistency is still
guaranteed (unlike setting fsync = off), but a crash may lose the effects
of the last few transactions.  Patch by Simon, some editorialization by Tom.
This commit is contained in:
Tom Lane
2007-08-01 22:45:09 +00:00
parent c722628a43
commit 4a78cdeb6b
25 changed files with 998 additions and 303 deletions
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90
src/backend/access/transam/xact.c
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@ -10,7 +10,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.245 2007/06/07 21:45:58 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.246 2007/08/01 22:45:07 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -55,6 +55,8 @@ int XactIsoLevel;
bool DefaultXactReadOnly = false;
bool XactReadOnly;
bool XactSyncCommit = true;
int CommitDelay = 0; /* precommit delay in microseconds */
int CommitSiblings = 5; /* # concurrent xacts needed to sleep */
@ -174,6 +176,11 @@ static TimestampTz xactStopTimestamp;
*/
static char *prepareGID;
/*
* Some commands want to force synchronous commit.
*/
static bool forceSyncCommit = false;
/*
* Private context for transaction-abort work --- we reserve space for this
* at startup to ensure that AbortTransaction and AbortSubTransaction can work
@ -554,6 +561,18 @@ CommandCounterIncrement(void)
AtStart_Cache();
}
/*
* ForceSyncCommit
*
* Interface routine to allow commands to force a synchronous commit of the
* current top-level transaction
*/
void
ForceSyncCommit(void)
{
forceSyncCommit = true;
}
/* ----------------------------------------------------------------
* StartTransaction stuff
@ -724,6 +743,7 @@ RecordTransactionCommit(void)
{
TransactionId xid = GetCurrentTransactionId();
bool madeTCentries;
bool isAsyncCommit = false;
XLogRecPtr recptr;
/* Tell bufmgr and smgr to prepare for commit */
@ -810,21 +830,44 @@ RecordTransactionCommit(void)
if (MyXactMadeXLogEntry)
{
/*
* Sleep before flush! So we can flush more than one commit
* records per single fsync. (The idea is some other backend may
* do the XLogFlush while we're sleeping. This needs work still,
* because on most Unixen, the minimum select() delay is 10msec or
* more, which is way too long.)
*
* We do not sleep if enableFsync is not turned on, nor if there
* are fewer than CommitSiblings other backends with active
* transactions.
* If the user has set synchronous_commit = off, and we're
* not doing cleanup of any rels nor committing any command
* that wanted to force sync commit, then we can defer fsync.
*/
if (CommitDelay > 0 && enableFsync &&
CountActiveBackends() >= CommitSiblings)
pg_usleep(CommitDelay);
if (XactSyncCommit || forceSyncCommit || nrels > 0)
{
/*
* Synchronous commit case.
*
* Sleep before flush! So we can flush more than one commit
* records per single fsync. (The idea is some other backend
* may do the XLogFlush while we're sleeping. This needs work
* still, because on most Unixen, the minimum select() delay
* is 10msec or more, which is way too long.)
*
* We do not sleep if enableFsync is not turned on, nor if
* there are fewer than CommitSiblings other backends with
* active transactions.
*/
if (CommitDelay > 0 && enableFsync &&
CountActiveBackends() >= CommitSiblings)
pg_usleep(CommitDelay);
XLogFlush(recptr);
XLogFlush(recptr);
}
else
{
/*
* Asynchronous commit case.
*/
isAsyncCommit = true;
/*
* Report the latest async commit LSN, so that
* the WAL writer knows to flush this commit.
*/
XLogSetAsyncCommitLSN(recptr);
}
}
/*
@ -835,12 +878,24 @@ RecordTransactionCommit(void)
* emitted an XLOG record for our commit, and so in the event of a
* crash the clog update might be lost. This is okay because no one
* else will ever care whether we committed.
*
* The recptr here refers to the last xlog entry by this transaction
* so is the correct value to use for setting the clog.
*/
if (madeTCentries || MyXactMadeTempRelUpdate)
{
TransactionIdCommit(xid);
/* to avoid race conditions, the parent must commit first */
TransactionIdCommitTree(nchildren, children);
if (isAsyncCommit)
{
TransactionIdAsyncCommit(xid, recptr);
/* to avoid race conditions, the parent must commit first */
TransactionIdAsyncCommitTree(nchildren, children, recptr);
}
else
{
TransactionIdCommit(xid);
/* to avoid race conditions, the parent must commit first */
TransactionIdCommitTree(nchildren, children);
}
}
/* Checkpoint can proceed now */
@ -1406,6 +1461,7 @@ StartTransaction(void)
FreeXactSnapshot();
XactIsoLevel = DefaultXactIsoLevel;
XactReadOnly = DefaultXactReadOnly;
forceSyncCommit = false;
/*
* reinitialize within-transaction counters