-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.272 2010/04/29 21:36:18 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.273 2010/05/02 02:10:32 tgl Exp $ -->
<chapter Id="runtime-config">
<title>Server Configuration</title>
<para>
When Hot Standby is active, this parameter specifies a wait policy
for applying WAL entries that conflict with active queries.
- If a conflict should occur the server will delay up to this number
- of seconds before it cancels conflicting queries, as
+ If a conflict should occur the server will delay up to this long
+ before it cancels conflicting queries, as
described in <xref linkend="hot-standby-conflict">.
- Typically, this parameter is used only during replication.
- The value is specified in seconds, and -1 causes the standby to wait
- forever for a conflicting query to complete.
The default is 30 seconds.
+ A value of -1 causes the standby to wait forever for a conflicting
+ query to complete.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</para>
<para>
While it is tempting to believe that <varname>max_standby_delay</>
- is the maximum number of seconds a query can run before
+ is the maximum length of time a query can run before
cancellation is possible, this is not true. When a long-running
query ends, there is a finite time required to apply backlogged
WAL logs. If a second long-running query appears before the
-<!-- $PostgreSQL: pgsql/doc/src/sgml/high-availability.sgml,v 1.65 2010/04/29 21:36:18 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/high-availability.sgml,v 1.66 2010/05/02 02:10:32 tgl Exp $ -->
<chapter id="high-availability">
<title>High Availability, Load Balancing, and Replication</title>
There are a number of choices for resolving query conflicts. The default
is to wait and hope the query finishes. The server will wait
automatically until the lag between primary and standby is at most
- <varname>max_standby_delay</> seconds. Once that grace period expires,
+ <xref linkend="guc-max-standby-delay"> (30 seconds by default).
+ Once that grace period expires,
one of the following actions is taken:
<itemizedlist>
</para>
<para>
- <varname>max_standby_delay</> is set in <filename>postgresql.conf</>.
- The parameter applies to the server as a whole, so if the delay is consumed
- by a single query then there may be little or no waiting for queries that
- follow, though they will have benefited equally from the initial
- waiting period. The server may take time to catch up again before the grace
- period is available again, though if there is a heavy and constant stream
- of conflicts it may seldom catch up fully.
+ Keep in mind that <varname>max_standby_delay</> is compared to the
+ difference between the standby server's clock and the transaction
+ commit timestamps read from the WAL log. Thus, the grace period
+ allowed to any one query on the standby is never more than
+ <varname>max_standby_delay</>, and could be considerably less if the
+ standby has already fallen behind as a result of waiting for previous
+ queries to complete, or as a result of being unable to keep up with a
+ heavy update load.
</para>
+ <caution>
+ <para>
+ Be sure that the primary and standby servers' clocks are kept in sync;
+ otherwise the values compared to <varname>max_standby_delay</> will be
+ erroneous, possibly leading to undesirable query cancellations.
+ If the clocks are intentionally not in sync, or if there is a large
+ propagation delay from primary to standby, it is advisable to set
+ <varname>max_standby_delay</> to -1. In any case the value should be
+ larger than the largest expected clock skew between primary and standby.
+ </para>
+ </caution>
+
<para>
Users should be clear that tables that are regularly and heavily updated on the
primary server will quickly cause cancellation of longer running queries on
<varname>max_standby_delay</> or even set it to zero, though that is a
very aggressive setting. If the standby server is tasked as an additional
server for decision support queries then it might be acceptable to set this
- to a value of many hours (in seconds). It is also possible to set
+ to a value of many hours. It is also possible to set
<varname>max_standby_delay</> to -1 which means wait forever for queries
to complete; this will be useful when performing
an archive recovery from a backup.
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/storage/ipc/standby.c,v 1.20 2010/04/28 16:10:42 heikki Exp $
+ * $PostgreSQL: pgsql/src/backend/storage/ipc/standby.c,v 1.21 2010/05/02 02:10:33 tgl Exp $
*
*-------------------------------------------------------------------------
*/
static bool
WaitExceedsMaxStandbyDelay(void)
{
- long delay_secs;
- int delay_usecs;
-
- if (MaxStandbyDelay == -1)
- return false;
-
/* Are we past max_standby_delay? */
- TimestampDifference(GetLatestXLogTime(), GetCurrentTimestamp(),
- &delay_secs, &delay_usecs);
- if (delay_secs > MaxStandbyDelay)
+ if (MaxStandbyDelay >= 0 &&
+ TimestampDifferenceExceeds(GetLatestXLogTime(), GetCurrentTimestamp(),
+ MaxStandbyDelay))
return true;
/*
- * Sleep, then do bookkeeping.
+ * Sleep a bit (this is essential to avoid busy-waiting).
*/
pg_usleep(standbyWait_us);
/*
- * Progressively increase the sleep times.
+ * Progressively increase the sleep times, but not to more than 1s,
+ * since pg_usleep isn't interruptable on some platforms.
*/
standbyWait_us *= 2;
if (standbyWait_us > 1000000)
standbyWait_us = 1000000;
- if (standbyWait_us > MaxStandbyDelay * 1000000 / 4)
- standbyWait_us = MaxStandbyDelay * 1000000 / 4;
return false;
}
ResolveRecoveryConflictWithVirtualXIDs(VirtualTransactionId *waitlist,
ProcSignalReason reason)
{
- char waitactivitymsg[100];
- char oldactivitymsg[101];
-
while (VirtualTransactionIdIsValid(*waitlist))
{
- long wait_s;
- int wait_us; /* wait in microseconds (us) */
TimestampTz waitStart;
- bool logged;
+ char *new_status;
+
+ pgstat_report_waiting(true);
waitStart = GetCurrentTimestamp();
+ new_status = NULL; /* we haven't changed the ps display */
+
+ /* reset standbyWait_us for each xact we wait for */
standbyWait_us = STANDBY_INITIAL_WAIT_US;
- logged = false;
/* wait until the virtual xid is gone */
while (!ConditionalVirtualXactLockTableWait(*waitlist))
{
/*
- * Report if we have been waiting for a while now...
+ * Report via ps if we have been waiting for more than 500 msec
+ * (should that be configurable?)
*/
- TimestampTz now = GetCurrentTimestamp();
-
- TimestampDifference(waitStart, now, &wait_s, &wait_us);
- if (!logged && (wait_s > 0 || wait_us > 500000))
+ if (update_process_title && new_status == NULL &&
+ TimestampDifferenceExceeds(waitStart, GetCurrentTimestamp(),
+ 500))
{
- const char *oldactivitymsgp;
+ const char *old_status;
int len;
- oldactivitymsgp = get_ps_display(&len);
-
- if (len > 100)
- len = 100;
-
- memcpy(oldactivitymsg, oldactivitymsgp, len);
- oldactivitymsg[len] = 0;
-
- snprintf(waitactivitymsg, sizeof(waitactivitymsg),
- "waiting for max_standby_delay (%u s)",
+ old_status = get_ps_display(&len);
+ new_status = (char *) palloc(len + 50);
+ memcpy(new_status, old_status, len);
+ snprintf(new_status + len, 50,
+ " waiting for max_standby_delay (%d ms)",
MaxStandbyDelay);
- set_ps_display(waitactivitymsg, false);
-
- pgstat_report_waiting(true);
-
- logged = true;
+ set_ps_display(new_status, false);
+ new_status[len] = '\0'; /* truncate off " waiting" */
}
/* Is it time to kill it? */
* unresponsive backend when system is heavily loaded.
*/
if (pid != 0)
- pg_usleep(5000);
+ pg_usleep(5000L);
}
}
- /* Reset ps display */
- if (logged)
+ /* Reset ps display if we changed it */
+ if (new_status)
{
- set_ps_display(oldactivitymsg, false);
- pgstat_report_waiting(false);
+ set_ps_display(new_status, false);
+ pfree(new_status);
}
+ pgstat_report_waiting(false);
/* The virtual transaction is gone now, wait for the next one */
waitlist++;
*/
SendRecoveryConflictWithBufferPin(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN);
}
- else if (MaxStandbyDelay == -1)
+ else if (MaxStandbyDelay < 0)
{
/*
* Send out a request to check for buffer pin deadlocks before we
}
else
{
- TimestampTz now;
- long standby_delay_secs; /* How far Startup process is lagging */
- int standby_delay_usecs;
-
- now = GetCurrentTimestamp();
+ TimestampTz then = GetLatestXLogTime();
+ TimestampTz now = GetCurrentTimestamp();
/* Are we past max_standby_delay? */
- TimestampDifference(GetLatestXLogTime(), now,
- &standby_delay_secs, &standby_delay_usecs);
-
- if (standby_delay_secs >= MaxStandbyDelay)
+ if (TimestampDifferenceExceeds(then, now, MaxStandbyDelay))
{
/*
* We're already behind, so clear a path as quickly as possible.
TimestampTz fin_time; /* Expected wake-up time by timer */
long timer_delay_secs; /* Amount of time we set timer
* for */
- int timer_delay_usecs = 0;
+ int timer_delay_usecs;
/*
* Send out a request to check for buffer pin deadlocks before we
/*
* How much longer we should wait?
*/
- timer_delay_secs = MaxStandbyDelay - standby_delay_secs;
- if (standby_delay_usecs > 0)
- {
- timer_delay_secs -= 1;
- timer_delay_usecs = 1000000 - standby_delay_usecs;
- }
+ fin_time = TimestampTzPlusMilliseconds(then, MaxStandbyDelay);
+
+ TimestampDifference(now, fin_time,
+ &timer_delay_secs, &timer_delay_usecs);
/*
* It's possible that the difference is less than a microsecond;
if (timer_delay_secs == 0 && timer_delay_usecs == 0)
timer_delay_usecs = 1;
- /*
- * When is the finish time? We recheck this if we are woken early.
- */
- fin_time = TimestampTzPlusMilliseconds(now,
- (timer_delay_secs * 1000) +
- (timer_delay_usecs / 1000));
-
if (enable_standby_sig_alarm(timer_delay_secs, timer_delay_usecs, fin_time))
sig_alarm_enabled = true;
else
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