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Add new replication mode synchronous_commit = 'write'.

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Replication occurs only to memory on standby, not to disk,
so provides additional performance if user wishes to
reduce durability level slightly. Adds concept of multiple
independent sync rep queues.

Fujii Masao and Simon Riggs
pull/1/head
Simon Riggs 14 years ago
parent 89dda5f297
commit 443b4821f1
8 changed files with 124 additions and 52 deletions
Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL
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  1. 18
      doc/src/sgml/config.sgml
  2. 16
      doc/src/sgml/high-availability.sgml
  3. 112
      src/backend/replication/syncrep.c
  4. 3
      src/backend/replication/walsender.c
  5. 5
      src/backend/utils/misc/guc.c
  6. 1
      src/include/access/xact.h
  7. 13
      src/include/replication/syncrep.h
  8. 8
      src/include/replication/walsender_private.h

18
doc/src/sgml/config.sgml
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@ -1560,7 +1560,7 @@ SET ENABLE_SEQSCAN TO OFF;
<para>
Specifies whether transaction commit will wait for WAL records
to be written to disk before the command returns a <quote>success</>
indication to the client. Valid values are <literal>on</>,
indication to the client. Valid values are <literal>on</>, <literal>write</>,
<literal>local</>, and <literal>off</>. The default, and safe, value
is <literal>on</>. When <literal>off</>, there can be a delay between
when success is reported to the client and when the transaction is
@ -1580,11 +1580,19 @@ SET ENABLE_SEQSCAN TO OFF;
If <xref linkend="guc-synchronous-standby-names"> is set, this
parameter also controls whether or not transaction commit will wait
for the transaction's WAL records to be flushed to disk and replicated
to the standby server. The commit wait will last until a reply from
the current synchronous standby indicates it has written the commit
record of the transaction to durable storage. If synchronous
to the standby server. When <literal>write</>, the commit wait will
last until a reply from the current synchronous standby indicates
it has received the commit record of the transaction to memory.
Normally this causes no data loss at the time of failover. However,
if both primary and standby crash, and the database cluster of
the primary gets corrupted, recent committed transactions might
be lost. When <literal>on</>, the commit wait will last until a reply
from the current synchronous standby indicates it has flushed
the commit record of the transaction to durable storage. This
avoids any data loss unless the database cluster of both primary and
standby gets corrupted simultaneously. If synchronous
replication is in use, it will normally be sensible either to wait
both for WAL records to reach both the local and remote disks, or
for both local flush and replication of WAL records, or
to allow the transaction to commit asynchronously. However, the
special value <literal>local</> is available for transactions that
wish to wait for local flush to disk, but not synchronous replication.

16
doc/src/sgml/high-availability.sgml
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@ -1010,6 +1010,16 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
standby servers using cascaded replication.
</para>
<para>
Setting <varname>synchronous_commit</> to <literal>write</> will
cause each commit to wait for confirmation that the standby has received
the commit record to memory. This provides a lower level of durability
than <literal>on</> does. However, it's a practically useful setting
because it can decrease the response time for the transaction, and causes
no data loss unless both the primary and the standby crashes and
the database of the primary gets corrupted at the same time.
</para>
<para>
Users will stop waiting if a fast shutdown is requested. However, as
when using asynchronous replication, the server will does not fully
@ -1065,13 +1075,13 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
<para>
Commits made when <varname>synchronous_commit</> is set to <literal>on</>
will wait until the sync standby responds. The response may never occur
if the last, or only, standby should crash.
or <literal>write</> will wait until the synchronous standby responds. The response
may never occur if the last, or only, standby should crash.
</para>
<para>
The best solution for avoiding data loss is to ensure you don't lose
your last remaining sync standby. This can be achieved by naming multiple
your last remaining synchronous standby. This can be achieved by naming multiple
potential synchronous standbys using <varname>synchronous_standby_names</>.
The first named standby will be used as the synchronous standby. Standbys
listed after this will take over the role of synchronous standby if the

112
src/backend/replication/syncrep.c
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@ -20,8 +20,8 @@
* per-transaction state information.
*
* Replication is either synchronous or not synchronous (async). If it is
* async, we just fastpath out of here. If it is sync, then in 9.1 we wait
* for the flush location on the standby before releasing the waiting backend.
* async, we just fastpath out of here. If it is sync, then we wait for
* the write or flush location on the standby before releasing the waiting backend.
* Further complexity in that interaction is expected in later releases.
*
* The best performing way to manage the waiting backends is to have a
@ -67,13 +67,15 @@ char *SyncRepStandbyNames;
static bool announce_next_takeover = true;
static void SyncRepQueueInsert(void);
static int SyncRepWaitMode = SYNC_REP_NO_WAIT;
static void SyncRepQueueInsert(int mode);
static void SyncRepCancelWait(void);
static int SyncRepGetStandbyPriority(void);
#ifdef USE_ASSERT_CHECKING
static bool SyncRepQueueIsOrderedByLSN(void);
static bool SyncRepQueueIsOrderedByLSN(int mode);
#endif
/*
@ -120,7 +122,7 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
* be a low cost check.
*/
if (!WalSndCtl->sync_standbys_defined ||
XLByteLE(XactCommitLSN, WalSndCtl->lsn))
XLByteLE(XactCommitLSN, WalSndCtl->lsn[SyncRepWaitMode]))
{
LWLockRelease(SyncRepLock);
return;
@ -132,8 +134,8 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
*/
MyProc->waitLSN = XactCommitLSN;
MyProc->syncRepState = SYNC_REP_WAITING;
SyncRepQueueInsert();
Assert(SyncRepQueueIsOrderedByLSN());
SyncRepQueueInsert(SyncRepWaitMode);
Assert(SyncRepQueueIsOrderedByLSN(SyncRepWaitMode));
LWLockRelease(SyncRepLock);
/* Alter ps display to show waiting for sync rep. */
@ -267,18 +269,19 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
}
/*
* Insert MyProc into SyncRepQueue, maintaining sorted invariant.
* Insert MyProc into the specified SyncRepQueue, maintaining sorted invariant.
*
* Usually we will go at tail of queue, though it's possible that we arrive
* here out of order, so start at tail and work back to insertion point.
*/
static void
SyncRepQueueInsert(void)
SyncRepQueueInsert(int mode)
{
PGPROC *proc;
proc = (PGPROC *) SHMQueuePrev(&(WalSndCtl->SyncRepQueue),
&(WalSndCtl->SyncRepQueue),
Assert(mode >= 0 && mode < NUM_SYNC_REP_WAIT_MODE);
proc = (PGPROC *) SHMQueuePrev(&(WalSndCtl->SyncRepQueue[mode]),
&(WalSndCtl->SyncRepQueue[mode]),
offsetof(PGPROC, syncRepLinks));
while (proc)
@ -290,7 +293,7 @@ SyncRepQueueInsert(void)
if (XLByteLT(proc->waitLSN, MyProc->waitLSN))
break;
proc = (PGPROC *) SHMQueuePrev(&(WalSndCtl->SyncRepQueue),
proc = (PGPROC *) SHMQueuePrev(&(WalSndCtl->SyncRepQueue[mode]),
&(proc->syncRepLinks),
offsetof(PGPROC, syncRepLinks));
}
@ -298,7 +301,7 @@ SyncRepQueueInsert(void)
if (proc)
SHMQueueInsertAfter(&(proc->syncRepLinks), &(MyProc->syncRepLinks));
else
SHMQueueInsertAfter(&(WalSndCtl->SyncRepQueue), &(MyProc->syncRepLinks));
SHMQueueInsertAfter(&(WalSndCtl->SyncRepQueue[mode]), &(MyProc->syncRepLinks));
}
/*
@ -368,7 +371,8 @@ SyncRepReleaseWaiters(void)
{
volatile WalSndCtlData *walsndctl = WalSndCtl;
volatile WalSnd *syncWalSnd = NULL;
int numprocs = 0;
int numwrite = 0;
int numflush = 0;
int priority = 0;
int i;
@ -419,20 +423,28 @@ SyncRepReleaseWaiters(void)
return;
}
if (XLByteLT(walsndctl->lsn, MyWalSnd->flush))
/*
* Set the lsn first so that when we wake backends they will release
* up to this location.
*/
if (XLByteLT(walsndctl->lsn[SYNC_REP_WAIT_WRITE], MyWalSnd->write))
{
/*
* Set the lsn first so that when we wake backends they will release
* up to this location.
*/
walsndctl->lsn = MyWalSnd->flush;
numprocs = SyncRepWakeQueue(false);
walsndctl->lsn[SYNC_REP_WAIT_WRITE] = MyWalSnd->write;
numwrite = SyncRepWakeQueue(false, SYNC_REP_WAIT_WRITE);
}
if (XLByteLT(walsndctl->lsn[SYNC_REP_WAIT_FLUSH], MyWalSnd->flush))
{
walsndctl->lsn[SYNC_REP_WAIT_FLUSH] = MyWalSnd->flush;
numflush = SyncRepWakeQueue(false, SYNC_REP_WAIT_FLUSH);
}
LWLockRelease(SyncRepLock);
elog(DEBUG3, "released %d procs up to %X/%X",
numprocs,
elog(DEBUG3, "released %d procs up to write %X/%X, %d procs up to flush %X/%X",
numwrite,
MyWalSnd->write.xlogid,
MyWalSnd->write.xrecoff,
numflush,
MyWalSnd->flush.xlogid,
MyWalSnd->flush.xrecoff);
@ -507,24 +519,26 @@ SyncRepGetStandbyPriority(void)
}
/*
* Walk queue from head. Set the state of any backends that need to be woken,
* remove them from the queue, and then wake them. Pass all = true to wake
* whole queue; otherwise, just wake up to the walsender's LSN.
* Walk the specified queue from head. Set the state of any backends that
* need to be woken, remove them from the queue, and then wake them.
* Pass all = true to wake whole queue; otherwise, just wake up to
* the walsender's LSN.
*
* Must hold SyncRepLock.
*/
int
SyncRepWakeQueue(bool all)
SyncRepWakeQueue(bool all, int mode)
{
volatile WalSndCtlData *walsndctl = WalSndCtl;
PGPROC *proc = NULL;
PGPROC *thisproc = NULL;
int numprocs = 0;
Assert(SyncRepQueueIsOrderedByLSN());
Assert(mode >= 0 && mode < NUM_SYNC_REP_WAIT_MODE);
Assert(SyncRepQueueIsOrderedByLSN(mode));
proc = (PGPROC *) SHMQueueNext(&(WalSndCtl->SyncRepQueue),
&(WalSndCtl->SyncRepQueue),
proc = (PGPROC *) SHMQueueNext(&(WalSndCtl->SyncRepQueue[mode]),
&(WalSndCtl->SyncRepQueue[mode]),
offsetof(PGPROC, syncRepLinks));
while (proc)
@ -532,7 +546,7 @@ SyncRepWakeQueue(bool all)
/*
* Assume the queue is ordered by LSN
*/
if (!all && XLByteLT(walsndctl->lsn, proc->waitLSN))
if (!all && XLByteLT(walsndctl->lsn[mode], proc->waitLSN))
return numprocs;
/*
@ -540,7 +554,7 @@ SyncRepWakeQueue(bool all)
* thisproc is valid, proc may be NULL after this.
*/
thisproc = proc;
proc = (PGPROC *) SHMQueueNext(&(WalSndCtl->SyncRepQueue),
proc = (PGPROC *) SHMQueueNext(&(WalSndCtl->SyncRepQueue[mode]),
&(proc->syncRepLinks),
offsetof(PGPROC, syncRepLinks));
@ -588,7 +602,12 @@ SyncRepUpdateSyncStandbysDefined(void)
* wants synchronous replication, we'd better wake them up.
*/
if (!sync_standbys_defined)
SyncRepWakeQueue(true);
{
int i;
for (i = 0; i < NUM_SYNC_REP_WAIT_MODE; i++)
SyncRepWakeQueue(true, i);
}
/*
* Only allow people to join the queue when there are synchronous
@ -605,16 +624,18 @@ SyncRepUpdateSyncStandbysDefined(void)
#ifdef USE_ASSERT_CHECKING
static bool
SyncRepQueueIsOrderedByLSN(void)
SyncRepQueueIsOrderedByLSN(int mode)
{
PGPROC *proc = NULL;
XLogRecPtr lastLSN;
Assert(mode >= 0 && mode < NUM_SYNC_REP_WAIT_MODE);
lastLSN.xlogid = 0;
lastLSN.xrecoff = 0;
proc = (PGPROC *) SHMQueueNext(&(WalSndCtl->SyncRepQueue),
&(WalSndCtl->SyncRepQueue),
proc = (PGPROC *) SHMQueueNext(&(WalSndCtl->SyncRepQueue[mode]),
&(WalSndCtl->SyncRepQueue[mode]),
offsetof(PGPROC, syncRepLinks));
while (proc)
@ -628,7 +649,7 @@ SyncRepQueueIsOrderedByLSN(void)
lastLSN = proc->waitLSN;
proc = (PGPROC *) SHMQueueNext(&(WalSndCtl->SyncRepQueue),
proc = (PGPROC *) SHMQueueNext(&(WalSndCtl->SyncRepQueue[mode]),
&(proc->syncRepLinks),
offsetof(PGPROC, syncRepLinks));
}
@ -675,3 +696,20 @@ check_synchronous_standby_names(char **newval, void **extra, GucSource source)
return true;
}
void
assign_synchronous_commit(int newval, void *extra)
{
switch (newval)
{
case SYNCHRONOUS_COMMIT_REMOTE_WRITE:
SyncRepWaitMode = SYNC_REP_WAIT_WRITE;
break;
case SYNCHRONOUS_COMMIT_REMOTE_FLUSH:
SyncRepWaitMode = SYNC_REP_WAIT_FLUSH;
break;
default:
SyncRepWaitMode = SYNC_REP_NO_WAIT;
break;
}
}

3
src/backend/replication/walsender.c
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@ -1410,7 +1410,8 @@ WalSndShmemInit(void)
/* First time through, so initialize */
MemSet(WalSndCtl, 0, WalSndShmemSize());
SHMQueueInit(&(WalSndCtl->SyncRepQueue));
for (i = 0; i < NUM_SYNC_REP_WAIT_MODE; i++)
SHMQueueInit(&(WalSndCtl->SyncRepQueue[i]));
for (i = 0; i < max_wal_senders; i++)
{

5
src/backend/utils/misc/guc.c
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@ -370,11 +370,12 @@ static const struct config_enum_entry constraint_exclusion_options[] = {
};
/*
* Although only "on", "off", and "local" are documented, we
* Although only "on", "off", "write", and "local" are documented, we
* accept all the likely variants of "on" and "off".
*/
static const struct config_enum_entry synchronous_commit_options[] = {
{"local", SYNCHRONOUS_COMMIT_LOCAL_FLUSH, false},
{"write", SYNCHRONOUS_COMMIT_REMOTE_WRITE, false},
{"on", SYNCHRONOUS_COMMIT_ON, false},
{"off", SYNCHRONOUS_COMMIT_OFF, false},
{"true", SYNCHRONOUS_COMMIT_ON, true},
@ -3164,7 +3165,7 @@ static struct config_enum ConfigureNamesEnum[] =
},
&synchronous_commit,
SYNCHRONOUS_COMMIT_ON, synchronous_commit_options,
NULL, NULL, NULL
NULL, assign_synchronous_commit, NULL
},
{

1
src/include/access/xact.h
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@ -55,6 +55,7 @@ typedef enum
{
SYNCHRONOUS_COMMIT_OFF, /* asynchronous commit */
SYNCHRONOUS_COMMIT_LOCAL_FLUSH, /* wait for local flush only */
SYNCHRONOUS_COMMIT_REMOTE_WRITE, /* wait for local flush and remote write */
SYNCHRONOUS_COMMIT_REMOTE_FLUSH /* wait for local and remote flush */
} SyncCommitLevel;

13
src/include/replication/syncrep.h
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@ -15,6 +15,16 @@
#include "utils/guc.h"
#define SyncRepRequested() \
(max_wal_senders > 0 && synchronous_commit > SYNCHRONOUS_COMMIT_LOCAL_FLUSH)
/* SyncRepWaitMode */
#define SYNC_REP_NO_WAIT -1
#define SYNC_REP_WAIT_WRITE 0
#define SYNC_REP_WAIT_FLUSH 1
#define NUM_SYNC_REP_WAIT_MODE 2
/* syncRepState */
#define SYNC_REP_NOT_WAITING 0
#define SYNC_REP_WAITING 1
@ -37,8 +47,9 @@ extern void SyncRepReleaseWaiters(void);
extern void SyncRepUpdateSyncStandbysDefined(void);
/* called by various procs */
extern int SyncRepWakeQueue(bool all);
extern int SyncRepWakeQueue(bool all, int mode);
extern bool check_synchronous_standby_names(char **newval, void **extra, GucSource source);
extern void assign_synchronous_commit(int newval, void *extra);
#endif /* _SYNCREP_H */

8
src/include/replication/walsender_private.h
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@ -14,6 +14,7 @@
#include "access/xlog.h"
#include "nodes/nodes.h"
#include "replication/syncrep.h"
#include "storage/latch.h"
#include "storage/shmem.h"
#include "storage/spin.h"
@ -68,15 +69,16 @@ extern WalSnd *MyWalSnd;
typedef struct
{
/*
* Synchronous replication queue. Protected by SyncRepLock.
* Synchronous replication queue with one queue per request type.
* Protected by SyncRepLock.
*/
SHM_QUEUE SyncRepQueue;
SHM_QUEUE SyncRepQueue[NUM_SYNC_REP_WAIT_MODE];
/*
* Current location of the head of the queue. All waiters should have a
* waitLSN that follows this value. Protected by SyncRepLock.
*/
XLogRecPtr lsn;
XLogRecPtr lsn[NUM_SYNC_REP_WAIT_MODE];
/*
* Are any sync standbys defined? Waiting backends can't reload the

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