On Thu, Jun 4, 2015 at 12:57 PM, Robert Haas <robertmhaas@xxxxxxxxx> wrote:
> On Thu, Jun 4, 2015 at 9:42 AM, Robert Haas <robertmhaas@xxxxxxxxx> wrote:
>> Thanks for the review.
>
> Here's a new version. I've fixed the things Alvaro and Noah noted,
> and some compiler warnings about set but unused variables.
>
> I also tested it, and it doesn't quite work as hoped. If started on a
> cluster where oldestMultiXid is incorrectly set to 1, it starts up and
> indicates that the member wraparound guards are disabled. But even
> after everything is fixed, they don't get enabled until after the next
> full restart. I think that's because TruncateMultiXact() bails out
> too early, without calling DetermineSafeOldestOffset.
>
> My attempt at a quick fix for that problem didn't work out, so I'm
> posting this version for now to facilitate further review and testing.
Here's a new version with some more fixes and improvements:
- SetOffsetVacuumLimit was failing to set MultiXactState->oldestOffset
when the oldest offset became known if the now-known value happened to
be zero. Fixed.
- SetOffsetVacuumLimit now logs useful information at the DEBUG1
level, so that you can see that it's doing what it's supposed to.
- TruncateMultiXact now calls DetermineSafeOldestOffset to adjust the
offsetStopLimit even if it can't truncate anything. This seems
useless, but it's not, because it may be that the last checkpoint
advanced lastCheckpointedOldest from a bogus value (i.e. 1) to a real
value, and now we can actually set offsetStopLimit properly.
- TruncateMultiXact no longer calls find_multixact_start when there
are no remaining multixacts. This is actually a completely separate
bug that goes all the way back to 9.3.0 and can potentially cause
TruncateMultiXact to remove every file in pg_multixact/offsets.
Restarting the cluster becomes impossible because TrimMultiXact barfs.
- TruncateMultiXact now logs a message if the oldest multixact does
not precede the earliest one on disk and is not equal to the next
multixact and yet does not exist. The value of the log message is
that it discovered the bug mentioned in the previous line, so I think
it's earning its keep.
With this version, I'm able to see that when you start up a
9.3.latest+this patch with a cluster that has a bogus value of 1 in
relminmxid, datminmxid, and the control file, autovacuum vacuums
everything in sight, all the values get set back to the right thing,
and the next checkpoint enables the member-wraparound guards. This
works with both autovacuum=on and autovacuum=off; the emergency
mechanism kicks in as intended. We'll want to warn people with big
databases who upgrade to 9.3.0 - 9.3.4 via pg_upgrade that they may
want to pre-vacuum those tables before upgrading to avoid a vacuum
storm. But generally I'm pretty happy with this: forcing those values
to get fixed so that we can guard against member-space wraparound
seems like the right thing to do.
So, to summarize, this patch does the following:
- Fixes the failure-to-start problems introduced in 9.4.2 in
complicated pg_upgrade scenarios.
- Prevents the new calls to find_multixact_start we added in 9.4.2
from happening during recovery, where they can only create failure
scenarios. The call in TruncateMultiXact that has been there all
along is not eliminated, but now handles failure more gracefully.
- Fixes possible incorrect removal of every single
pg_multixact/offsets file when no multixacts exist; one file should be
kept.
- Forces aggressive autovacuuming when the control file's
oldestMultiXid doesn't point to a valid MultiXact and enables member
wraparound at the next checkpoint following the correction of that
problem.
Thanks,
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
commit 87aa15fe5257060e0c971e135dd9f460fdc00bd0
Author: Robert Haas <rhaas@postgresql.org>
Date: Thu Jun 4 11:58:49 2015 -0400
bar
diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index 9568ff1..7c457a6 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -198,13 +198,24 @@ typedef struct MultiXactStateData
/* next-to-be-assigned offset */
MultiXactOffset nextOffset;
+ /* Have we completed multixact startup? */
+ bool finishedStartup;
+
/*
- * Oldest multixact that is still on disk. Anything older than this
- * should not be consulted. These values are updated by vacuum.
+ * Oldest multixact that is still potentially referenced by a relation.
+ * Anything older than this should not be consulted. These values are
+ * updated by vacuum.
*/
MultiXactId oldestMultiXactId;
Oid oldestMultiXactDB;
+
+ /*
+ * Oldest multixact offset that is potentially referenced by a
+ * multixact referenced by a relation. We don't always know this value,
+ * so there's a flag here to indicate whether or not we currently do.
+ */
MultiXactOffset oldestOffset;
+ bool oldestOffsetKnown;
/*
* This is what the previous checkpoint stored as the truncate position.
@@ -221,6 +232,7 @@ typedef struct MultiXactStateData
/* support for members anti-wraparound measures */
MultiXactOffset offsetStopLimit;
+ bool offsetStopLimitKnown;
/*
* Per-backend data starts here. We have two arrays stored in the area
@@ -353,7 +365,8 @@ static void ExtendMultiXactMember(MultiXactOffset offset, int nmembers);
static void DetermineSafeOldestOffset(MultiXactId oldestMXact);
static bool MultiXactOffsetWouldWrap(MultiXactOffset boundary,
MultiXactOffset start, uint32 distance);
-static MultiXactOffset find_multixact_start(MultiXactId multi);
+static bool SetOffsetVacuumLimit(bool finish_setup);
+static bool find_multixact_start(MultiXactId multi, MultiXactOffset *result);
static void WriteMZeroPageXlogRec(int pageno, uint8 info);
@@ -955,7 +968,8 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
* against catastrophic data loss due to multixact wraparound. The basic
* rules are:
*
- * If we're past multiVacLimit or the safe threshold for member storage space,
+ * If we're past multiVacLimit or the safe threshold for member storage
+ * space, or we don't know what the safe threshold for member storage is,
* start trying to force autovacuum cycles.
* If we're past multiWarnLimit, start issuing warnings.
* If we're past multiStopLimit, refuse to create new MultiXactIds.
@@ -964,6 +978,7 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
*----------
*/
if (!MultiXactIdPrecedes(result, MultiXactState->multiVacLimit) ||
+ !MultiXactState->oldestOffsetKnown ||
(MultiXactState->nextOffset - MultiXactState->oldestOffset
> MULTIXACT_MEMBER_SAFE_THRESHOLD))
{
@@ -1078,7 +1093,8 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
*----------
*/
#define OFFSET_WARN_SEGMENTS 20
- if (MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit, nextOffset,
+ if (MultiXactState->offsetStopLimitKnown &&
+ MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit, nextOffset,
nmembers))
ereport(ERROR,
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
@@ -1090,7 +1106,8 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
MultiXactState->offsetStopLimit - nextOffset - 1),
errhint("Execute a database-wide VACUUM in database with OID %u with reduced vacuum_multixact_freeze_min_age and vacuum_multixact_freeze_table_age settings.",
MultiXactState->oldestMultiXactDB)));
- else if (MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit,
+ else if (MultiXactState->offsetStopLimitKnown &&
+ MultiXactOffsetWouldWrap(MultiXactState->offsetStopLimit,
nextOffset,
nmembers + MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT * OFFSET_WARN_SEGMENTS))
ereport(WARNING,
@@ -1956,12 +1973,6 @@ StartupMultiXact(void)
*/
pageno = MXOffsetToMemberPage(offset);
MultiXactMemberCtl->shared->latest_page_number = pageno;
-
- /*
- * compute the oldest member we need to keep around to avoid old member
- * data overrun.
- */
- DetermineSafeOldestOffset(MultiXactState->oldestMultiXactId);
}
/*
@@ -1975,6 +1986,7 @@ TrimMultiXact(void)
{
MultiXactId multi = MultiXactState->nextMXact;
MultiXactOffset offset = MultiXactState->nextOffset;
+ MultiXactId oldestMXact;
int pageno;
int entryno;
int flagsoff;
@@ -2047,6 +2059,13 @@ TrimMultiXact(void)
}
LWLockRelease(MultiXactMemberControlLock);
+
+ if (SetOffsetVacuumLimit(true) && IsUnderPostmaster)
+ SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
+ LWLockAcquire(MultiXactGenLock, LW_SHARED);
+ oldestMXact = MultiXactState->lastCheckpointedOldest;
+ LWLockRelease(MultiXactGenLock);
+ DetermineSafeOldestOffset(oldestMXact);
}
/*
@@ -2146,8 +2165,7 @@ SetMultiXactIdLimit(MultiXactId oldest_datminmxid, Oid oldest_datoid)
MultiXactId multiStopLimit;
MultiXactId multiWrapLimit;
MultiXactId curMulti;
- MultiXactOffset oldestOffset;
- MultiXactOffset nextOffset;
+ bool needs_offset_vacuum;
Assert(MultiXactIdIsValid(oldest_datminmxid));
@@ -2200,35 +2218,6 @@ SetMultiXactIdLimit(MultiXactId oldest_datminmxid, Oid oldest_datoid)
if (multiVacLimit < FirstMultiXactId)
multiVacLimit += FirstMultiXactId;
- /*
- * Determine the offset of the oldest multixact that might still be
- * referenced. Normally, we can read the offset from the multixact
- * itself, but there's an important special case: if there are no
- * multixacts in existence at all, oldest_datminmxid obviously can't point
- * to one. It will instead point to the multixact ID that will be
- * assigned the next time one is needed.
- *
- * NB: oldest_dataminmxid is the oldest multixact that might still be
- * referenced from a table, unlike in DetermineSafeOldestOffset, where we
- * do this same computation based on the oldest value that might still
- * exist in the SLRU. This is because here we're trying to compute a
- * threshold for activating autovacuum, which can only remove references
- * to multixacts, whereas there we are computing a threshold for creating
- * new multixacts, which requires the old ones to have first been
- * truncated away by a checkpoint.
- */
- LWLockAcquire(MultiXactGenLock, LW_SHARED);
- if (MultiXactState->nextMXact == oldest_datminmxid)
- {
- oldestOffset = MultiXactState->nextOffset;
- LWLockRelease(MultiXactGenLock);
- }
- else
- {
- LWLockRelease(MultiXactGenLock);
- oldestOffset = find_multixact_start(oldest_datminmxid);
- }
-
/* Grab lock for just long enough to set the new limit values */
LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
MultiXactState->oldestMultiXactId = oldest_datminmxid;
@@ -2237,9 +2226,7 @@ SetMultiXactIdLimit(MultiXactId oldest_datminmxid, Oid oldest_datoid)
MultiXactState->multiWarnLimit = multiWarnLimit;
MultiXactState->multiStopLimit = multiStopLimit;
MultiXactState->multiWrapLimit = multiWrapLimit;
- MultiXactState->oldestOffset = oldestOffset;
curMulti = MultiXactState->nextMXact;
- nextOffset = MultiXactState->nextOffset;
LWLockRelease(MultiXactGenLock);
/* Log the info */
@@ -2247,6 +2234,9 @@ SetMultiXactIdLimit(MultiXactId oldest_datminmxid, Oid oldest_datoid)
(errmsg("MultiXactId wrap limit is %u, limited by database with OID %u",
multiWrapLimit, oldest_datoid)));
+ /* Set limits for offset vacuum. */
+ needs_offset_vacuum = SetOffsetVacuumLimit(false);
+
/*
* If past the autovacuum force point, immediately signal an autovac
* request. The reason for this is that autovac only processes one
@@ -2255,8 +2245,7 @@ SetMultiXactIdLimit(MultiXactId oldest_datminmxid, Oid oldest_datoid)
* another iteration immediately if there are still any old databases.
*/
if ((MultiXactIdPrecedes(multiVacLimit, curMulti) ||
- (nextOffset - oldestOffset > MULTIXACT_MEMBER_SAFE_THRESHOLD)) &&
- IsUnderPostmaster && !InRecovery)
+ needs_offset_vacuum) && IsUnderPostmaster && !InRecovery)
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
/* Give an immediate warning if past the wrap warn point */
@@ -2512,6 +2501,25 @@ static void
DetermineSafeOldestOffset(MultiXactId oldestMXact)
{
MultiXactOffset oldestOffset;
+ MultiXactOffset nextOffset;
+ MultiXactOffset offsetStopLimit;
+ MultiXactOffset prevOffsetStopLimit;
+ MultiXactId nextMXact;
+ bool finishedStartup;
+ bool prevOffsetStopLimitKnown;
+
+ /* Fetch values from shared memory. */
+ LWLockAcquire(MultiXactGenLock, LW_SHARED);
+ finishedStartup = MultiXactState->finishedStartup;
+ nextMXact = MultiXactState->nextMXact;
+ nextOffset = MultiXactState->nextOffset;
+ prevOffsetStopLimit = MultiXactState->offsetStopLimit;
+ prevOffsetStopLimitKnown = MultiXactState->offsetStopLimitKnown;
+ LWLockRelease(MultiXactGenLock);
+
+ /* Don't worry about this until after we've started up. */
+ if (!finishedStartup)
+ return;
/*
* Determine the offset of the oldest multixact. Normally, we can read
@@ -2521,30 +2529,143 @@ DetermineSafeOldestOffset(MultiXactId oldestMXact)
* ID that will be assigned the next time one is needed.
*
* NB: oldestMXact should be the oldest multixact that still exists in the
- * SLRU, unlike in SetMultiXactIdLimit, where we do this same computation
+ * SLRU, unlike in SetOffsetVacuumLimit, where we do this same computation
* based on the oldest value that might be referenced in a table.
*/
- LWLockAcquire(MultiXactGenLock, LW_SHARED);
- if (MultiXactState->nextMXact == oldestMXact)
- {
- oldestOffset = MultiXactState->nextOffset;
- LWLockRelease(MultiXactGenLock);
- }
+ if (nextMXact == oldestMXact)
+ oldestOffset = nextOffset;
else
{
- LWLockRelease(MultiXactGenLock);
- oldestOffset = find_multixact_start(oldestMXact);
+ bool oldestOffsetKnown;
+
+ oldestOffsetKnown = find_multixact_start(oldestMXact, &oldestOffset);
+ if (!oldestOffsetKnown)
+ {
+ ereport(LOG,
+ (errmsg("MultiXact member wraparound protections are disabled because oldest checkpointed MultiXact %u does not exist on disk",
+ oldestMXact)));
+ return;
+ }
}
/* move back to start of the corresponding segment */
- oldestOffset -= oldestOffset %
- (MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT);
+ offsetStopLimit = oldestOffset - (oldestOffset %
+ (MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT));
+ /* always leave one segment before the wraparound point */
+ offsetStopLimit -= (MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT);
+
+ /* if nothing has changed, we're done */
+ if (prevOffsetStopLimitKnown && offsetStopLimit == prevOffsetStopLimit)
+ return;
LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
- /* always leave one segment before the wraparound point */
- MultiXactState->offsetStopLimit = oldestOffset -
- (MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT);
+ MultiXactState->offsetStopLimit = offsetStopLimit;
+ MultiXactState->offsetStopLimitKnown = true;
+ LWLockRelease(MultiXactGenLock);
+
+ if (!prevOffsetStopLimitKnown && IsUnderPostmaster)
+ ereport(LOG,
+ (errmsg("MultiXact member wraparound protections are now enabled")));
+ ereport(DEBUG1,
+ (errmsg("MultiXact member stop limit is now %u based on MultiXact %u",
+ offsetStopLimit, oldestMXact)));
+}
+
+/*
+ * Determine how aggressively we need to vacuum in order to prevent member
+ * wraparound.
+ *
+ * To determine the oldest multixact ID, we look at oldestMultiXactId, not
+ * lastCheckpointedOldest. That's because vacuuming can't help with anything
+ * older than oldestMultiXactId; anything older than that isn't referenced
+ * by any table. Offsets older than oldestMultiXactId but not as old as
+ * lastCheckpointedOldest will go away after the next checkpoint.
+ *
+ * The return value is true if emergency autovacuum is required and false
+ * otherwise.
+ */
+static bool
+SetOffsetVacuumLimit(bool finish_setup)
+{
+ MultiXactId oldestMultiXactId;
+ MultiXactId nextMXact;
+ bool finishedStartup;
+ MultiXactOffset oldestOffset = 0; /* placate compiler */
+ MultiXactOffset nextOffset;
+ bool oldestOffsetKnown = false;
+ MultiXactOffset prevOldestOffset;
+ bool prevOldestOffsetKnown;
+
+ /* Read relevant fields from shared memory. */
+ LWLockAcquire(MultiXactGenLock, LW_SHARED);
+ oldestMultiXactId = MultiXactState->oldestMultiXactId;
+ nextMXact = MultiXactState->nextMXact;
+ nextOffset = MultiXactState->nextOffset;
+ finishedStartup = MultiXactState->finishedStartup;
+ prevOldestOffset = MultiXactState->oldestOffset;
+ prevOldestOffsetKnown = MultiXactState->oldestOffsetKnown;
LWLockRelease(MultiXactGenLock);
+
+ /* Don't do this until after any recovery is complete. */
+ if (!finishedStartup && !finish_setup)
+ return false;
+
+ /*
+ * If no multixacts exist, then oldestMultiXactId will be the next
+ * multixact that will be created, rather than an existing multixact.
+ */
+ if (oldestMultiXactId == nextMXact)
+ {
+ /*
+ * When the next multixact gets created, it will be stored at the
+ * next offset.
+ */
+ oldestOffset = nextOffset;
+ oldestOffsetKnown = true;
+ }
+ else
+ {
+ /*
+ * Figure out where the oldest existing multixact's offsets are stored.
+ * Due to bugs in early release of PostgreSQL 9.3.X and 9.4.X, the
+ * supposedly-earliest multixact might not really exist. We are
+ * careful not to fail in that case.
+ */
+ oldestOffsetKnown =
+ find_multixact_start(oldestMultiXactId, &oldestOffset);
+ }
+
+ /*
+ * Except when initializing the system for the first time, there's no
+ * need to update anything if we don't know the oldest offset or if it
+ * hasn't changed.
+ */
+ if (finish_setup ||
+ (oldestOffsetKnown && !prevOldestOffsetKnown) ||
+ (oldestOffsetKnown && prevOldestOffset != oldestOffset))
+ {
+ /* Install the new limits. */
+ LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
+ MultiXactState->oldestOffset = oldestOffset;
+ MultiXactState->oldestOffsetKnown = oldestOffsetKnown;
+ MultiXactState->finishedStartup = true;
+ LWLockRelease(MultiXactGenLock);
+
+ /* Log the info */
+ if (oldestOffsetKnown)
+ ereport(DEBUG1,
+ (errmsg("oldest MultiXactId member is at offset %u",
+ oldestOffset)));
+ else
+ ereport(DEBUG1,
+ (errmsg("oldest MultiXactId member offset unknown")));
+ }
+
+ /*
+ * Do we need an emergency autovacuum? If we're not sure, assume yes.
+ */
+ return !oldestOffsetKnown ||
+ (nextOffset - oldestOffset > MULTIXACT_MEMBER_SAFE_THRESHOLD);
}
/*
@@ -2597,9 +2718,12 @@ MultiXactOffsetWouldWrap(MultiXactOffset boundary, MultiXactOffset start,
/*
* Find the starting offset of the given MultiXactId.
+ *
+ * Returns false if the file containing the multi does not exist on disk.
+ * Otherwise, returns true and sets *result to the starting member offset.
*/
-static MultiXactOffset
-find_multixact_start(MultiXactId multi)
+static bool
+find_multixact_start(MultiXactId multi, MultiXactOffset *result)
{
MultiXactOffset offset;
int pageno;
@@ -2610,6 +2734,9 @@ find_multixact_start(MultiXactId multi)
pageno = MultiXactIdToOffsetPage(multi);
entryno = MultiXactIdToOffsetEntry(multi);
+ if (!SimpleLruDoesPhysicalPageExist(MultiXactOffsetCtl, pageno))
+ return false;
+
/* lock is acquired by SimpleLruReadPage_ReadOnly */
slotno = SimpleLruReadPage_ReadOnly(MultiXactOffsetCtl, pageno, multi);
offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
@@ -2622,25 +2749,31 @@ find_multixact_start(MultiXactId multi)
/*
* Determine how many multixacts, and how many multixact members, currently
- * exist.
+ * exist. Return false if unable to determine.
*/
-static void
+static bool
ReadMultiXactCounts(uint32 *multixacts, MultiXactOffset *members)
{
MultiXactOffset nextOffset;
MultiXactOffset oldestOffset;
MultiXactId oldestMultiXactId;
MultiXactId nextMultiXactId;
+ bool oldestOffsetKnown;
LWLockAcquire(MultiXactGenLock, LW_SHARED);
nextOffset = MultiXactState->nextOffset;
oldestMultiXactId = MultiXactState->oldestMultiXactId;
nextMultiXactId = MultiXactState->nextMXact;
oldestOffset = MultiXactState->oldestOffset;
+ oldestOffsetKnown = MultiXactState->oldestOffsetKnown;
LWLockRelease(MultiXactGenLock);
+ if (!oldestOffsetKnown)
+ return false;
+
*members = nextOffset - oldestOffset;
*multixacts = nextMultiXactId - oldestMultiXactId;
+ return true;
}
/*
@@ -2682,7 +2815,9 @@ MultiXactMemberFreezeThreshold(void)
uint32 victim_multixacts;
double fraction;
- ReadMultiXactCounts(&multixacts, &members);
+ /* If we can't determine member space utilization, assume the worst. */
+ if (!ReadMultiXactCounts(&multixacts, &members))
+ return 0;
/* If member space utilization is low, no special action is required. */
if (members <= MULTIXACT_MEMBER_SAFE_THRESHOLD)
@@ -2801,7 +2936,8 @@ TruncateMultiXact(void)
{
MultiXactId oldestMXact;
MultiXactOffset oldestOffset;
- MultiXactOffset nextOffset;
+ MultiXactId nextMXact;
+ MultiXactOffset nextOffset;
mxtruncinfo trunc;
MultiXactId earliest;
MembersLiveRange range;
@@ -2811,6 +2947,8 @@ TruncateMultiXact(void)
LWLockAcquire(MultiXactGenLock, LW_SHARED);
oldestMXact = MultiXactState->lastCheckpointedOldest;
+ nextMXact = MultiXactState->nextMXact;
+ nextOffset = MultiXactState->nextOffset;
LWLockRelease(MultiXactGenLock);
Assert(MultiXactIdIsValid(oldestMXact));
@@ -2827,15 +2965,39 @@ TruncateMultiXact(void)
if (earliest < FirstMultiXactId)
earliest = FirstMultiXactId;
- /* nothing to do */
+ /*
+ * If there's nothing to remove, we can bail out early.
+ *
+ * Due to bugs in early releases of PostgreSQL 9.3.X and 9.4.X,
+ * oldestMXact might point to a multixact that does not exist.
+ * Autovacuum will eventually advance it to a value that does exist,
+ * and we want to set a proper offsetStopLimit when that happens,
+ * so call DetermineSafeOldestOffset here even if we're not actually
+ * truncating.
+ */
if (MultiXactIdPrecedes(oldestMXact, earliest))
+ {
+ DetermineSafeOldestOffset(oldestMXact);
return;
+ }
/*
* First, compute the safe truncation point for MultiXactMember. This is
* the starting offset of the oldest multixact.
+ *
+ * Hopefully, find_multixact_start will always work here, because we've
+ * already checked that it doesn't precede the earliest MultiXact on
+ * disk. But if it fails, don't truncate anything, and log a message.
*/
- oldestOffset = find_multixact_start(oldestMXact);
+ if (oldestMXact == nextMXact)
+ oldestOffset = nextOffset; /* there are NO MultiXacts */
+ else if (!find_multixact_start(oldestMXact, &oldestOffset))
+ {
+ ereport(LOG,
+ (errmsg("oldest MultiXact %u not found, earliest MultiXact %u, skipping truncation",
+ oldestMXact, earliest)));
+ return;
+ }
/*
* To truncate MultiXactMembers, we need to figure out the active page
@@ -2847,10 +3009,6 @@ TruncateMultiXact(void)
range.rangeStart = MXOffsetToMemberPage(oldestOffset);
range.rangeStart -= range.rangeStart % SLRU_PAGES_PER_SEGMENT;
- LWLockAcquire(MultiXactGenLock, LW_SHARED);
- nextOffset = MultiXactState->nextOffset;
- LWLockRelease(MultiXactGenLock);
-
range.rangeEnd = MXOffsetToMemberPage(nextOffset);
SlruScanDirectory(MultiXactMemberCtl, SlruScanDirCbRemoveMembers, &range);
--
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