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Avoid pin scan for replay of XLOG_BTREE_VACUUM in all cases

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Replay of XLOG_BTREE_VACUUM during Hot Standby was previously thought to require
complex interlocking that matched the requirements on the master. This required
an O(N) operation that became a significant problem with large indexes, causing
replication delays of seconds or in some cases minutes while the
XLOG_BTREE_VACUUM was replayed.

This commit skips the pin scan that was previously required, by observing in
detail when and how it is safe to do so, with full documentation. The pin
scan is skipped only in replay; the VACUUM code path on master is not
touched here and WAL is identical.

The current commit applies in all cases, effectively replacing commit
687f2cd7a0.
This commit is contained in:
Simon Riggs 2016-04-03 17:46:09 +01:00
parent 3cc38ca7d2
commit 3e4b7d8798
3 changed files with 23 additions and 35 deletions
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15
src/backend/access/nbtree/README
View File

@ -525,8 +525,12 @@ MVCC scans is not required on standby nodes. That is because
HeapTupleSatisfiesUpdate(), HeapTupleSatisfiesSelf(),
HeapTupleSatisfiesDirty() and HeapTupleSatisfiesVacuum() are only
ever used during write transactions, which cannot exist on the standby.
This leaves HeapTupleSatisfiesMVCC() and HeapTupleSatisfiesToast(), so
HeapTupleSatisfiesToast() is the only non-MVCC scan type used on standbys.
This leaves HeapTupleSatisfiesMVCC() and HeapTupleSatisfiesToast().
HeapTupleSatisfiesToast() doesn't use MVCC semantics, though that's
because it doesn't need to - if the main heap row is visible then the
toast rows will also be visible. So as long as we follow a toast
pointer from a visible (live) tuple the corresponding toast rows
will also be visible, so we do not need to recheck MVCC on them.
There is one minor exception, which is that the optimizer sometimes
looks at the boundaries of value ranges using SnapshotDirty, which
could result in returning a newer value for query statistics; this
@ -536,13 +540,6 @@ in the index, so the scan retrieves a tid then immediately uses it
to look in the heap. It is unlikely that the tid could have been
deleted, vacuumed and re-inserted in the time taken to look in the heap
via direct tid access. So we ignore that scan type as a problem.
This means if we re-check the results of any scan of a toast index we
will be able to completely avoid performing the "pin scan" operation
during replay of VACUUM WAL records.
XXX FIXME: Toast re-checks are not yet added, so we still perform the
pin scan when replaying vacuum records of toast indexes.
Other Things That Are Handy to Know
-----------------------------------

32
src/backend/access/nbtree/nbtree.c
View File

@ -22,7 +22,6 @@
#include "access/relscan.h"
#include "access/xlog.h"
#include "catalog/index.h"
#include "catalog/pg_namespace.h"
#include "commands/vacuum.h"
#include "storage/indexfsm.h"
#include "storage/ipc.h"
@ -833,8 +832,7 @@ btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
/*
* Check to see if we need to issue one final WAL record for this index,
* which may be needed for correctness on a hot standby node when
* non-MVCC index scans could take place. This now only occurs when we
* perform a TOAST scan, so only occurs for TOAST indexes.
* non-MVCC index scans could take place.
*
* If the WAL is replayed in hot standby, the replay process needs to get
* cleanup locks on all index leaf pages, just as we've been doing here.
@ -846,7 +844,6 @@ btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
* against the last leaf page in the index, if that one wasn't vacuumed.
*/
if (XLogStandbyInfoActive() &&
rel->rd_rel->relnamespace == PG_TOAST_NAMESPACE &&
vstate.lastBlockVacuumed < vstate.lastBlockLocked)
{
Buffer buf;
@ -1045,25 +1042,14 @@ restart:
*/
if (ndeletable > 0)
{
BlockNumber lastBlockVacuumed = InvalidBlockNumber;
/*
* We may need to record the lastBlockVacuumed for use when
* non-MVCC scans might be performed on the index on a
* hot standby. See explanation in btree_xlog_vacuum().
*
* On a hot standby, a non-MVCC scan can only take place
* when we access a Toast Index, so we need only record
* the lastBlockVacuumed if we are vacuuming a Toast Index.
*/
if (rel->rd_rel->relnamespace == PG_TOAST_NAMESPACE)
lastBlockVacuumed = vstate->lastBlockVacuumed;
/*
* Notice that the issued XLOG_BTREE_VACUUM WAL record includes an
* instruction to the replay code to get cleanup lock on all pages
* between the previous lastBlockVacuumed and this page. This
* ensures that WAL replay locks all leaf pages at some point.
* Notice that the issued XLOG_BTREE_VACUUM WAL record includes all
* information to the replay code to allow it to get a cleanup lock
* on all pages between the previous lastBlockVacuumed and this page.
* This ensures that WAL replay locks all leaf pages at some point,
* which is important should non-MVCC scans be requested.
* This is currently unused on standby, but we record it anyway, so
* that the WAL contains the required information.
*
* Since we can visit leaf pages out-of-order when recursing,
* replay might end up locking such pages an extra time, but it
@ -1071,7 +1057,7 @@ restart:
* that.
*/
_bt_delitems_vacuum(rel, buf, deletable, ndeletable,
lastBlockVacuumed);
vstate->lastBlockVacuumed);
/*
* Remember highest leaf page number we've issued a

11
src/backend/access/nbtree/nbtxlog.c
View File

@ -385,17 +385,21 @@ static void
btree_xlog_vacuum(XLogReaderState *record)
{
XLogRecPtr lsn = record->EndRecPtr;
xl_btree_vacuum *xlrec = (xl_btree_vacuum *) XLogRecGetData(record);
Buffer buffer;
Page page;
BTPageOpaque opaque;
#ifdef UNUSED
xl_btree_vacuum *xlrec = (xl_btree_vacuum *) XLogRecGetData(record);
/*
* This section of code is thought to be no longer needed, after
* analysis of the calling paths. It is retained to allow the code
* to be reinstated if a flaw is revealed in that thinking.
*
* If we are running non-MVCC scans using this index we need to do some
* additional work to ensure correctness, which is known as a "pin scan"
* described in more detail in next paragraphs. We used to do the extra
* work in all cases, whereas we now avoid that work except when the index
* is a toast index, since toast scans aren't fully MVCC compliant.
* work in all cases, whereas we now avoid that work in most cases.
* If lastBlockVacuumed is set to InvalidBlockNumber then we skip the
* additional work required for the pin scan.
*
@ -458,6 +462,7 @@ btree_xlog_vacuum(XLogReaderState *record)
}
}
}
#endif
/*
* Like in btvacuumpage(), we need to take a cleanup lock on every leaf