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Remove unneeded "pin scan" nbtree VACUUM code.

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The REDO routine for nbtree's xl_btree_vacuum record type hasn't
performed a "pin scan" since commit 3e4b7d87 went in, so clearly there
isn't any point in VACUUM WAL-logging information that won't actually be
used.  Finish off the work of commit 3e4b7d87 (and the closely related
preceding commit 687f2cd7) by removing the code that generates this
unused information.  Also remove the REDO routine code disabled by
commit 3e4b7d87.

Replace the unneeded lastBlockVacuumed field in xl_btree_vacuum with a
new "ndeleted" field.  The new field isn't actually needed right now,
since we could continue to infer the array length from the overall
record length.  However, an upcoming patch to add deduplication to
nbtree needs to add an "items updated" field to xl_btree_vacuum, so we
might as well start being explicit about the number of items now.
(Besides, it doesn't seem like a good idea to leave the xl_btree_vacuum
struct without any fields; the C standard says that that's undefined.)

nbtree VACUUM no longer forces writing a WAL record for the last block
in the index.  Writing out a WAL record with no items for the final
block was supposed to force processing of a lastBlockVacuumed field by a
pin scan.

Bump XLOG_PAGE_MAGIC because xl_btree_vacuum changed.

Discussion: https://postgr.es/m/CAH2-WzmY_mT7UnTzFB5LBQDBkKpdV5UxP3B5bLb7uP%3D%3D6UQJRQ%40mail.gmail.com
pull/48/head
Peter Geoghegan 6 years ago
parent b93e9a5c94
commit 9f83468b35
8 changed files with 101 additions and 245 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. 63
      src/backend/access/nbtree/README
  2. 34
      src/backend/access/nbtree/nbtpage.c
  3. 111
      src/backend/access/nbtree/nbtree.c
  4. 99
      src/backend/access/nbtree/nbtxlog.c
  5. 3
      src/backend/access/rmgrdesc/nbtdesc.c
  6. 3
      src/include/access/nbtree.h
  7. 31
      src/include/access/nbtxlog.h
  8. 2
      src/include/access/xlog_internal.h

63
src/backend/access/nbtree/README
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@ -508,7 +508,9 @@ the parent is finished and the flag in the child cleared, but can be
released immediately after that, before recursing up the tree if the parent
also needs to be split. This ensures that incompletely split pages should
not be seen under normal circumstances; only if insertion to the parent
has failed for some reason.
has failed for some reason. (It's also possible for a reader to observe
a page with the incomplete split flag set during recovery; see later
section on "Scans during Recovery" for details.)
We flag the left page, even though it's the right page that's missing the
downlink, because it's more convenient to know already when following the
@ -528,7 +530,7 @@ next VACUUM will find the half-dead leaf page and continue the deletion.
Before 9.4, we used to keep track of incomplete splits and page deletions
during recovery and finish them immediately at end of recovery, instead of
doing it lazily at the next insertion or vacuum. However, that made the
doing it lazily at the next insertion or vacuum. However, that made the
recovery much more complicated, and only fixed the problem when crash
recovery was performed. An incomplete split can also occur if an otherwise
recoverable error, like out-of-memory or out-of-disk-space, happens while
@ -537,23 +539,41 @@ inserting the downlink to the parent.
Scans during Recovery
---------------------
The btree index type can be safely used during recovery. During recovery
we have at most one writer and potentially many readers. In that
situation the locking requirements can be relaxed and we do not need
double locking during block splits. Each WAL record makes changes to a
single level of the btree using the correct locking sequence and so
is safe for concurrent readers. Some readers may observe a block split
in progress as they descend the tree, but they will simply move right
onto the correct page.
nbtree indexes support read queries in Hot Standby mode. Every atomic
action/WAL record makes isolated changes that leave the tree in a
consistent state for readers. Readers lock pages according to the same
rules that readers follow on the primary. (Readers may have to move
right to recover from a "concurrent" page split or page deletion, just
like on the primary.)
However, there are a couple of differences in how pages are locked by
replay/the startup process as compared to the original write operation
on the primary. The exceptions involve page splits and page deletions.
The first phase and second phase of a page split are processed
independently during replay, since they are independent atomic actions.
We do not attempt to recreate the coupling of parent and child page
write locks that took place on the primary. This is safe because readers
never care about the incomplete split flag anyway. Holding on to an
extra write lock on the primary is only necessary so that a second
writer cannot observe the incomplete split flag before the first writer
finishes the split. If we let concurrent writers on the primary observe
an incomplete split flag on the same page, each writer would attempt to
complete the unfinished split, corrupting the parent page. (Similarly,
replay of page deletion records does not hold a write lock on the leaf
page throughout; only the primary needs to blocks out concurrent writers
that insert on to the page being deleted.)
During recovery all index scans start with ignore_killed_tuples = false
and we never set kill_prior_tuple. We do this because the oldest xmin
on the standby server can be older than the oldest xmin on the master
server, which means tuples can be marked as killed even when they are
still visible on the standby. We don't WAL log tuple killed bits, but
server, which means tuples can be marked LP_DEAD even when they are
still visible on the standby. We don't WAL log tuple LP_DEAD bits, but
they can still appear in the standby because of full page writes. So
we must always ignore them in standby, and that means it's not worth
setting them either.
setting them either. (When LP_DEAD-marked tuples are eventually deleted
on the primary, the deletion is WAL-logged. Queries that run on a
standby therefore get much of the benefit of any LP_DEAD setting that
takes place on the primary.)
Note that we talk about scans that are started during recovery. We go to
a little trouble to allow a scan to start during recovery and end during
@ -562,14 +582,17 @@ because it allows running applications to continue while the standby
changes state into a normally running server.
The interlocking required to avoid returning incorrect results from
non-MVCC scans is not required on standby nodes. That is because
non-MVCC scans is not required on standby nodes. We still get a
super-exclusive lock ("cleanup lock") when replaying VACUUM records
during recovery, but recovery does not need to lock every leaf page
(only those leaf pages that have items to delete). That is safe because
HeapTupleSatisfiesUpdate(), HeapTupleSatisfiesSelf(),
HeapTupleSatisfiesDirty() and HeapTupleSatisfiesVacuum() are only
ever used during write transactions, which cannot exist on the standby.
MVCC scans are already protected by definition, so HeapTupleSatisfiesMVCC()
is not a problem. The optimizer looks at the boundaries of value ranges
using HeapTupleSatisfiesNonVacuumable() with an index-only scan, which is
also safe. That leaves concern only for HeapTupleSatisfiesToast().
HeapTupleSatisfiesDirty() and HeapTupleSatisfiesVacuum() are only ever
used during write transactions, which cannot exist on the standby. MVCC
scans are already protected by definition, so HeapTupleSatisfiesMVCC()
is not a problem. The optimizer looks at the boundaries of value ranges
using HeapTupleSatisfiesNonVacuumable() with an index-only scan, which
is also safe. That leaves concern only for 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

34
src/backend/access/nbtree/nbtpage.c
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@ -968,32 +968,28 @@ _bt_page_recyclable(Page page)
* deleting the page it points to.
*
* This routine assumes that the caller has pinned and locked the buffer.
* Also, the given itemnos *must* appear in increasing order in the array.
* Also, the given deletable array *must* be sorted in ascending order.
*
* We record VACUUMs and b-tree deletes differently in WAL. InHotStandby
* we need to be able to pin all of the blocks in the btree in physical
* order when replaying the effects of a VACUUM, just as we do for the
* original VACUUM itself. lastBlockVacuumed allows us to tell whether an
* intermediate range of blocks has had no changes at all by VACUUM,
* and so must be scanned anyway during replay. We always write a WAL record
* for the last block in the index, whether or not it contained any items
* to be removed. This allows us to scan right up to end of index to
* ensure correct locking.
* We record VACUUMs and b-tree deletes differently in WAL. Deletes must
* generate recovery conflicts by accessing the heap inline, whereas VACUUMs
* can rely on the initial heap scan taking care of the problem (pruning would
* have generated the conflicts needed for hot standby already).
*/
void
_bt_delitems_vacuum(Relation rel, Buffer buf,
OffsetNumber *itemnos, int nitems,
BlockNumber lastBlockVacuumed)
OffsetNumber *deletable, int ndeletable)
{
Page page = BufferGetPage(buf);
BTPageOpaque opaque;
/* Shouldn't be called unless there's something to do */
Assert(ndeletable > 0);
/* No ereport(ERROR) until changes are logged */
START_CRIT_SECTION();
/* Fix the page */
if (nitems > 0)
PageIndexMultiDelete(page, itemnos, nitems);
PageIndexMultiDelete(page, deletable, ndeletable);
/*
* We can clear the vacuum cycle ID since this page has certainly been
@ -1019,7 +1015,7 @@ _bt_delitems_vacuum(Relation rel, Buffer buf,
XLogRecPtr recptr;
xl_btree_vacuum xlrec_vacuum;
xlrec_vacuum.lastBlockVacuumed = lastBlockVacuumed;
xlrec_vacuum.ndeleted = ndeletable;
XLogBeginInsert();
XLogRegisterBuffer(0, buf, REGBUF_STANDARD);
@ -1030,8 +1026,8 @@ _bt_delitems_vacuum(Relation rel, Buffer buf,
* is. When XLogInsert stores the whole buffer, the offsets array
* need not be stored too.
*/
if (nitems > 0)
XLogRegisterBufData(0, (char *) itemnos, nitems * sizeof(OffsetNumber));
XLogRegisterBufData(0, (char *) deletable,
ndeletable * sizeof(OffsetNumber));
recptr = XLogInsert(RM_BTREE_ID, XLOG_BTREE_VACUUM);
@ -1050,8 +1046,8 @@ _bt_delitems_vacuum(Relation rel, Buffer buf,
* Also, the given itemnos *must* appear in increasing order in the array.
*
* This is nearly the same as _bt_delitems_vacuum as far as what it does to
* the page, but the WAL logging considerations are quite different. See
* comments for _bt_delitems_vacuum.
* the page, but it needs to generate its own recovery conflicts by accessing
* the heap. See comments for _bt_delitems_vacuum.
*/
void
_bt_delitems_delete(Relation rel, Buffer buf,

111
src/backend/access/nbtree/nbtree.c
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@ -46,8 +46,6 @@ typedef struct
IndexBulkDeleteCallback callback;
void *callback_state;
BTCycleId cycleid;
BlockNumber lastBlockVacuumed; /* highest blkno actually vacuumed */
BlockNumber lastBlockLocked; /* highest blkno we've cleanup-locked */
BlockNumber totFreePages; /* true total # of free pages */
TransactionId oldestBtpoXact;
MemoryContext pagedelcontext;
@ -978,8 +976,6 @@ btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
vstate.callback = callback;
vstate.callback_state = callback_state;
vstate.cycleid = cycleid;
vstate.lastBlockVacuumed = BTREE_METAPAGE; /* Initialise at first block */
vstate.lastBlockLocked = BTREE_METAPAGE;
vstate.totFreePages = 0;
vstate.oldestBtpoXact = InvalidTransactionId;
@ -1040,39 +1036,6 @@ 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.
*
* 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.
* However, we won't issue any WAL records about pages that have no items
* to be deleted. For pages between pages we've vacuumed, the replay code
* will take locks under the direction of the lastBlockVacuumed fields in
* the XLOG_BTREE_VACUUM WAL records. To cover pages after the last one
* we vacuum, we need to issue a dummy XLOG_BTREE_VACUUM WAL record
* against the last leaf page in the index, if that one wasn't vacuumed.
*/
if (XLogStandbyInfoActive() &&
vstate.lastBlockVacuumed < vstate.lastBlockLocked)
{
Buffer buf;
/*
* The page should be valid, but we can't use _bt_getbuf() because we
* want to use a nondefault buffer access strategy. Since we aren't
* going to delete any items, getting cleanup lock again is probably
* overkill, but for consistency do that anyway.
*/
buf = ReadBufferExtended(rel, MAIN_FORKNUM, vstate.lastBlockLocked,
RBM_NORMAL, info->strategy);
LockBufferForCleanup(buf);
_bt_checkpage(rel, buf);
_bt_delitems_vacuum(rel, buf, NULL, 0, vstate.lastBlockVacuumed);
_bt_relbuf(rel, buf);
}
MemoryContextDelete(vstate.pagedelcontext);
/*
@ -1203,13 +1166,6 @@ restart:
LockBuffer(buf, BUFFER_LOCK_UNLOCK);
LockBufferForCleanup(buf);
/*
* Remember highest leaf page number we've taken cleanup lock on; see
* notes in btvacuumscan
*/
if (blkno > vstate->lastBlockLocked)
vstate->lastBlockLocked = blkno;
/*
* Check whether we need to recurse back to earlier pages. What we
* are concerned about is a page split that happened since we started
@ -1225,8 +1181,10 @@ restart:
recurse_to = opaque->btpo_next;
/*
* Scan over all items to see which ones need deleted according to the
* callback function.
* When each VACUUM begins, it determines an OldestXmin cutoff value.
* Tuples before the cutoff are removed by VACUUM. Scan over all
* items to see which ones need to be deleted according to cutoff
* point using callback.
*/
ndeletable = 0;
minoff = P_FIRSTDATAKEY(opaque);
@ -1245,25 +1203,24 @@ restart:
htup = &(itup->t_tid);
/*
* During Hot Standby we currently assume that
* XLOG_BTREE_VACUUM records do not produce conflicts. That is
* only true as long as the callback function depends only
* upon whether the index tuple refers to heap tuples removed
* in the initial heap scan. When vacuum starts it derives a
* value of OldestXmin. Backends taking later snapshots could
* have a RecentGlobalXmin with a later xid than the vacuum's
* OldestXmin, so it is possible that row versions deleted
* after OldestXmin could be marked as killed by other
* backends. The callback function *could* look at the index
* tuple state in isolation and decide to delete the index
* tuple, though currently it does not. If it ever did, we
* would need to reconsider whether XLOG_BTREE_VACUUM records
* should cause conflicts. If they did cause conflicts they
* would be fairly harsh conflicts, since we haven't yet
* worked out a way to pass a useful value for
* latestRemovedXid on the XLOG_BTREE_VACUUM records. This
* applies to *any* type of index that marks index tuples as
* killed.
* Hot Standby assumes that it's okay that XLOG_BTREE_VACUUM
* records do not produce their own conflicts. This is safe
* as long as the callback function only considers whether the
* index tuple refers to pre-cutoff heap tuples that were
* certainly already pruned away during VACUUM's initial heap
* scan by the time we get here. (We can rely on conflicts
* produced by heap pruning, rather than producing our own
* now.)
*
* Backends with snapshots acquired after a VACUUM starts but
* before it finishes could have a RecentGlobalXmin with a
* later xid than the VACUUM's OldestXmin cutoff. These
* backends might happen to opportunistically mark some index
* tuples LP_DEAD before we reach them, even though they may
* be after our cutoff. We don't try to kill these "extra"
* index tuples in _bt_delitems_vacuum(). This keep things
* simple, and allows us to always avoid generating our own
* conflicts.
*/
if (callback(htup, callback_state))
deletable[ndeletable++] = offnum;
@ -1276,29 +1233,7 @@ restart:
*/
if (ndeletable > 0)
{
/*
* 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
* doesn't seem worth the amount of bookkeeping it'd take to avoid
* that.
*/
_bt_delitems_vacuum(rel, buf, deletable, ndeletable,
vstate->lastBlockVacuumed);
/*
* Remember highest leaf page number we've issued a
* XLOG_BTREE_VACUUM WAL record for.
*/
if (blkno > vstate->lastBlockVacuumed)
vstate->lastBlockVacuumed = blkno;
_bt_delitems_vacuum(rel, buf, deletable, ndeletable);
stats->tuples_removed += ndeletable;
/* must recompute maxoff */

99
src/backend/access/nbtree/nbtxlog.c
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@ -383,110 +383,25 @@ 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 in most cases. If
* lastBlockVacuumed is set to InvalidBlockNumber then we skip the
* additional work required for the pin scan.
*
* Avoiding this extra work is important since it requires us to touch
* every page in the index, so is an O(N) operation. Worse, it is an
* operation performed in the foreground during redo, so it delays
* replication directly.
*
* If queries might be active then we need to ensure every leaf page is
* unpinned between the lastBlockVacuumed and the current block, if there
* are any. This prevents replay of the VACUUM from reaching the stage of
* removing heap tuples while there could still be indexscans "in flight"
* to those particular tuples for those scans which could be confused by
* finding new tuples at the old TID locations (see nbtree/README).
*
* It might be worth checking if there are actually any backends running;
* if not, we could just skip this.
*
* Since VACUUM can visit leaf pages out-of-order, it might issue records
* with lastBlockVacuumed >= block; that's not an error, it just means
* nothing to do now.
*
* Note: since we touch all pages in the range, we will lock non-leaf
* pages, and also any empty (all-zero) pages that may be in the index. It
* doesn't seem worth the complexity to avoid that. But it's important
* that HotStandbyActiveInReplay() will not return true if the database
* isn't yet consistent; so we need not fear reading still-corrupt blocks
* here during crash recovery.
*/
if (HotStandbyActiveInReplay() && BlockNumberIsValid(xlrec->lastBlockVacuumed))
{
RelFileNode thisrnode;
BlockNumber thisblkno;
BlockNumber blkno;
XLogRecGetBlockTag(record, 0, &thisrnode, NULL, &thisblkno);
for (blkno = xlrec->lastBlockVacuumed + 1; blkno < thisblkno; blkno++)
{
/*
* We use RBM_NORMAL_NO_LOG mode because it's not an error
* condition to see all-zero pages. The original btvacuumpage
* scan would have skipped over all-zero pages, noting them in FSM
* but not bothering to initialize them just yet; so we mustn't
* throw an error here. (We could skip acquiring the cleanup lock
* if PageIsNew, but it's probably not worth the cycles to test.)
*
* XXX we don't actually need to read the block, we just need to
* confirm it is unpinned. If we had a special call into the
* buffer manager we could optimise this so that if the block is
* not in shared_buffers we confirm it as unpinned. Optimizing
* this is now moot, since in most cases we avoid the scan.
*/
buffer = XLogReadBufferExtended(thisrnode, MAIN_FORKNUM, blkno,
RBM_NORMAL_NO_LOG);
if (BufferIsValid(buffer))
{
LockBufferForCleanup(buffer);
UnlockReleaseBuffer(buffer);
}
}
}
#endif
/*
* Like in btvacuumpage(), we need to take a cleanup lock on every leaf
* page. See nbtree/README for details.
* We need to take a cleanup lock here, just like btvacuumpage(). However,
* it isn't necessary to exhaustively get a cleanup lock on every block in
* the index during recovery (just getting a cleanup lock on pages with
* items to kill suffices). See nbtree/README for details.
*/
if (XLogReadBufferForRedoExtended(record, 0, RBM_NORMAL, true, &buffer)
== BLK_NEEDS_REDO)
{
char *ptr;
Size len;
ptr = XLogRecGetBlockData(record, 0, &len);
char *ptr = XLogRecGetBlockData(record, 0, NULL);
page = (Page) BufferGetPage(buffer);
if (len > 0)
{
OffsetNumber *unused;
OffsetNumber *unend;
unused = (OffsetNumber *) ptr;
unend = (OffsetNumber *) ((char *) ptr + len);
if ((unend - unused) > 0)
PageIndexMultiDelete(page, unused, unend - unused);
}
PageIndexMultiDelete(page, (OffsetNumber *) ptr, xlrec->ndeleted);
/*
* Mark the page as not containing any LP_DEAD items --- see comments

3
src/backend/access/rmgrdesc/nbtdesc.c
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@ -46,8 +46,7 @@ btree_desc(StringInfo buf, XLogReaderState *record)
{
xl_btree_vacuum *xlrec = (xl_btree_vacuum *) rec;
appendStringInfo(buf, "lastBlockVacuumed %u",
xlrec->lastBlockVacuumed);
appendStringInfo(buf, "ndeleted %u", xlrec->ndeleted);
break;
}
case XLOG_BTREE_DELETE:

3
src/include/access/nbtree.h
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@ -779,8 +779,7 @@ extern bool _bt_page_recyclable(Page page);
extern void _bt_delitems_delete(Relation rel, Buffer buf,
OffsetNumber *itemnos, int nitems, Relation heapRel);
extern void _bt_delitems_vacuum(Relation rel, Buffer buf,
OffsetNumber *itemnos, int nitems,
BlockNumber lastBlockVacuumed);
OffsetNumber *deletable, int ndeletable);
extern int _bt_pagedel(Relation rel, Buffer buf);
/*

31
src/include/access/nbtxlog.h
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@ -134,7 +134,11 @@ typedef struct xl_btree_delete
#define SizeOfBtreeDelete (offsetof(xl_btree_delete, nitems) + sizeof(int))
/*
* This is what we need to know about page reuse within btree.
* This is what we need to know about page reuse within btree. This record
* only exists to generate a conflict point for Hot Standby.
*
* Note that we must include a RelFileNode in the record because we don't
* actually register the buffer with the record.
*/
typedef struct xl_btree_reuse_page
{
@ -150,32 +154,17 @@ typedef struct xl_btree_reuse_page
* The WAL record can represent deletion of any number of index tuples on a
* single index page when executed by VACUUM.
*
* For MVCC scans, lastBlockVacuumed will be set to InvalidBlockNumber.
* For a non-MVCC index scans there is an additional correctness requirement
* for applying these changes during recovery, which is that we must do one
* of these two things for every block in the index:
* * lock the block for cleanup and apply any required changes
* * EnsureBlockUnpinned()
* The purpose of this is to ensure that no index scans started before we
* finish scanning the index are still running by the time we begin to remove
* heap tuples.
*
* Any changes to any one block are registered on just one WAL record. All
* blocks that we need to run EnsureBlockUnpinned() are listed as a block range
* starting from the last block vacuumed through until this one. Individual
* block numbers aren't given.
*
* Note that the *last* WAL record in any vacuum of an index is allowed to
* have a zero length array of offsets. Earlier records must have at least one.
* Note that the WAL record in any vacuum of an index must have at least one
* item to delete.
*/
typedef struct xl_btree_vacuum
{
BlockNumber lastBlockVacuumed;
uint32 ndeleted;
/* TARGET OFFSET NUMBERS FOLLOW */
/* DELETED TARGET OFFSET NUMBERS FOLLOW */
} xl_btree_vacuum;
#define SizeOfBtreeVacuum (offsetof(xl_btree_vacuum, lastBlockVacuumed) + sizeof(BlockNumber))
#define SizeOfBtreeVacuum (offsetof(xl_btree_vacuum, ndeleted) + sizeof(uint32))
/*
* This is what we need to know about marking an empty branch for deletion.

2
src/include/access/xlog_internal.h
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@ -31,7 +31,7 @@
/*
* Each page of XLOG file has a header like this:
*/
#define XLOG_PAGE_MAGIC 0xD102 /* can be used as WAL version indicator */
#define XLOG_PAGE_MAGIC 0xD103 /* can be used as WAL version indicator */
typedef struct XLogPageHeaderData
{

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