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Rewrite the FSM. Instead of relying on a fixed-size shared memory segment, the

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free space information is stored in a dedicated FSM relation fork, with each
relation (except for hash indexes; they don't use FSM).

This eliminates the max_fsm_relations and max_fsm_pages GUC options; remove any
trace of them from the backend, initdb, and documentation.

Rewrite contrib/pg_freespacemap to match the new FSM implementation. Also
introduce a new variant of the get_raw_page(regclass, int4, int4) function in
contrib/pageinspect that let's you to return pages from any relation fork, and
a new fsm_page_contents() function to inspect the new FSM pages.
This commit is contained in:
Heikki Linnakangas
2008-09-30 10:52:14 +00:00
parent 2dbc0ca937
commit 15c121b3ed
53 changed files with 1844 additions and 2720 deletions
Download Patch File Download Diff File Expand all files Collapse all files

4
src/backend/storage/freespace/Makefile
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@@ -4,7 +4,7 @@
# Makefile for storage/freespace
#
# IDENTIFICATION
# $PostgreSQL: pgsql/src/backend/storage/freespace/Makefile,v 1.4 2008/02/19 10:30:08 petere Exp $
# $PostgreSQL: pgsql/src/backend/storage/freespace/Makefile,v 1.5 2008/09/30 10:52:13 heikki Exp $
#
#-------------------------------------------------------------------------
@@ -12,6 +12,6 @@ subdir = src/backend/storage/freespace
top_builddir = ../../../..
include $(top_builddir)/src/Makefile.global
OBJS = freespace.o
OBJS = freespace.o fsmpage.o indexfsm.o
include $(top_srcdir)/src/backend/common.mk

195
src/backend/storage/freespace/README Normal file
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@@ -0,0 +1,195 @@
$PostgreSQL: pgsql/src/backend/storage/freespace/README,v 1.1 2008/09/30 10:52:13 heikki Exp $
Free Space Map
--------------
The purpose of the free space map is to quickly locate a page with enough
free space to hold a tuple to be stored; or to determine that no such page
exists and the relation must be extended by one page. As of PostgreSQL 8.4
each relation has its own, extensible free space map stored in a separate
"fork" of its relation. This eliminates the disadvantages of the former
fixed-size FSM.
It is important to keep the map small so that it can be searched rapidly.
Therefore, we don't attempt to record the exact free space on a page.
We allocate one map byte to each page, allowing us to record free space
at a granularity of 1/256th of a page. Another way to say it is that
the stored value is the free space divided by BLCKSZ/256 (rounding down).
We assume that the free space must always be less than BLCKSZ, since
all pages have some overhead; so the maximum map value is 255.
To assist in fast searching, the map isn't simply an array of per-page
entries, but has a tree structure above those entries. There is a tree
structure of pages, and a tree structure within each page, as described
below.
FSM page structure
------------------
Within each FSM page, we use a binary tree structure where leaf nodes store
the amount of free space on heap pages (or lower level FSM pages, see
"Higher-level structure" below), with one leaf node per heap page. A non-leaf
node stores the max amount of free space on any of its children.
For example:
4
4 2
3 4 0 2 <- This level represents heap pages
We need two basic operations: search and update.
To search for a page with X amount of free space, traverse down the tree
along a path where n >= X, until you hit the bottom. If both children of a
node satisfy the condition, you can pick either one arbitrarily.
To update the amount of free space on a page to X, first update the leaf node
corresponding to the heap page, then "bubble up" the change to upper nodes,
by walking up to each parent and recomputing its value as the max of its
two children. Repeat until reaching the root or a parent whose value
doesn't change.
This data structure has a couple of nice properties:
- to discover that there is no page with X bytes of free space, you only
need to look at the root node
- by varying which child to traverse to in the search algorithm, when you have
a choice, we can implement various strategies, like preferring pages closer
to a given page, or spreading the load across the table.
Higher-level routines that use FSM pages access them through the fsm_set_avail()
and fsm_search_avail() functions. The interface to those functions hides the
page's internal tree structure, treating the FSM page as a black box that has
a certain number of "slots" for storing free space information. (However,
the higher routines have to be aware of the tree structure of the whole map.)
The binary tree is stored on each FSM page as an array. Because the page
header takes some space on a page, the binary tree isn't perfect. That is,
a few right-most leaf nodes are missing, and there are some useless non-leaf
nodes at the right. So the tree looks something like this:
0
1 2
3 4 5 6
7 8 9 A B
where the numbers denote each node's position in the array. Note that the
tree is guaranteed complete above the leaf level; only some leaf nodes are
missing. This is reflected in the number of usable "slots" per page not
being an exact power of 2.
A FSM page also has a next slot pointer, fp_next_slot, that determines where
to start the next search for free space within that page. The reason for that
is to spread out the pages that are returned by FSM searches. When several
backends are concurrently inserting into a relation, contention can be avoided
by having them insert into different pages. But it is also desirable to fill
up pages in sequential order, to get the benefit of OS prefetching and batched
writes. The FSM is responsible for making that happen, and the next slot
pointer helps provide the desired behavior.
Higher-level structure
----------------------
To scale up the data structure described above beyond a single page, we
maintain a similar tree-structure across pages. Leaf nodes in higher level
pages correspond to lower level FSM pages. The root node within each page
has the same value as the corresponding leaf node on its parent page.
The root page is always stored at physical block 0.
For example, assuming each FSM page can hold information about 4 pages (in
reality, it holds (BLCKSZ - headers) / 2, or ~4000 with default BLCKSZ),
we get a disk layout like this:
0 <-- page 0 at level 2 (root page)
0 <-- page 0 at level 1
0 <-- page 0 at level 0
1 <-- page 1 at level 0
2 <-- ...
3
1 <-- page 1 at level 1
4
5
6
7
2
8
9
10
11
3
12
13
14
15
where the numbers are page numbers *at that level*, starting from 0.
To find the physical block # corresponding to leaf page n, we need to
count the number number of leaf and upper-level pages preceding page n.
This turns out to be
y = n + (n / F + 1) + (n / F^2 + 1) + ... + 1
where F is the fanout (4 in the above example). The first term n is the number
of preceding leaf pages, the second term is the number of pages at level 1,
and so forth.
To keep things simple, the tree is always constant height. To cover the
maximum relation size of 2^32-1 blocks, three levels is enough with the default
BLCKSZ (4000^3 > 2^32).
Addressing
----------
The higher-level routines operate on "logical" addresses, consisting of
- level,
- logical page number, and
- slot (if applicable)
Bottom level FSM pages have level of 0, the level above that 1, and root 2.
As in the diagram above, logical page number is the page number at that level,
starting from 0.
Locking
-------
When traversing down to search for free space, only one page is locked at a
time: the parent page is released before locking the child. If the child page
is concurrently modified, and there no longer is free space on the child page
when you land on it, you need to start from scratch (after correcting the
parent page, so that you don't get into an infinite loop).
We use shared buffer locks when searching, but exclusive buffer lock when
updating a page. However, the next slot search pointer is updated during
searches even though we have only a shared lock. fp_next_slot is just a hint
and we can easily reset it if it gets corrupted; so it seems better to accept
some risk of that type than to pay the overhead of exclusive locking.
Recovery
--------
The FSM is not explicitly WAL-logged. Instead, we rely on a bunch of
self-correcting measures to repair possible corruption.
First of all, whenever a value is set on an FSM page, the root node of the
page is compared against the new value after bubbling up the change is
finished. It should be greater than or equal to the value just set, or we
have a corrupted page, with a parent somewhere with too small a value.
Secondly, if we detect corrupted pages while we search, traversing down
the tree. That check will notice if a parent node is set to too high a value.
In both cases, the upper nodes on the page are immediately rebuilt, fixing
the corruption.
Vacuum updates all the bottom level pages with correct amount of free space
on the heap pages, fixing any outdated values there. After the heap and
index passes are done, FreeSpaceMapVacuum is called, and the FSM tree is
scanned in depth-first order. This fixes any discrepancies between upper
and lower level FSM pages.
TODO
----
- fastroot to avoid traversing upper nodes with just 1 child
- use a different system for tables that fit into one FSM page, with a
mechanism to switch to the real thing as it grows.

2308
src/backend/storage/freespace/freespace.c
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352
src/backend/storage/freespace/fsmpage.c Normal file
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@@ -0,0 +1,352 @@
/*-------------------------------------------------------------------------
*
* fsmpage.c
* routines to search and manipulate one FSM page.
*
*
* Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/storage/freespace/fsmpage.c,v 1.1 2008/09/30 10:52:13 heikki Exp $
*
* NOTES:
*
* The public functions in this file form an API that hides the internal
* structure of a FSM page. This allows freespace.c to treat each FSM page
* as a black box with SlotsPerPage "slots". fsm_set_avail() and
* fsm_get_avail() let's you get/set the value of a slot, and
* fsm_search_avail() let's you search for a slot with value >= X.
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "storage/bufmgr.h"
#include "storage/fsm_internals.h"
/* macros to navigate the tree within a page. */
#define leftchild(x) (2 * (x) + 1)
#define rightchild(x) (2 * (x) + 2)
#define parentof(x) (((x) - 1) / 2)
/* returns right sibling of x, wrapping around within the level */
static int
rightsibling(int x)
{
/*
* Move right. This might wrap around, stepping to the leftmost node at
* the next level.
*/
x++;
/*
* Check if we stepped to the leftmost node at next level, and correct
* if so. The leftmost nodes at each level are of form x = 2^level - 1, so
* check if (x + 1) is a power of two.
*/
if (((x + 1) & x) == 0)
x = parentof(x);
return x;
}
/*
* Sets the value of a slot on page. Returns true if the page was
* modified.
*
* The caller must hold an exclusive lock on the page.
*/
bool
fsm_set_avail(Page page, int slot, uint8 value)
{
int nodeno = NonLeafNodesPerPage + slot;
FSMPage fsmpage = (FSMPage) PageGetContents(page);
uint8 oldvalue;
Assert(slot < LeafNodesPerPage);
oldvalue = fsmpage->fp_nodes[nodeno];
/* If the value hasn't changed, we don't need to do anything */
if (oldvalue == value && value <= fsmpage->fp_nodes[0])
return false;
fsmpage->fp_nodes[nodeno] = value;
/*
* Propagate up, until we hit the root or a node that doesn't
* need to be updated.
*/
do
{
uint8 newvalue = 0;
int lchild;
int rchild;
nodeno = parentof(nodeno);
lchild = leftchild(nodeno);
rchild = lchild + 1;
newvalue = fsmpage->fp_nodes[lchild];
if (rchild < NodesPerPage)
newvalue = Max(newvalue,
fsmpage->fp_nodes[rchild]);
oldvalue = fsmpage->fp_nodes[nodeno];
if (oldvalue == newvalue)
break;
fsmpage->fp_nodes[nodeno] = newvalue;
} while (nodeno > 0);
/*
* sanity check: if the new value value is higher than the value
* at the top, the tree is corrupt.
*/
if (value > fsmpage->fp_nodes[0])
fsm_rebuild_page(page);
return true;
}
/*
* Returns the value of given slot on page.
*
* Since this is just a read-only access of a single byte, the page doesn't
* need to be locked.
*/
uint8
fsm_get_avail(Page page, int slot)
{
FSMPage fsmpage = (FSMPage) PageGetContents(page);
return fsmpage->fp_nodes[NonLeafNodesPerPage + slot];
}
/*
* Returns the value at the root of a page.
* Since this is just a read-only access of a single byte, the page doesn't
* need to be locked.
*/
uint8
fsm_get_max_avail(Page page)
{
FSMPage fsmpage = (FSMPage) PageGetContents(page);
return fsmpage->fp_nodes[0];
}
/*
* Searches for a slot with min. category. Returns slot number, or -1 if
* none found.
*
* The caller must hold at least a shared lock on the page, and this
* function can unlock and lock the page again in exclusive mode if it
* needs to be updated. exclusive_lock_held should be set to true if the
* caller is already holding an exclusive lock, to avoid extra work.
*
* If advancenext is false, fp_next_slot is set to point to the returned
* slot, and if it's true, to the slot next to the returned slot.
*/
int
fsm_search_avail(Buffer buf, uint8 minvalue, bool advancenext,
bool exclusive_lock_held)
{
Page page = BufferGetPage(buf);
FSMPage fsmpage = (FSMPage) PageGetContents(page);
int nodeno;
int target;
uint16 slot;
restart:
/*
* Check the root first, and exit quickly if there's no page with
* enough free space
*/
if (fsmpage->fp_nodes[0] < minvalue)
return -1;
/* fp_next_slot is just a hint, so check that it's sane */
target = fsmpage->fp_next_slot;
if (target < 0 || target >= LeafNodesPerPage)
target = 0;
target += NonLeafNodesPerPage;
/*
* Start the search from the target slot. At every step, move one
* node to the right, and climb up to the parent. Stop when we reach a
* node with enough free space. (note that moving to the right only
* makes a difference if we're on the right child of the parent)
*
* The idea is to graduall expand our "search triangle", that is, all
* nodes covered by the current node. In the beginning, just the target
* node is included, and more nodes to the right of the target node,
* taking wrap-around into account, is included at each step. Nodes are
* added to the search triangle in left-to-right order, starting from
* the target node. This ensures that we'll find the first suitable node
* to the right of the target node, and not some other node with enough
* free space.
*
* For example, consider this tree:
*
* 7
* 7 6
* 5 7 6 5
* 4 5 5 7 2 6 5 2
* T
*
* Imagine that target node is the node indicated by the letter T, and
* we're searching for a node with value of 6 or higher. The search
* begins at T. At first iteration, we move to the right, and to the
* parent, arriving the rightmost 5. At the 2nd iteration, we move to the
* right, wrapping around, and climb up, arriving at the 7 at the 2nd
* level. 7 satisfies our search, so we descend down to the bottom,
* following the path of sevens.
*/
nodeno = target;
while (nodeno > 0)
{
if (fsmpage->fp_nodes[nodeno] >= minvalue)
break;
/*
* Move to the right, wrapping around at the level if necessary, and
* climb up.
*/
nodeno = parentof(rightsibling(nodeno));
}
/*
* We're now at a node with enough free space, somewhere in the middle of
* the tree. Descend to the bottom, following a path with enough free
* space, preferring to move left if there's a choice.
*/
while (nodeno < NonLeafNodesPerPage)
{
int leftnodeno = leftchild(nodeno);
int rightnodeno = leftnodeno + 1;
bool leftok = (leftnodeno < NodesPerPage) &&
(fsmpage->fp_nodes[leftnodeno] >= minvalue);
bool rightok = (rightnodeno < NodesPerPage) &&
(fsmpage->fp_nodes[rightnodeno] >= minvalue);
if (leftok)
nodeno = leftnodeno;
else if (rightok)
nodeno = rightnodeno;
else
{
/*
* Oops. The parent node promised that either left or right
* child has enough space, but neither actually did. This can
* happen in case of a "torn page", IOW if we crashed earlier
* while writing the page to disk, and only part of the page
* made it to disk.
*
* Fix the corruption and restart.
*/
RelFileNode rnode;
ForkNumber forknum;
BlockNumber blknum;
BufferGetTag(buf, &rnode, &forknum, &blknum);
elog(DEBUG1, "fixing corrupt FSM block %u, relation %u/%u/%u",
blknum, rnode.spcNode, rnode.dbNode, rnode.relNode);
/* make sure we hold an exclusive lock */
if (!exclusive_lock_held)
{
LockBuffer(buf, BUFFER_LOCK_UNLOCK);
LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
exclusive_lock_held = true;
}
fsm_rebuild_page(page);
MarkBufferDirty(buf);
goto restart;
}
}
/* We're now at the bottom level, at a node with enough space. */
slot = nodeno - NonLeafNodesPerPage;
/*
* Update the next slot pointer. Note that we do this even if we're only
* holding a shared lock, on the grounds that it's better to use a shared
* lock and get a garbled next pointer every now and then, than take the
* concurrency hit of an exlusive lock.
*
* Wrap-around is handled at the beginning of this function.
*/
fsmpage->fp_next_slot = slot + (advancenext ? 1 : 0);
return slot;
}
/*
* Sets the available space to zero for all slots numbered >= nslots.
* Returns true if the page was modified.
*/
bool
fsm_truncate_avail(Page page, int nslots)
{
FSMPage fsmpage = (FSMPage) PageGetContents(page);
uint8 *ptr;
bool changed = false;
Assert(nslots >= 0 && nslots < LeafNodesPerPage);
/* Clear all truncated leaf nodes */
ptr = &fsmpage->fp_nodes[NonLeafNodesPerPage + nslots];
for (; ptr < &fsmpage->fp_nodes[NodesPerPage]; ptr++)
{
if (*ptr != 0)
changed = true;
*ptr = 0;
}
/* Fix upper nodes. */
if (changed)
fsm_rebuild_page(page);
return changed;
}
/*
* Reconstructs the upper levels of a page. Returns true if the page
* was modified.
*/
bool
fsm_rebuild_page(Page page)
{
FSMPage fsmpage = (FSMPage) PageGetContents(page);
bool changed = false;
int nodeno;
/*
* Start from the lowest non-leaflevel, at last node, working our way
* backwards, through all non-leaf nodes at all levels, up to the root.
*/
for (nodeno = NonLeafNodesPerPage - 1; nodeno >= 0; nodeno--)
{
int lchild = leftchild(nodeno);
int rchild = lchild + 1;
uint8 newvalue = 0;
if (lchild < NodesPerPage)
newvalue = fsmpage->fp_nodes[lchild];
if (rchild < NodesPerPage)
newvalue = Max(newvalue,
fsmpage->fp_nodes[rchild]);
if (fsmpage->fp_nodes[nodeno] != newvalue)
{
fsmpage->fp_nodes[nodeno] = newvalue;
changed = true;
}
}
return changed;
}

92
src/backend/storage/freespace/indexfsm.c Normal file
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@@ -0,0 +1,92 @@
/*-------------------------------------------------------------------------
*
* indexfsm.c
* POSTGRES free space map for quickly finding free pages in relations
*
*
* Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/storage/freespace/indexfsm.c,v 1.1 2008/09/30 10:52:13 heikki Exp $
*
*
* NOTES:
*
* This is similar to the FSM used for heap, in freespace.c, but instead
* of tracking the amount of free space on pages, we only track whether
* pages are completely free or in-use. We use the same FSM implementation
* as for heaps, using BLCKSZ - 1 to denote used pages, and 0 for unused.
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "storage/freespace.h"
#include "storage/indexfsm.h"
#include "storage/smgr.h"
/*
* Exported routines
*/
/*
* InitIndexFreeSpaceMap - Create or reset the FSM fork for relation.
*/
void
InitIndexFreeSpaceMap(Relation rel)
{
/* Create FSM fork if it doesn't exist yet, or truncate it if it does */
RelationOpenSmgr(rel);
if (!smgrexists(rel->rd_smgr, FSM_FORKNUM))
smgrcreate(rel->rd_smgr, FSM_FORKNUM, rel->rd_istemp, false);
else
smgrtruncate(rel->rd_smgr, FSM_FORKNUM, 0, rel->rd_istemp);
}
/*
* GetFreeIndexPage - return a free page from the FSM
*
* As a side effect, the page is marked as used in the FSM.
*/
BlockNumber
GetFreeIndexPage(Relation rel)
{
BlockNumber blkno = GetPageWithFreeSpace(rel, BLCKSZ/2);
if (blkno != InvalidBlockNumber)
RecordUsedIndexPage(rel, blkno);
return blkno;
}
/*
* RecordFreeIndexPage - mark a page as free in the FSM
*/
void
RecordFreeIndexPage(Relation rel, BlockNumber freeBlock)
{
RecordPageWithFreeSpace(rel, freeBlock, BLCKSZ - 1);
}
/*
* RecordUsedIndexPage - mark a page as used in the FSM
*/
void
RecordUsedIndexPage(Relation rel, BlockNumber usedBlock)
{
RecordPageWithFreeSpace(rel, usedBlock, 0);
}
/*
* IndexFreeSpaceMapTruncate - adjust for truncation of a relation.
*
* We need to delete any stored data past the new relation length, so that
* we don't bogusly return removed block numbers.
*/
void
IndexFreeSpaceMapTruncate(Relation rel, BlockNumber nblocks)
{
FreeSpaceMapTruncateRel(rel, nblocks);
}

9
src/backend/storage/ipc/ipci.c
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@@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/storage/ipc/ipci.c,v 1.96 2008/05/12 00:00:50 alvherre Exp $
* $PostgreSQL: pgsql/src/backend/storage/ipc/ipci.c,v 1.97 2008/09/30 10:52:13 heikki Exp $
*
*-------------------------------------------------------------------------
*/
@@ -26,7 +26,6 @@
#include "postmaster/bgwriter.h"
#include "postmaster/postmaster.h"
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "storage/ipc.h"
#include "storage/pg_shmem.h"
#include "storage/pmsignal.h"
@@ -110,7 +109,6 @@ CreateSharedMemoryAndSemaphores(bool makePrivate, int port)
size = add_size(size, ProcArrayShmemSize());
size = add_size(size, BackendStatusShmemSize());
size = add_size(size, SInvalShmemSize());
size = add_size(size, FreeSpaceShmemSize());
size = add_size(size, BgWriterShmemSize());
size = add_size(size, AutoVacuumShmemSize());
size = add_size(size, BTreeShmemSize());
@@ -203,11 +201,6 @@ CreateSharedMemoryAndSemaphores(bool makePrivate, int port)
*/
CreateSharedInvalidationState();
/*
* Set up free-space map
*/
InitFreeSpaceMap();
/*
* Set up interprocess signaling mechanisms
*/

24
src/backend/storage/smgr/smgr.c
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@@ -11,7 +11,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/storage/smgr/smgr.c,v 1.111 2008/08/11 11:05:11 heikki Exp $
* $PostgreSQL: pgsql/src/backend/storage/smgr/smgr.c,v 1.112 2008/09/30 10:52:13 heikki Exp $
*
*-------------------------------------------------------------------------
*/
@@ -21,7 +21,6 @@
#include "access/xlogutils.h"
#include "commands/tablespace.h"
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "storage/ipc.h"
#include "storage/smgr.h"
#include "utils/hsearch.h"
@@ -474,13 +473,6 @@ smgr_internal_unlink(RelFileNode rnode, ForkNumber forknum,
*/
DropRelFileNodeBuffers(rnode, forknum, isTemp, 0);
/*
* Tell the free space map to forget this relation. It won't be accessed
* any more anyway, but we may as well recycle the map space quickly.
*/
if (forknum == MAIN_FORKNUM)
FreeSpaceMapForgetRel(&rnode);
/*
* It'd be nice to tell the stats collector to forget it immediately, too.
* But we can't because we don't know the OID (and in cases involving
@@ -577,13 +569,6 @@ smgrtruncate(SMgrRelation reln, ForkNumber forknum, BlockNumber nblocks,
*/
DropRelFileNodeBuffers(reln->smgr_rnode, forknum, isTemp, nblocks);
/*
* Tell the free space map to forget anything it may have stored for the
* about-to-be-deleted blocks. We want to be sure it won't return bogus
* block numbers later on.
*/
FreeSpaceMapTruncateRel(&reln->smgr_rnode, nblocks);
/* Do the truncation */
(*(smgrsw[reln->smgr_which].smgr_truncate)) (reln, forknum, nblocks,
isTemp);
@@ -905,13 +890,6 @@ smgr_redo(XLogRecPtr lsn, XLogRecord *record)
DropRelFileNodeBuffers(xlrec->rnode, xlrec->forknum, false,
xlrec->blkno);
/*
* Tell the free space map to forget anything it may have stored for
* the about-to-be-deleted blocks. We want to be sure it won't return
* bogus block numbers later on.
*/
FreeSpaceMapTruncateRel(&reln->smgr_rnode, xlrec->blkno);
/* Do the truncation */
(*(smgrsw[reln->smgr_which].smgr_truncate)) (reln,
xlrec->forknum,