greenplumn clog 源码
greenplumn clog 代码
文件路径：/src/backend/access/transam/clog.c
/*-------------------------------------------------------------------------
*
* clog.c
* PostgreSQL transaction-commit-log manager
*
* This module replaces the old "pg_log" access code, which treated pg_log
* essentially like a relation, in that it went through the regular buffer
* manager. The problem with that was that there wasn't any good way to
* recycle storage space for transactions so old that they'll never be
* looked up again. Now we use specialized access code so that the commit
* log can be broken into relatively small, independent segments.
*
* XLOG interactions: this module generates an XLOG record whenever a new
* CLOG page is initialized to zeroes. Other writes of CLOG come from
* recording of transaction commit or abort in xact.c, which generates its
* own XLOG records for these events and will re-perform the status update
* on redo; so we need make no additional XLOG entry here. For synchronous
* transaction commits, the XLOG is guaranteed flushed through the XLOG commit
* record before we are called to log a commit, so the WAL rule "write xlog
* before data" is satisfied automatically. However, for async commits we
* must track the latest LSN affecting each CLOG page, so that we can flush
* XLOG that far and satisfy the WAL rule. We don't have to worry about this
* for aborts (whether sync or async), since the post-crash assumption would
* be that such transactions failed anyway.
*
* Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* src/backend/access/transam/clog.c
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "access/clog.h"
#include "access/slru.h"
#include "access/transam.h"
#include "access/xlog.h"
#include "access/xloginsert.h"
#include "access/xlogutils.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "pg_trace.h"
#include "storage/proc.h"
/*
* Defines for CLOG page sizes. A page is the same BLCKSZ as is used
* everywhere else in Postgres.
*
* Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
* CLOG page numbering also wraps around at 0xFFFFFFFF/CLOG_XACTS_PER_PAGE,
* and CLOG segment numbering at
* 0xFFFFFFFF/CLOG_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT. We need take no
* explicit notice of that fact in this module, except when comparing segment
* and page numbers in TruncateCLOG (see CLOGPagePrecedes).
*/
/* We need two bits per xact, so four xacts fit in a byte */
#define CLOG_BITS_PER_XACT 2
#define CLOG_XACTS_PER_BYTE 4
#define CLOG_XACTS_PER_PAGE (BLCKSZ * CLOG_XACTS_PER_BYTE)
#define CLOG_XACT_BITMASK ((1 << CLOG_BITS_PER_XACT) - 1)
#define TransactionIdToPage(xid) ((xid) / (TransactionId) CLOG_XACTS_PER_PAGE)
#define TransactionIdToPgIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE)
#define TransactionIdToByte(xid) (TransactionIdToPgIndex(xid) / CLOG_XACTS_PER_BYTE)
#define TransactionIdToBIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_BYTE)
/* We store the latest async LSN for each group of transactions */
#define CLOG_XACTS_PER_LSN_GROUP 32 /* keep this a power of 2 */
#define CLOG_LSNS_PER_PAGE (CLOG_XACTS_PER_PAGE / CLOG_XACTS_PER_LSN_GROUP)
#define GetLSNIndex(slotno, xid) ((slotno) * CLOG_LSNS_PER_PAGE + \
((xid) % (TransactionId) CLOG_XACTS_PER_PAGE) / CLOG_XACTS_PER_LSN_GROUP)
/*
* The number of subtransactions below which we consider to apply clog group
* update optimization. Testing reveals that the number higher than this can
* hurt performance.
*/
#define THRESHOLD_SUBTRANS_CLOG_OPT 5
/*
* Link to shared-memory data structures for CLOG control
*/
static SlruCtlData ClogCtlData;
#define ClogCtl (&ClogCtlData)
static int ZeroCLOGPage(int pageno, bool writeXlog);
static bool CLOGPagePrecedes(int page1, int page2);
static void WriteZeroPageXlogRec(int pageno);
static void WriteTruncateXlogRec(int pageno, TransactionId oldestXact,
Oid oldestXidDb);
static void TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
TransactionId *subxids, XidStatus status,
XLogRecPtr lsn, int pageno,
bool all_xact_same_page);
static void TransactionIdSetStatusBit(TransactionId xid, XidStatus status,
XLogRecPtr lsn, int slotno);
static void set_status_by_pages(int nsubxids, TransactionId *subxids,
XidStatus status, XLogRecPtr lsn);
static bool TransactionGroupUpdateXidStatus(TransactionId xid,
XidStatus status, XLogRecPtr lsn, int pageno);
static void TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids,
TransactionId *subxids, XidStatus status,
XLogRecPtr lsn, int pageno);
/*
* TransactionIdSetTreeStatus
*
* Record the final state of transaction entries in the commit log for
* a transaction and its subtransaction tree. Take care to ensure this is
* efficient, and as atomic as possible.
*
* xid is a single xid to set status for. This will typically be
* the top level transactionid for a top level commit or abort. It can
* also be a subtransaction when we record transaction aborts.
*
* subxids is an array of xids of length nsubxids, representing subtransactions
* in the tree of xid. In various cases nsubxids may be zero.
*
* lsn must be the WAL location of the commit record when recording an async
* commit. For a synchronous commit it can be InvalidXLogRecPtr, since the
* caller guarantees the commit record is already flushed in that case. It
* should be InvalidXLogRecPtr for abort cases, too.
*
* In the commit case, atomicity is limited by whether all the subxids are in
* the same CLOG page as xid. If they all are, then the lock will be grabbed
* only once, and the status will be set to committed directly. Otherwise
* we must
* 1. set sub-committed all subxids that are not on the same page as the
* main xid
* 2. atomically set committed the main xid and the subxids on the same page
* 3. go over the first bunch again and set them committed
* Note that as far as concurrent checkers are concerned, main transaction
* commit as a whole is still atomic.
*
* Example:
* TransactionId t commits and has subxids t1, t2, t3, t4
* t is on page p1, t1 is also on p1, t2 and t3 are on p2, t4 is on p3
* 1. update pages2-3:
* page2: set t2,t3 as sub-committed
* page3: set t4 as sub-committed
* 2. update page1:
* set t1 as sub-committed,
* then set t as committed,
then set t1 as committed
* 3. update pages2-3:
* page2: set t2,t3 as committed
* page3: set t4 as committed
*
* NB: this is a low-level routine and is NOT the preferred entry point
* for most uses; functions in transam.c are the intended callers.
*
* XXX Think about issuing FADVISE_WILLNEED on pages that we will need,
* but aren't yet in cache, as well as hinting pages not to fall out of
* cache yet.
*/
void
TransactionIdSetTreeStatus(TransactionId xid, int nsubxids,
TransactionId *subxids, XidStatus status, XLogRecPtr lsn)
{
int pageno = TransactionIdToPage(xid); /* get page of parent */
int i;
Assert(status == TRANSACTION_STATUS_COMMITTED ||
status == TRANSACTION_STATUS_ABORTED);
/*
* See how many subxids, if any, are on the same page as the parent, if
* any.
*/
for (i = 0; i < nsubxids; i++)
{
if (TransactionIdToPage(subxids[i]) != pageno)
break;
}
/*
* Do all items fit on a single page?
*/
if (i == nsubxids)
{
/*
* Set the parent and all subtransactions in a single call
*/
TransactionIdSetPageStatus(xid, nsubxids, subxids, status, lsn,
pageno, true);
}
else
{
int nsubxids_on_first_page = i;
/*
* If this is a commit then we care about doing this correctly (i.e.
* using the subcommitted intermediate status). By here, we know
* we're updating more than one page of clog, so we must mark entries
* that are *not* on the first page so that they show as subcommitted
* before we then return to update the status to fully committed.
*
* To avoid touching the first page twice, skip marking subcommitted
* for the subxids on that first page.
*/
if (status == TRANSACTION_STATUS_COMMITTED)
set_status_by_pages(nsubxids - nsubxids_on_first_page,
subxids + nsubxids_on_first_page,
TRANSACTION_STATUS_SUB_COMMITTED, lsn);
/*
* Now set the parent and subtransactions on same page as the parent,
* if any
*/
pageno = TransactionIdToPage(xid);
TransactionIdSetPageStatus(xid, nsubxids_on_first_page, subxids, status,
lsn, pageno, false);
/*
* Now work through the rest of the subxids one clog page at a time,
* starting from the second page onwards, like we did above.
*/
set_status_by_pages(nsubxids - nsubxids_on_first_page,
subxids + nsubxids_on_first_page,
status, lsn);
}
}
/*
* Helper for TransactionIdSetTreeStatus: set the status for a bunch of
* transactions, chunking in the separate CLOG pages involved. We never
* pass the whole transaction tree to this function, only subtransactions
* that are on different pages to the top level transaction id.
*/
static void
set_status_by_pages(int nsubxids, TransactionId *subxids,
XidStatus status, XLogRecPtr lsn)
{
int pageno = TransactionIdToPage(subxids[0]);
int offset = 0;
int i = 0;
Assert(nsubxids > 0); /* else the pageno fetch above is unsafe */
while (i < nsubxids)
{
int num_on_page = 0;
int nextpageno;
do
{
nextpageno = TransactionIdToPage(subxids[i]);
if (nextpageno != pageno)
break;
num_on_page++;
i++;
} while (i < nsubxids);
TransactionIdSetPageStatus(InvalidTransactionId,
num_on_page, subxids + offset,
status, lsn, pageno, false);
offset = i;
pageno = nextpageno;
}
}
/*
* Record the final state of transaction entries in the commit log for all
* entries on a single page. Atomic only on this page.
*/
static void
TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
TransactionId *subxids, XidStatus status,
XLogRecPtr lsn, int pageno,
bool all_xact_same_page)
{
/* Can't use group update when PGPROC overflows. */
StaticAssertStmt(THRESHOLD_SUBTRANS_CLOG_OPT <= PGPROC_MAX_CACHED_SUBXIDS,
"group clog threshold less than PGPROC cached subxids");
/*
* When there is contention on CLogControlLock, we try to group multiple
* updates; a single leader process will perform transaction status
* updates for multiple backends so that the number of times
* CLogControlLock needs to be acquired is reduced.
*
* For this optimization to be safe, the XID in MyPgXact and the subxids
* in MyProc must be the same as the ones for which we're setting the
* status. Check that this is the case.
*
* For this optimization to be efficient, we shouldn't have too many
* sub-XIDs and all of the XIDs for which we're adjusting clog should be
* on the same page. Check those conditions, too.
*/
if (all_xact_same_page && xid == MyPgXact->xid &&
nsubxids <= THRESHOLD_SUBTRANS_CLOG_OPT &&
nsubxids == MyPgXact->nxids &&
memcmp(subxids, MyProc->subxids.xids,
nsubxids * sizeof(TransactionId)) == 0)
{
/*
* We don't try to do group update optimization if a process has
* overflowed the subxids array in its PGPROC, since in that case we
* don't have a complete list of XIDs for it.
*/
Assert(THRESHOLD_SUBTRANS_CLOG_OPT <= PGPROC_MAX_CACHED_SUBXIDS);
/*
* If we can immediately acquire CLogControlLock, we update the status
* of our own XID and release the lock. If not, try use group XID
* update. If that doesn't work out, fall back to waiting for the
* lock to perform an update for this transaction only.
*/
if (LWLockConditionalAcquire(CLogControlLock, LW_EXCLUSIVE))
{
/* Got the lock without waiting! Do the update. */
TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
lsn, pageno);
LWLockRelease(CLogControlLock);
return;
}
else if (TransactionGroupUpdateXidStatus(xid, status, lsn, pageno))
{
/* Group update mechanism has done the work. */
return;
}
/* Fall through only if update isn't done yet. */
}
/* Group update not applicable, or couldn't accept this page number. */
LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
lsn, pageno);
LWLockRelease(CLogControlLock);
}
/*
* Record the final state of transaction entry in the commit log
*
* We don't do any locking here; caller must handle that.
*/
static void
TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids,
TransactionId *subxids, XidStatus status,
XLogRecPtr lsn, int pageno)
{
int slotno;
int i;
Assert(status == TRANSACTION_STATUS_COMMITTED ||
status == TRANSACTION_STATUS_ABORTED ||
(status == TRANSACTION_STATUS_SUB_COMMITTED && !TransactionIdIsValid(xid)));
Assert(LWLockHeldByMeInMode(CLogControlLock, LW_EXCLUSIVE));
/*
* If we're doing an async commit (ie, lsn is valid), then we must wait
* for any active write on the page slot to complete. Otherwise our
* update could reach disk in that write, which will not do since we
* mustn't let it reach disk until we've done the appropriate WAL flush.
* But when lsn is invalid, it's OK to scribble on a page while it is
* write-busy, since we don't care if the update reaches disk sooner than
* we think.
*/
slotno = SimpleLruReadPage(ClogCtl, pageno, XLogRecPtrIsInvalid(lsn), xid);
/*
* Set the main transaction id, if any.
*
* If we update more than one xid on this page while it is being written
* out, we might find that some of the bits go to disk and others don't.
* If we are updating commits on the page with the top-level xid that
* could break atomicity, so we subcommit the subxids first before we mark
* the top-level commit.
*/
if (TransactionIdIsValid(xid))
{
/* Subtransactions first, if needed ... */
if (status == TRANSACTION_STATUS_COMMITTED)
{
for (i = 0; i < nsubxids; i++)
{
Assert(ClogCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
TransactionIdSetStatusBit(subxids[i],
TRANSACTION_STATUS_SUB_COMMITTED,
lsn, slotno);
}
}
/* ... then the main transaction */
TransactionIdSetStatusBit(xid, status, lsn, slotno);
}
/* Set the subtransactions */
for (i = 0; i < nsubxids; i++)
{
Assert(ClogCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
TransactionIdSetStatusBit(subxids[i], status, lsn, slotno);
}
ClogCtl->shared->page_dirty[slotno] = true;
}
/*
* When we cannot immediately acquire CLogControlLock in exclusive mode at
* commit time, add ourselves to a list of processes that need their XIDs
* status update. The first process to add itself to the list will acquire
* CLogControlLock in exclusive mode and set transaction status as required
* on behalf of all group members. This avoids a great deal of contention
* around CLogControlLock when many processes are trying to commit at once,
* since the lock need not be repeatedly handed off from one committing
* process to the next.
*
* Returns true when transaction status has been updated in clog; returns
* false if we decided against applying the optimization because the page
* number we need to update differs from those processes already waiting.
*/
static bool
TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status,
XLogRecPtr lsn, int pageno)
{
volatile PROC_HDR *procglobal = ProcGlobal;
PGPROC *proc = MyProc;
uint32 nextidx;
uint32 wakeidx;
/* We should definitely have an XID whose status needs to be updated. */
Assert(TransactionIdIsValid(xid));
/*
* Add ourselves to the list of processes needing a group XID status
* update.
*/
proc->clogGroupMember = true;
proc->clogGroupMemberXid = xid;
proc->clogGroupMemberXidStatus = status;
proc->clogGroupMemberPage = pageno;
proc->clogGroupMemberLsn = lsn;
nextidx = pg_atomic_read_u32(&procglobal->clogGroupFirst);
while (true)
{
/*
* Add the proc to list, if the clog page where we need to update the
* current transaction status is same as group leader's clog page.
*
* There is a race condition here, which is that after doing the below
* check and before adding this proc's clog update to a group, the
* group leader might have already finished the group update for this
* page and becomes group leader of another group. This will lead to a
* situation where a single group can have different clog page
* updates. This isn't likely and will still work, just maybe a bit
* less efficiently.
*/
if (nextidx != INVALID_PGPROCNO &&
ProcGlobal->allProcs[nextidx].clogGroupMemberPage != proc->clogGroupMemberPage)
{
proc->clogGroupMember = false;
return false;
}
pg_atomic_write_u32(&proc->clogGroupNext, nextidx);
if (pg_atomic_compare_exchange_u32(&procglobal->clogGroupFirst,
&nextidx,
(uint32) proc->pgprocno))
break;
}
/*
* If the list was not empty, the leader will update the status of our
* XID. It is impossible to have followers without a leader because the
* first process that has added itself to the list will always have
* nextidx as INVALID_PGPROCNO.
*/
if (nextidx != INVALID_PGPROCNO)
{
int extraWaits = 0;
/* Sleep until the leader updates our XID status. */
pgstat_report_wait_start(WAIT_EVENT_CLOG_GROUP_UPDATE);
for (;;)
{
/* acts as a read barrier */
PGSemaphoreLock(proc->sem);
if (!proc->clogGroupMember)
break;
extraWaits++;
}
pgstat_report_wait_end();
Assert(pg_atomic_read_u32(&proc->clogGroupNext) == INVALID_PGPROCNO);
/* Fix semaphore count for any absorbed wakeups */
while (extraWaits-- > 0)
PGSemaphoreUnlock(proc->sem);
return true;
}
/* We are the leader. Acquire the lock on behalf of everyone. */
LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
/*
* Now that we've got the lock, clear the list of processes waiting for
* group XID status update, saving a pointer to the head of the list.
* Trying to pop elements one at a time could lead to an ABA problem.
*/
nextidx = pg_atomic_exchange_u32(&procglobal->clogGroupFirst,
INVALID_PGPROCNO);
/* Remember head of list so we can perform wakeups after dropping lock. */
wakeidx = nextidx;
/* Walk the list and update the status of all XIDs. */
while (nextidx != INVALID_PGPROCNO)
{
PGPROC *proc = &ProcGlobal->allProcs[nextidx];
PGXACT *pgxact = &ProcGlobal->allPgXact[nextidx];
/*
* Overflowed transactions should not use group XID status update
* mechanism.
*/
Assert(!pgxact->overflowed);
TransactionIdSetPageStatusInternal(proc->clogGroupMemberXid,
pgxact->nxids,
proc->subxids.xids,
proc->clogGroupMemberXidStatus,
proc->clogGroupMemberLsn,
proc->clogGroupMemberPage);
/* Move to next proc in list. */
nextidx = pg_atomic_read_u32(&proc->clogGroupNext);
}
/* We're done with the lock now. */
LWLockRelease(CLogControlLock);
/*
* Now that we've released the lock, go back and wake everybody up. We
* don't do this under the lock so as to keep lock hold times to a
* minimum.
*/
while (wakeidx != INVALID_PGPROCNO)
{
PGPROC *proc = &ProcGlobal->allProcs[wakeidx];
wakeidx = pg_atomic_read_u32(&proc->clogGroupNext);
pg_atomic_write_u32(&proc->clogGroupNext, INVALID_PGPROCNO);
/* ensure all previous writes are visible before follower continues. */
pg_write_barrier();
proc->clogGroupMember = false;
if (proc != MyProc)
PGSemaphoreUnlock(proc->sem);
}
return true;
}
/*
* Sets the commit status of a single transaction.
*
* Must be called with CLogControlLock held
*/
static void
TransactionIdSetStatusBit(TransactionId xid, XidStatus status, XLogRecPtr lsn, int slotno)
{
int byteno = TransactionIdToByte(xid);
int bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
char *byteptr;
char byteval;
char curval;
byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;
curval = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
/*
* When replaying transactions during recovery we still need to perform
* the two phases of subcommit and then commit. However, some transactions
* are already correctly marked, so we just treat those as a no-op which
* allows us to keep the following Assert as restrictive as possible.
*/
if (InRecovery && status == TRANSACTION_STATUS_SUB_COMMITTED &&
curval == TRANSACTION_STATUS_COMMITTED)
return;
/*
* Current state change should be from 0 or subcommitted to target state
* or we should already be there when replaying changes during recovery.
*/
Assert(curval == 0 ||
(curval == TRANSACTION_STATUS_SUB_COMMITTED &&
status != TRANSACTION_STATUS_IN_PROGRESS) ||
curval == status);
/* note this assumes exclusive access to the clog page */
byteval = *byteptr;
byteval &= ~(((1 << CLOG_BITS_PER_XACT) - 1) << bshift);
byteval |= (status << bshift);
*byteptr = byteval;
/*
* Update the group LSN if the transaction completion LSN is higher.
*
* Note: lsn will be invalid when supplied during InRecovery processing,
* so we don't need to do anything special to avoid LSN updates during
* recovery. After recovery completes the next clog change will set the
* LSN correctly.
*/
if (!XLogRecPtrIsInvalid(lsn))
{
int lsnindex = GetLSNIndex(slotno, xid);
if (ClogCtl->shared->group_lsn[lsnindex] < lsn)
ClogCtl->shared->group_lsn[lsnindex] = lsn;
}
}
/*
* Interrogate the state of a transaction in the commit log.
*
* Aside from the actual commit status, this function returns (into *lsn)
* an LSN that is late enough to be able to guarantee that if we flush up to
* that LSN then we will have flushed the transaction's commit record to disk.
* The result is not necessarily the exact LSN of the transaction's commit
* record! For example, for long-past transactions (those whose clog pages
* already migrated to disk), we'll return InvalidXLogRecPtr. Also, because
* we group transactions on the same clog page to conserve storage, we might
* return the LSN of a later transaction that falls into the same group.
*
* NB: this is a low-level routine and is NOT the preferred entry point
* for most uses; TransactionLogFetch() in transam.c is the intended caller.
*/
XidStatus
TransactionIdGetStatus(TransactionId xid, XLogRecPtr *lsn)
{
int pageno = TransactionIdToPage(xid);
int byteno = TransactionIdToByte(xid);
int bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
int slotno;
int lsnindex;
char *byteptr;
XidStatus status;
/* lock is acquired by SimpleLruReadPage_ReadOnly */
slotno = SimpleLruReadPage_ReadOnly(ClogCtl, pageno, xid);
byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;
status = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
lsnindex = GetLSNIndex(slotno, xid);
*lsn = ClogCtl->shared->group_lsn[lsnindex];
LWLockRelease(CLogControlLock);
return status;
}
/*
* Find the next lowest transaction with a logged or recorded status.
* I.e. One that does not have a status of default (0) -- i.e: in-progress.
*/
bool
CLOGScanForPrevStatus(
TransactionId *indexXid,
XidStatus *status)
{
TransactionId highXid;
int pageno;
TransactionId lowXid;
int slotno;
int byteno;
int bshift;
TransactionId xid;
char *byteptr;
*status = TRANSACTION_STATUS_IN_PROGRESS; // Set it to something.
if ((*indexXid) == InvalidTransactionId)
return false;
highXid = (*indexXid) - 1;
if (highXid < FirstNormalTransactionId)
return false;
while (true)
{
pageno = TransactionIdToPage(highXid);
/*
* Compute the xid floor for the page.
*/
lowXid = pageno * (TransactionId) CLOG_XACTS_PER_PAGE;
if (lowXid == InvalidTransactionId)
lowXid = FirstNormalTransactionId;
LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
/*
* Peek to see if page exists.
*/
if (!SimpleLruDoesPhysicalPageExist(ClogCtl, pageno))
{
LWLockRelease(CLogControlLock);
*indexXid = InvalidTransactionId;
*status = TRANSACTION_STATUS_IN_PROGRESS; // Set it to something.
return false;
}
slotno = SimpleLruReadPage(ClogCtl, pageno, false, highXid);
for (xid = highXid; xid >= lowXid; xid--)
{
byteno = TransactionIdToByte(xid);
bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;
*status = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
if (*status != TRANSACTION_STATUS_IN_PROGRESS)
{
LWLockRelease(CLogControlLock);
*indexXid = xid;
return true;
}
}
LWLockRelease(CLogControlLock);
if (lowXid == FirstNormalTransactionId)
{
*indexXid = InvalidTransactionId;
*status = TRANSACTION_STATUS_IN_PROGRESS; // Set it to something.
return false;
}
highXid = lowXid - 1; // Go to last xid of previous page.
}
return false; // We'll never reach this.
}
/*
* Determine the "age" of a transaction id.
*/
bool
CLOGTransactionIsOld(TransactionId xid)
{
TransactionId nextXid;
int pagesBack;
if (ShmemVariableCache == NULL)
return false; // In case we are called very early in the life of the backend process, etc.
nextXid = XidFromFullTransactionId(ShmemVariableCache->nextFullXid);
if (nextXid < xid)
return false; // Not sure what is going on.
pagesBack = (nextXid - xid) / CLOG_XACTS_PER_PAGE;
/*
* Declare the transaction old if it is in the bottom older half of the hot CLOG cache window, or
* before the window.
*/
return (pagesBack > CLOGShmemBuffers()/2);
}
/*
* Number of shared CLOG buffers.
*
* On larger multi-processor systems, it is possible to have many CLOG page
* requests in flight at one time which could lead to disk access for CLOG
* page if the required page is not found in memory. Testing revealed that we
* can get the best performance by having 128 CLOG buffers, more than that it
* doesn't improve performance.
*
* Unconditionally keeping the number of CLOG buffers to 128 did not seem like
* a good idea, because it would increase the minimum amount of shared memory
* required to start, which could be a problem for people running very small
* configurations. The following formula seems to represent a reasonable
* compromise: people with very low values for shared_buffers will get fewer
* CLOG buffers as well, and everyone else will get 128.
*/
Size
CLOGShmemBuffers(void)
{
return Min(128, Max(4, NBuffers / 512));
}
/*
* Initialization of shared memory for CLOG
*/
Size
CLOGShmemSize(void)
{
return SimpleLruShmemSize(CLOGShmemBuffers(), CLOG_LSNS_PER_PAGE);
}
void
CLOGShmemInit(void)
{
ClogCtl->PagePrecedes = CLOGPagePrecedes;
SimpleLruInit(ClogCtl, "clog", CLOGShmemBuffers(), CLOG_LSNS_PER_PAGE,
CLogControlLock, "pg_xact", LWTRANCHE_CLOG_BUFFERS);
}
/*
* This func must be called ONCE on system install. It creates
* the initial CLOG segment. (The CLOG directory is assumed to
* have been created by initdb, and CLOGShmemInit must have been
* called already.)
*/
void
BootStrapCLOG(void)
{
int slotno;
LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
/* Create and zero the first page of the commit log */
slotno = ZeroCLOGPage(0, false);
/* Make sure it's written out */
SimpleLruWritePage(ClogCtl, slotno);
Assert(!ClogCtl->shared->page_dirty[slotno]);
LWLockRelease(CLogControlLock);
}
/*
* Initialize (or reinitialize) a page of CLOG to zeroes.
* If writeXlog is true, also emit an XLOG record saying we did this.
*
* The page is not actually written, just set up in shared memory.
* The slot number of the new page is returned.
*
* Control lock must be held at entry, and will be held at exit.
*/
static int
ZeroCLOGPage(int pageno, bool writeXlog)
{
int slotno;
slotno = SimpleLruZeroPage(ClogCtl, pageno);
if (writeXlog)
WriteZeroPageXlogRec(pageno);
return slotno;
}
/*
* This must be called ONCE during postmaster or standalone-backend startup,
* after StartupXLOG has initialized ShmemVariableCache->nextFullXid.
*/
void
StartupCLOG(void)
{
TransactionId xid = XidFromFullTransactionId(ShmemVariableCache->nextFullXid);
int pageno = TransactionIdToPage(xid);
LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
/*
* Initialize our idea of the latest page number.
*/
ClogCtl->shared->latest_page_number = pageno;
LWLockRelease(CLogControlLock);
}
/*
* This must be called ONCE at the end of startup/recovery.
*/
void
TrimCLOG(void)
{
TransactionId xid = XidFromFullTransactionId(ShmemVariableCache->nextFullXid);
int pageno = TransactionIdToPage(xid);
LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
/*
* Re-Initialize our idea of the latest page number.
*/
ClogCtl->shared->latest_page_number = pageno;
/*
* Zero out the remainder of the current clog page. Under normal
* circumstances it should be zeroes already, but it seems at least
* theoretically possible that XLOG replay will have settled on a nextXID
* value that is less than the last XID actually used and marked by the
* previous database lifecycle (since subtransaction commit writes clog
* but makes no WAL entry). Let's just be safe. (We need not worry about
* pages beyond the current one, since those will be zeroed when first
* used. For the same reason, there is no need to do anything when
* nextFullXid is exactly at a page boundary; and it's likely that the
* "current" page doesn't exist yet in that case.)
*/
if (TransactionIdToPgIndex(xid) != 0)
{
int byteno = TransactionIdToByte(xid);
int bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
int slotno;
char *byteptr;
slotno = SimpleLruReadPage(ClogCtl, pageno, false, xid);
byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;
/* Zero so-far-unused positions in the current byte */
*byteptr &= (1 << bshift) - 1;
/* Zero the rest of the page */
MemSet(byteptr + 1, 0, BLCKSZ - byteno - 1);
ClogCtl->shared->page_dirty[slotno] = true;
}
LWLockRelease(CLogControlLock);
}
/*
* This must be called ONCE during postmaster or standalone-backend shutdown
*/
void
ShutdownCLOG(void)
{
/* Flush dirty CLOG pages to disk */
TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(false);
SimpleLruFlush(ClogCtl, false);
/*
* fsync pg_xact to ensure that any files flushed previously are durably
* on disk.
*/
fsync_fname("pg_xact", true);
TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(false);
}
/*
* Perform a checkpoint --- either during shutdown, or on-the-fly
*/
void
CheckPointCLOG(void)
{
/* Flush dirty CLOG pages to disk */
TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(true);
SimpleLruFlush(ClogCtl, true);
/*
* fsync pg_xact to ensure that any files flushed previously are durably
* on disk.
*/
fsync_fname("pg_xact", true);
TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(true);
}
/*
* Make sure that CLOG has room for a newly-allocated XID.
*
* NB: this is called while holding XidGenLock. We want it to be very fast
* most of the time; even when it's not so fast, no actual I/O need happen
* unless we're forced to write out a dirty clog or xlog page to make room
* in shared memory.
*/
void
ExtendCLOG(TransactionId newestXact)
{
int pageno;
/*
* No work except at first XID of a page. But beware: just after
* wraparound, the first XID of page zero is FirstNormalTransactionId.
*/
if (TransactionIdToPgIndex(newestXact) != 0 &&
!TransactionIdEquals(newestXact, FirstNormalTransactionId))
return;
pageno = TransactionIdToPage(newestXact);
LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
/* Zero the page and make an XLOG entry about it */
ZeroCLOGPage(pageno, true);
LWLockRelease(CLogControlLock);
}
/*
* Remove all CLOG segments before the one holding the passed transaction ID
*
* Before removing any CLOG data, we must flush XLOG to disk, to ensure
* that any recently-emitted HEAP_FREEZE records have reached disk; otherwise
* a crash and restart might leave us with some unfrozen tuples referencing
* removed CLOG data. We choose to emit a special TRUNCATE XLOG record too.
* Replaying the deletion from XLOG is not critical, since the files could
* just as well be removed later, but doing so prevents a long-running hot
* standby server from acquiring an unreasonably bloated CLOG directory.
*
* Since CLOG segments hold a large number of transactions, the opportunity to
* actually remove a segment is fairly rare, and so it seems best not to do
* the XLOG flush unless we have confirmed that there is a removable segment.
*/
void
TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid)
{
int cutoffPage;
/*
* The cutoff point is the start of the segment containing oldestXact. We
* pass the *page* containing oldestXact to SimpleLruTruncate.
*/
cutoffPage = TransactionIdToPage(oldestXact);
/* Check to see if there's any files that could be removed */
if (!SlruScanDirectory(ClogCtl, SlruScanDirCbReportPresence, &cutoffPage))
return; /* nothing to remove */
/*
* Advance oldestClogXid before truncating clog, so concurrent xact status
* lookups can ensure they don't attempt to access truncated-away clog.
*
* It's only necessary to do this if we will actually truncate away clog
* pages.
*/
AdvanceOldestClogXid(oldestXact);
/*
* Write XLOG record and flush XLOG to disk. We record the oldest xid
* we're keeping information about here so we can ensure that it's always
* ahead of clog truncation in case we crash, and so a standby finds out
* the new valid xid before the next checkpoint.
*/
WriteTruncateXlogRec(cutoffPage, oldestXact, oldestxid_datoid);
/* Now we can remove the old CLOG segment(s) */
SimpleLruTruncate(ClogCtl, cutoffPage);
}
/*
* Decide which of two CLOG page numbers is "older" for truncation purposes.
*
* We need to use comparison of TransactionIds here in order to do the right
* thing with wraparound XID arithmetic. However, if we are asked about
* page number zero, we don't want to hand InvalidTransactionId to
* TransactionIdPrecedes: it'll get weird about permanent xact IDs. So,
* offset both xids by FirstNormalTransactionId to avoid that.
*/
static bool
CLOGPagePrecedes(int page1, int page2)
{
TransactionId xid1;
TransactionId xid2;
xid1 = ((TransactionId) page1) * CLOG_XACTS_PER_PAGE;
xid1 += FirstNormalTransactionId;
xid2 = ((TransactionId) page2) * CLOG_XACTS_PER_PAGE;
xid2 += FirstNormalTransactionId;
return TransactionIdPrecedes(xid1, xid2);
}
/*
* Write a ZEROPAGE xlog record
*/
static void
WriteZeroPageXlogRec(int pageno)
{
XLogBeginInsert();
XLogRegisterData((char *) (&pageno), sizeof(int));
(void) XLogInsert(RM_CLOG_ID, CLOG_ZEROPAGE);
}
/*
* Write a TRUNCATE xlog record
*
* We must flush the xlog record to disk before returning --- see notes
* in TruncateCLOG().
*/
static void
WriteTruncateXlogRec(int pageno, TransactionId oldestXact, Oid oldestXactDb)
{
XLogRecPtr recptr;
xl_clog_truncate xlrec;
xlrec.pageno = pageno;
xlrec.oldestXact = oldestXact;
xlrec.oldestXactDb = oldestXactDb;
XLogBeginInsert();
XLogRegisterData((char *) (&xlrec), sizeof(xl_clog_truncate));
recptr = XLogInsert(RM_CLOG_ID, CLOG_TRUNCATE);
XLogFlush(recptr);
}
/*
* CLOG resource manager's routines
*/
void
clog_redo(XLogReaderState *record)
{
uint8 info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
/* Backup blocks are not used in clog records */
Assert(!XLogRecHasAnyBlockRefs(record));
if (info == CLOG_ZEROPAGE)
{
int pageno;
int slotno;
memcpy(&pageno, XLogRecGetData(record), sizeof(int));
LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
slotno = ZeroCLOGPage(pageno, false);
SimpleLruWritePage(ClogCtl, slotno);
Assert(!ClogCtl->shared->page_dirty[slotno]);
LWLockRelease(CLogControlLock);
}
else if (info == CLOG_TRUNCATE)
{
xl_clog_truncate xlrec;
memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_clog_truncate));
/*
* During XLOG replay, latest_page_number isn't set up yet; insert a
* suitable value to bypass the sanity test in SimpleLruTruncate.
*/
ClogCtl->shared->latest_page_number = xlrec.pageno;
AdvanceOldestClogXid(xlrec.oldestXact);
SimpleLruTruncate(ClogCtl, xlrec.pageno);
}
else
elog(PANIC, "clog_redo: unknown op code %u", info);
}
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