/*------------------------------------------------------------------------- * * slru.c * Simple LRU buffering for transaction status logfiles * * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * * $Header: /cvsroot/pgsql/src/backend/access/transam/slru.c,v 1.2 2003/07/19 21:37:37 tgl Exp $ * *------------------------------------------------------------------------- */ #include "postgres.h" #include #include #include #include #include #include "access/slru.h" #include "storage/lwlock.h" #include "miscadmin.h" /* * Define segment size. A page is the same BLCKSZ as is used everywhere * else in Postgres. The segment size can be chosen somewhat arbitrarily; * we make it 32 pages by default, or 256Kb, i.e. 1M transactions for CLOG * or 64K transactions for SUBTRANS. * * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF, * page numbering also wraps around at 0xFFFFFFFF/xxxx_XACTS_PER_PAGE (where * xxxx is CLOG or SUBTRANS, respectively), and segment numbering at * 0xFFFFFFFF/xxxx_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 SimpleLruTruncate (see PagePrecedes()). */ #define SLRU_PAGES_PER_SEGMENT 32 /*---------- * Shared-memory data structures for SLRU control * * We use a simple least-recently-used scheme to manage a pool of page * buffers. Under ordinary circumstances we expect that write * traffic will occur mostly to the latest page (and to the just-prior * page, soon after a page transition). Read traffic will probably touch * a larger span of pages, but in any case a fairly small number of page * buffers should be sufficient. So, we just search the buffers using plain * linear search; there's no need for a hashtable or anything fancy. * The management algorithm is straight LRU except that we will never swap * out the latest page (since we know it's going to be hit again eventually). * * We use a control LWLock to protect the shared data structures, plus * per-buffer LWLocks that synchronize I/O for each buffer. A process * that is reading in or writing out a page buffer does not hold the control * lock, only the per-buffer lock for the buffer it is working on. * * To change the page number or state of a buffer, one must normally hold * the control lock. (The sole exception to this rule is that a writer * process changes the state from DIRTY to WRITE_IN_PROGRESS while holding * only the per-buffer lock.) If the buffer's state is neither EMPTY nor * CLEAN, then there may be processes doing (or waiting to do) I/O on the * buffer, so the page number may not be changed, and the only allowed state * transition is to change WRITE_IN_PROGRESS to DIRTY after dirtying the page. * To do any other state transition involving a buffer with potential I/O * processes, one must hold both the per-buffer lock and the control lock. * (Note the control lock must be acquired second; do not wait on a buffer * lock while holding the control lock.) A process wishing to read a page * marks the buffer state as READ_IN_PROGRESS, then drops the control lock, * acquires the per-buffer lock, and rechecks the state before proceeding. * This recheck takes care of the possibility that someone else already did * the read, while the early marking prevents someone else from trying to * read the same page into a different buffer. * * Note we are assuming that read and write of the state value is atomic, * since I/O processes may examine and change the state while not holding * the control lock. * * As with the regular buffer manager, it is possible for another process * to re-dirty a page that is currently being written out. This is handled * by setting the page's state from WRITE_IN_PROGRESS to DIRTY. The writing * process must notice this and not mark the page CLEAN when it's done. *---------- */ typedef enum { SLRU_PAGE_EMPTY, /* buffer is not in use */ SLRU_PAGE_READ_IN_PROGRESS, /* page is being read in */ SLRU_PAGE_CLEAN, /* page is valid and not dirty */ SLRU_PAGE_DIRTY, /* page is valid but needs write */ SLRU_PAGE_WRITE_IN_PROGRESS /* page is being written out */ } SlruPageStatus; /* * Shared-memory state */ typedef struct SlruSharedData { /* * Info for each buffer slot. Page number is undefined when status is * EMPTY. lru_count is essentially the number of page switches since * last use of this page; the page with highest lru_count is the best * candidate to replace. */ char *page_buffer[NUM_CLOG_BUFFERS]; SlruPageStatus page_status[NUM_CLOG_BUFFERS]; int page_number[NUM_CLOG_BUFFERS]; unsigned int page_lru_count[NUM_CLOG_BUFFERS]; /* * latest_page_number is the page number of the current end of the * CLOG; this is not critical data, since we use it only to avoid * swapping out the latest page. */ int latest_page_number; } SlruSharedData; typedef SlruSharedData *SlruShared; #define SlruFileName(ctl, path, seg) \ snprintf(path, MAXPGPATH, "%s/%04X", (ctl)->Dir, seg) /* * Macro to mark a buffer slot "most recently used". */ #define SlruRecentlyUsed(shared, slotno) \ do { \ if ((shared)->page_lru_count[slotno] != 0) { \ int iilru; \ for (iilru = 0; iilru < NUM_CLOG_BUFFERS; iilru++) \ (shared)->page_lru_count[iilru]++; \ (shared)->page_lru_count[slotno] = 0; \ } \ } while (0) /* Saved info for SlruReportIOError */ typedef enum { SLRU_OPEN_FAILED, SLRU_CREATE_FAILED, SLRU_SEEK_FAILED, SLRU_READ_FAILED, SLRU_WRITE_FAILED } SlruErrorCause; static SlruErrorCause slru_errcause; static int slru_errno; static bool SlruPhysicalReadPage(SlruCtl ctl, int pageno, int slotno); static bool SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno); static void SlruReportIOError(SlruCtl ctl, int pageno, TransactionId xid); static int SlruSelectLRUPage(SlruCtl ctl, int pageno); static bool SlruScanDirectory(SlruCtl ctl, int cutoffPage, bool doDeletions); /* * Initialization of shared memory */ int SimpleLruShmemSize(void) { return MAXALIGN(sizeof(SlruSharedData)) + BLCKSZ * NUM_CLOG_BUFFERS #ifdef EXEC_BACKEND + MAXALIGN(sizeof(SlruLockData)) #endif ; } void SimpleLruInit(SlruCtl ctl, const char *name, const char *subdir) { bool found; char *ptr; SlruShared shared; SlruLock locks; ptr = ShmemInitStruct(name, SimpleLruShmemSize(), &found); shared = (SlruShared) ptr; #ifdef EXEC_BACKEND /* * Locks are in shared memory */ locks = (SlruLock)(ptr + MAXALIGN(sizeof(SlruSharedData)) + BLCKSZ * NUM_CLOG_BUFFERS); #else /* * Locks are in private memory */ Assert(!IsUnderPostmaster); locks = malloc(sizeof(SlruLockData)); Assert(locks); #endif if (!IsUnderPostmaster) /* Initialize locks and shared memory area */ { char *bufptr; int slotno; Assert(!found); locks->ControlLock = LWLockAssign(); memset(shared, 0, sizeof(SlruSharedData)); bufptr = (char *)shared + MAXALIGN(sizeof(SlruSharedData)); for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++) { locks->BufferLocks[slotno] = LWLockAssign(); shared->page_buffer[slotno] = bufptr; shared->page_status[slotno] = SLRU_PAGE_EMPTY; shared->page_lru_count[slotno] = 1; bufptr += BLCKSZ; } /* shared->latest_page_number will be set later */ } else Assert(found); ctl->locks = locks; ctl->shared = shared; /* Init directory path */ snprintf(ctl->Dir, MAXPGPATH, "%s/%s", DataDir, subdir); } /* * Initialize (or reinitialize) a page to zeroes. * * 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. */ int SimpleLruZeroPage(SlruCtl ctl, int pageno) { int slotno; SlruShared shared = (SlruShared) ctl->shared; /* Find a suitable buffer slot for the page */ slotno = SlruSelectLRUPage(ctl, pageno); Assert(shared->page_status[slotno] == SLRU_PAGE_EMPTY || shared->page_status[slotno] == SLRU_PAGE_CLEAN || shared->page_number[slotno] == pageno); /* Mark the slot as containing this page */ shared->page_number[slotno] = pageno; shared->page_status[slotno] = SLRU_PAGE_DIRTY; SlruRecentlyUsed(shared, slotno); /* Set the buffer to zeroes */ MemSet(shared->page_buffer[slotno], 0, BLCKSZ); /* Assume this page is now the latest active page */ shared->latest_page_number = pageno; return slotno; } /* * Find a page in a shared buffer, reading it in if necessary. * The page number must correspond to an already-initialized page. * * The passed-in xid is used only for error reporting, and may be * InvalidTransactionId if no specific xid is associated with the action. * * Return value is the shared-buffer address of the page. * The buffer's LRU access info is updated. * If forwrite is true, the buffer is marked as dirty. * * Control lock must be held at entry, and will be held at exit. */ char * SimpleLruReadPage(SlruCtl ctl, int pageno, TransactionId xid, bool forwrite) { SlruShared shared = (SlruShared) ctl->shared; /* Outer loop handles restart if we lose the buffer to someone else */ for (;;) { int slotno; bool ok; /* See if page already is in memory; if not, pick victim slot */ slotno = SlruSelectLRUPage(ctl, pageno); /* Did we find the page in memory? */ if (shared->page_number[slotno] == pageno && shared->page_status[slotno] != SLRU_PAGE_EMPTY) { /* If page is still being read in, we cannot use it yet */ if (shared->page_status[slotno] != SLRU_PAGE_READ_IN_PROGRESS) { /* otherwise, it's ready to use */ SlruRecentlyUsed(shared, slotno); if (forwrite) shared->page_status[slotno] = SLRU_PAGE_DIRTY; return shared->page_buffer[slotno]; } } else { /* We found no match; assert we selected a freeable slot */ Assert(shared->page_status[slotno] == SLRU_PAGE_EMPTY || shared->page_status[slotno] == SLRU_PAGE_CLEAN); } /* Mark the slot read-busy (no-op if it already was) */ shared->page_number[slotno] = pageno; shared->page_status[slotno] = SLRU_PAGE_READ_IN_PROGRESS; /* * Temporarily mark page as recently-used to discourage * SlruSelectLRUPage from selecting it again for someone else. */ SlruRecentlyUsed(shared, slotno); /* Release shared lock, grab per-buffer lock instead */ LWLockRelease(ctl->locks->ControlLock); LWLockAcquire(ctl->locks->BufferLocks[slotno], LW_EXCLUSIVE); /* * Check to see if someone else already did the read, or took the * buffer away from us. If so, restart from the top. */ if (shared->page_number[slotno] != pageno || shared->page_status[slotno] != SLRU_PAGE_READ_IN_PROGRESS) { LWLockRelease(ctl->locks->BufferLocks[slotno]); LWLockAcquire(ctl->locks->ControlLock, LW_EXCLUSIVE); continue; } /* Okay, do the read */ ok = SlruPhysicalReadPage(ctl, pageno, slotno); /* Re-acquire shared control lock and update page state */ LWLockAcquire(ctl->locks->ControlLock, LW_EXCLUSIVE); Assert(shared->page_number[slotno] == pageno && shared->page_status[slotno] == SLRU_PAGE_READ_IN_PROGRESS); shared->page_status[slotno] = ok ? SLRU_PAGE_CLEAN : SLRU_PAGE_EMPTY; LWLockRelease(ctl->locks->BufferLocks[slotno]); /* Now it's okay to ereport if we failed */ if (!ok) SlruReportIOError(ctl, pageno, xid); SlruRecentlyUsed(shared, slotno); if (forwrite) shared->page_status[slotno] = SLRU_PAGE_DIRTY; return shared->page_buffer[slotno]; } } /* * Write a page from a shared buffer, if necessary. * Does nothing if the specified slot is not dirty. * * NOTE: only one write attempt is made here. Hence, it is possible that * the page is still dirty at exit (if someone else re-dirtied it during * the write). However, we *do* attempt a fresh write even if the page * is already being written; this is for checkpoints. * * Control lock must be held at entry, and will be held at exit. */ void SimpleLruWritePage(SlruCtl ctl, int slotno) { int pageno; bool ok; SlruShared shared = (SlruShared) ctl->shared; /* Do nothing if page does not need writing */ if (shared->page_status[slotno] != SLRU_PAGE_DIRTY && shared->page_status[slotno] != SLRU_PAGE_WRITE_IN_PROGRESS) return; pageno = shared->page_number[slotno]; /* Release shared lock, grab per-buffer lock instead */ LWLockRelease(ctl->locks->ControlLock); LWLockAcquire(ctl->locks->BufferLocks[slotno], LW_EXCLUSIVE); /* * Check to see if someone else already did the write, or took the * buffer away from us. If so, do nothing. NOTE: we really should * never see WRITE_IN_PROGRESS here, since that state should only * occur while the writer is holding the buffer lock. But accept it * so that we have a recovery path if a writer aborts. */ if (shared->page_number[slotno] != pageno || (shared->page_status[slotno] != SLRU_PAGE_DIRTY && shared->page_status[slotno] != SLRU_PAGE_WRITE_IN_PROGRESS)) { LWLockRelease(ctl->locks->BufferLocks[slotno]); LWLockAcquire(ctl->locks->ControlLock, LW_EXCLUSIVE); return; } /* * Mark the slot write-busy. After this point, a transaction status * update on this page will mark it dirty again. NB: we are assuming * that read/write of the page status field is atomic, since we change * the state while not holding control lock. However, we cannot set * this state any sooner, or we'd possibly fool a previous writer into * thinking he's successfully dumped the page when he hasn't. * (Scenario: other writer starts, page is redirtied, we come along * and set WRITE_IN_PROGRESS again, other writer completes and sets * CLEAN because redirty info has been lost, then we think it's clean * too.) */ shared->page_status[slotno] = SLRU_PAGE_WRITE_IN_PROGRESS; /* Okay, do the write */ ok = SlruPhysicalWritePage(ctl, pageno, slotno); /* Re-acquire shared control lock and update page state */ LWLockAcquire(ctl->locks->ControlLock, LW_EXCLUSIVE); Assert(shared->page_number[slotno] == pageno && (shared->page_status[slotno] == SLRU_PAGE_WRITE_IN_PROGRESS || shared->page_status[slotno] == SLRU_PAGE_DIRTY)); /* Cannot set CLEAN if someone re-dirtied page since write started */ if (shared->page_status[slotno] == SLRU_PAGE_WRITE_IN_PROGRESS) shared->page_status[slotno] = ok ? SLRU_PAGE_CLEAN : SLRU_PAGE_DIRTY; LWLockRelease(ctl->locks->BufferLocks[slotno]); /* Now it's okay to ereport if we failed */ if (!ok) SlruReportIOError(ctl, pageno, InvalidTransactionId); } /* * Physical read of a (previously existing) page into a buffer slot * * On failure, we cannot just ereport(ERROR) since caller has put state in * shared memory that must be undone. So, we return FALSE and save enough * info in static variables to let SlruReportIOError make the report. * * For now, assume it's not worth keeping a file pointer open across * read/write operations. We could cache one virtual file pointer ... */ static bool SlruPhysicalReadPage(SlruCtl ctl, int pageno, int slotno) { SlruShared shared = (SlruShared) ctl->shared; int segno = pageno / SLRU_PAGES_PER_SEGMENT; int rpageno = pageno % SLRU_PAGES_PER_SEGMENT; int offset = rpageno * BLCKSZ; char path[MAXPGPATH]; int fd; SlruFileName(ctl, path, segno); /* * In a crash-and-restart situation, it's possible for us to receive * commands to set the commit status of transactions whose bits are in * already-truncated segments of the commit log (see notes in * SlruPhysicalWritePage). Hence, if we are InRecovery, allow the * case where the file doesn't exist, and return zeroes instead. */ fd = BasicOpenFile(path, O_RDWR | PG_BINARY, S_IRUSR | S_IWUSR); if (fd < 0) { if (errno != ENOENT || !InRecovery) { slru_errcause = SLRU_OPEN_FAILED; slru_errno = errno; return false; } ereport(LOG, (errmsg("file \"%s\" doesn't exist, reading as zeroes", path))); MemSet(shared->page_buffer[slotno], 0, BLCKSZ); return true; } if (lseek(fd, (off_t) offset, SEEK_SET) < 0) { slru_errcause = SLRU_SEEK_FAILED; slru_errno = errno; return false; } errno = 0; if (read(fd, shared->page_buffer[slotno], BLCKSZ) != BLCKSZ) { slru_errcause = SLRU_READ_FAILED; slru_errno = errno; return false; } close(fd); return true; } /* * Physical write of a page from a buffer slot * * On failure, we cannot just ereport(ERROR) since caller has put state in * shared memory that must be undone. So, we return FALSE and save enough * info in static variables to let SlruReportIOError make the report. * * For now, assume it's not worth keeping a file pointer open across * read/write operations. We could cache one virtual file pointer ... */ static bool SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno) { SlruShared shared = (SlruShared) ctl->shared; int segno = pageno / SLRU_PAGES_PER_SEGMENT; int rpageno = pageno % SLRU_PAGES_PER_SEGMENT; int offset = rpageno * BLCKSZ; char path[MAXPGPATH]; int fd; SlruFileName(ctl, path, segno); /* * If the file doesn't already exist, we should create it. It is * possible for this to need to happen when writing a page that's not * first in its segment; we assume the OS can cope with that. (Note: * it might seem that it'd be okay to create files only when * SimpleLruZeroPage is called for the first page of a segment. However, * if after a crash and restart the REDO logic elects to replay the * log from a checkpoint before the latest one, then it's possible * that we will get commands to set transaction status of transactions * that have already been truncated from the commit log. Easiest way * to deal with that is to accept references to nonexistent files here * and in SlruPhysicalReadPage.) */ fd = BasicOpenFile(path, O_RDWR | PG_BINARY, S_IRUSR | S_IWUSR); if (fd < 0) { if (errno != ENOENT) { slru_errcause = SLRU_OPEN_FAILED; slru_errno = errno; return false; } fd = BasicOpenFile(path, O_RDWR | O_CREAT | O_EXCL | PG_BINARY, S_IRUSR | S_IWUSR); if (fd < 0) { slru_errcause = SLRU_CREATE_FAILED; slru_errno = errno; return false; } } if (lseek(fd, (off_t) offset, SEEK_SET) < 0) { slru_errcause = SLRU_SEEK_FAILED; slru_errno = errno; return false; } errno = 0; if (write(fd, shared->page_buffer[slotno], BLCKSZ) != BLCKSZ) { /* if write didn't set errno, assume problem is no disk space */ if (errno == 0) errno = ENOSPC; slru_errcause = SLRU_WRITE_FAILED; slru_errno = errno; return false; } close(fd); return true; } /* * Issue the error message after failure of SlruPhysicalReadPage or * SlruPhysicalWritePage. Call this after cleaning up shared-memory state. */ static void SlruReportIOError(SlruCtl ctl, int pageno, TransactionId xid) { int segno = pageno / SLRU_PAGES_PER_SEGMENT; int rpageno = pageno % SLRU_PAGES_PER_SEGMENT; int offset = rpageno * BLCKSZ; char path[MAXPGPATH]; SlruFileName(ctl, path, segno); errno = slru_errno; switch (slru_errcause) { case SLRU_OPEN_FAILED: ereport(ERROR, (errcode_for_file_access(), errmsg("could not access status of transaction %u", xid), errdetail("open of file \"%s\" failed: %m", path))); break; case SLRU_CREATE_FAILED: ereport(ERROR, (errcode_for_file_access(), errmsg("could not access status of transaction %u", xid), errdetail("creation of file \"%s\" failed: %m", path))); break; case SLRU_SEEK_FAILED: ereport(ERROR, (errcode_for_file_access(), errmsg("could not access status of transaction %u", xid), errdetail("lseek of file \"%s\", offset %u failed: %m", path, offset))); break; case SLRU_READ_FAILED: ereport(ERROR, (errcode_for_file_access(), errmsg("could not access status of transaction %u", xid), errdetail("read of file \"%s\", offset %u failed: %m", path, offset))); break; case SLRU_WRITE_FAILED: ereport(ERROR, (errcode_for_file_access(), errmsg("could not access status of transaction %u", xid), errdetail("write of file \"%s\", offset %u failed: %m", path, offset))); break; default: /* can't get here, we trust */ elog(ERROR, "unrecognized SimpleLru error cause: %d", (int) slru_errcause); break; } } /* * Select the slot to re-use when we need a free slot. * * The target page number is passed because we need to consider the * possibility that some other process reads in the target page while * we are doing I/O to free a slot. Hence, check or recheck to see if * any slot already holds the target page, and return that slot if so. * Thus, the returned slot is *either* a slot already holding the pageno * (could be any state except EMPTY), *or* a freeable slot (state EMPTY * or CLEAN). * * Control lock must be held at entry, and will be held at exit. */ static int SlruSelectLRUPage(SlruCtl ctl, int pageno) { SlruShared shared = (SlruShared) ctl->shared; /* Outer loop handles restart after I/O */ for (;;) { int slotno; int bestslot = 0; unsigned int bestcount = 0; /* See if page already has a buffer assigned */ for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++) { if (shared->page_number[slotno] == pageno && shared->page_status[slotno] != SLRU_PAGE_EMPTY) return slotno; } /* * If we find any EMPTY slot, just select that one. Else locate * the least-recently-used slot that isn't the latest page. */ for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++) { if (shared->page_status[slotno] == SLRU_PAGE_EMPTY) return slotno; if (shared->page_lru_count[slotno] > bestcount && shared->page_number[slotno] != shared->latest_page_number) { bestslot = slotno; bestcount = shared->page_lru_count[slotno]; } } /* * If the selected page is clean, we're set. */ if (shared->page_status[bestslot] == SLRU_PAGE_CLEAN) return bestslot; /* * We need to do I/O. Normal case is that we have to write it * out, but it's possible in the worst case to have selected a * read-busy page. In that case we use SimpleLruReadPage to wait for * the read to complete. */ if (shared->page_status[bestslot] == SLRU_PAGE_READ_IN_PROGRESS) (void) SimpleLruReadPage(ctl, shared->page_number[bestslot], InvalidTransactionId, false); else SimpleLruWritePage(ctl, bestslot); /* * Now loop back and try again. This is the easiest way of * dealing with corner cases such as the victim page being * re-dirtied while we wrote it. */ } } /* * This must be called ONCE during postmaster or standalone-backend startup */ void SimpleLruSetLatestPage(SlruCtl ctl, int pageno) { SlruShared shared = (SlruShared) ctl->shared; shared->latest_page_number = pageno; } /* * This is called during checkpoint and postmaster/standalone-backend shutdown */ void SimpleLruFlush(SlruCtl ctl, bool checkpoint) { SlruShared shared = (SlruShared) ctl->shared; int slotno; LWLockAcquire(ctl->locks->ControlLock, LW_EXCLUSIVE); for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++) { SimpleLruWritePage(ctl, slotno); /* * When called during a checkpoint, * we cannot assert that the slot is clean now, since another * process might have re-dirtied it already. That's okay. */ Assert(checkpoint || shared->page_status[slotno] == SLRU_PAGE_EMPTY || shared->page_status[slotno] == SLRU_PAGE_CLEAN); } LWLockRelease(ctl->locks->ControlLock); } /* * Remove all segments before the one holding the passed page number * * When this is called, we know that the database logically contains no * reference to transaction IDs older than oldestXact. However, we must * not remove any segment until we have performed a checkpoint, to ensure * that no such references remain on disk either; else a crash just after * the truncation might leave us with a problem. 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 checkpoint * unless we have confirmed that there is a removable segment. Therefore * we issue the checkpoint command here, not in higher-level code as might * seem cleaner. */ void SimpleLruTruncate(SlruCtl ctl, int cutoffPage) { int slotno; SlruShared shared = (SlruShared) ctl->shared; /* * The cutoff point is the start of the segment containing cutoffPage. */ cutoffPage -= cutoffPage % SLRU_PAGES_PER_SEGMENT; if (!SlruScanDirectory(ctl, cutoffPage, false)) return; /* nothing to remove */ /* Perform a forced CHECKPOINT */ CreateCheckPoint(false, true); /* * Scan shared memory and remove any pages preceding the cutoff * page, to ensure we won't rewrite them later. (Any dirty pages * should have been flushed already during the checkpoint, we're just * being extra careful here.) */ LWLockAcquire(ctl->locks->ControlLock, LW_EXCLUSIVE); restart:; /* * While we are holding the lock, make an important safety check: the * planned cutoff point must be <= the current endpoint page. * Otherwise we have already wrapped around, and proceeding with the * truncation would risk removing the current segment. */ if (ctl->PagePrecedes(shared->latest_page_number, cutoffPage)) { LWLockRelease(ctl->locks->ControlLock); ereport(LOG, (errmsg("unable to truncate \"%s\": apparent wraparound", ctl->Dir))); return; } for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++) { if (shared->page_status[slotno] == SLRU_PAGE_EMPTY) continue; if (!ctl->PagePrecedes(shared->page_number[slotno], cutoffPage)) continue; /* * If page is CLEAN, just change state to EMPTY (expected case). */ if (shared->page_status[slotno] == SLRU_PAGE_CLEAN) { shared->page_status[slotno] = SLRU_PAGE_EMPTY; continue; } /* * Hmm, we have (or may have) I/O operations acting on the page, * so we've got to wait for them to finish and then start again. * This is the same logic as in SlruSelectLRUPage. */ if (shared->page_status[slotno] == SLRU_PAGE_READ_IN_PROGRESS) (void) SimpleLruReadPage(ctl, shared->page_number[slotno], InvalidTransactionId, false); else SimpleLruWritePage(ctl, slotno); goto restart; } LWLockRelease(ctl->locks->ControlLock); /* Now we can remove the old segment(s) */ (void) SlruScanDirectory(ctl, cutoffPage, true); } /* * SlruTruncate subroutine: scan directory for removable segments. * Actually remove them iff doDeletions is true. Return TRUE iff any * removable segments were found. Note: no locking is needed. */ static bool SlruScanDirectory(SlruCtl ctl, int cutoffPage, bool doDeletions) { bool found = false; DIR *cldir; struct dirent *clde; int segno; int segpage; char path[MAXPGPATH]; cldir = opendir(ctl->Dir); if (cldir == NULL) ereport(ERROR, (errcode_for_file_access(), errmsg("could not open directory \"%s\": %m", ctl->Dir))); errno = 0; while ((clde = readdir(cldir)) != NULL) { if (strlen(clde->d_name) == 4 && strspn(clde->d_name, "0123456789ABCDEF") == 4) { segno = (int) strtol(clde->d_name, NULL, 16); segpage = segno * SLRU_PAGES_PER_SEGMENT; if (ctl->PagePrecedes(segpage, cutoffPage)) { found = true; if (doDeletions) { ereport(LOG, (errmsg("removing file \"%s/%s\"", ctl->Dir, clde->d_name))); snprintf(path, MAXPGPATH, "%s/%s", ctl->Dir, clde->d_name); unlink(path); } } } errno = 0; } if (errno) ereport(ERROR, (errcode_for_file_access(), errmsg("could not read directory \"%s\": %m", ctl->Dir))); closedir(cldir); return found; }