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Diffstat (limited to 'src/backend/access/transam/slru.c')
| -rw-r--r-- | src/backend/access/transam/slru.c | 886 |
1 files changed, 886 insertions, 0 deletions
diff --git a/src/backend/access/transam/slru.c b/src/backend/access/transam/slru.c new file mode 100644 index 00000000000..85e20e0e5b3 --- /dev/null +++ b/src/backend/access/transam/slru.c @@ -0,0 +1,886 @@ +/*------------------------------------------------------------------------- + * + * slru.c + * Simple LRU + * + * 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. + * + * Portions Copyright (c) 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 1.1 2003/06/11 22:37:45 momjian Exp $ + * + *------------------------------------------------------------------------- + */ +#include "postgres.h" + +#include <fcntl.h> +#include <dirent.h> +#include <errno.h> +#include <sys/stat.h> +#include <unistd.h> + +#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 elog 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 elog 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 elog(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; + } + + elog(LOG, "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 elog(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]; + + /* XXX TODO: provide xid as context in error messages */ + + SlruFileName(ctl, path, segno); + errno = slru_errno; + switch (slru_errcause) + { + case SLRU_OPEN_FAILED: + elog(ERROR, "open of %s failed: %m", path); + break; + case SLRU_CREATE_FAILED: + elog(ERROR, "creation of file %s failed: %m", path); + break; + case SLRU_SEEK_FAILED: + elog(ERROR, "lseek of file %s, offset %u failed: %m", + path, offset); + break; + case SLRU_READ_FAILED: + elog(ERROR, "read of file %s, offset %u failed: %m", + path, offset); + break; + case SLRU_WRITE_FAILED: + elog(ERROR, "write of file %s, offset %u failed: %m", + path, offset); + break; + default: + /* can't get here, we trust */ + elog(ERROR, "unknown SimpleLru I/O error"); + 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); + elog(LOG, "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) + elog(ERROR, "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) + { + elog(LOG, "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) + elog(ERROR, "could not read directory (%s): %m", ctl->Dir); + closedir(cldir); + + return found; +} |