slru.c

postgresql8.3.4源码,开源数据库

	/*
	 * If not part of Flush, need to fsync now.  We assume this happens
	 * infrequently enough that it's not a performance issue.
	 */
	if (!fdata)
	{
		if (ctl->do_fsync && pg_fsync(fd))
		{
			slru_errcause = SLRU_FSYNC_FAILED;
			slru_errno = errno;
			close(fd);
			return false;
		}

		if (close(fd))
		{
			slru_errcause = SLRU_CLOSE_FAILED;
			slru_errno = errno;
			return false;
		}
	}

	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("Could not open file \"%s\": %m.", path)));
			break;
		case SLRU_SEEK_FAILED:
			ereport(ERROR,
					(errcode_for_file_access(),
					 errmsg("could not access status of transaction %u", xid),
				 errdetail("Could not seek in file \"%s\" to offset %u: %m.",
						   path, offset)));
			break;
		case SLRU_READ_FAILED:
			ereport(ERROR,
					(errcode_for_file_access(),
					 errmsg("could not access status of transaction %u", xid),
			   errdetail("Could not read from file \"%s\" at offset %u: %m.",
						 path, offset)));
			break;
		case SLRU_WRITE_FAILED:
			ereport(ERROR,
					(errcode_for_file_access(),
					 errmsg("could not access status of transaction %u", xid),
				errdetail("Could not write to file \"%s\" at offset %u: %m.",
						  path, offset)));
			break;
		case SLRU_FSYNC_FAILED:
			ereport(ERROR,
					(errcode_for_file_access(),
					 errmsg("could not access status of transaction %u", xid),
					 errdetail("Could not fsync file \"%s\": %m.",
							   path)));
			break;
		case SLRU_CLOSE_FAILED:
			ereport(ERROR,
					(errcode_for_file_access(),
					 errmsg("could not access status of transaction %u", xid),
					 errdetail("Could not close file \"%s\": %m.",
							   path)));
			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 = ctl->shared;

	/* Outer loop handles restart after I/O */
	for (;;)
	{
		int			slotno;
		int			cur_count;
		int			bestslot;
		int			best_delta;
		int			best_page_number;

		/* See if page already has a buffer assigned */
		for (slotno = 0; slotno < shared->num_slots; 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 to replace.
		 *
		 * Normally the page_lru_count values will all be different and so
		 * there will be a well-defined LRU page.  But since we allow
		 * concurrent execution of SlruRecentlyUsed() within
		 * SimpleLruReadPage_ReadOnly(), it is possible that multiple pages
		 * acquire the same lru_count values.  In that case we break ties by
		 * choosing the furthest-back page.
		 *
		 * In no case will we select the slot containing latest_page_number
		 * for replacement, even if it appears least recently used.
		 *
		 * Notice that this next line forcibly advances cur_lru_count to a
		 * value that is certainly beyond any value that will be in the
		 * page_lru_count array after the loop finishes.  This ensures that
		 * the next execution of SlruRecentlyUsed will mark the page newly
		 * used, even if it's for a page that has the current counter value.
		 * That gets us back on the path to having good data when there are
		 * multiple pages with the same lru_count.
		 */
		cur_count = (shared->cur_lru_count)++;
		best_delta = -1;
		bestslot = 0;			/* no-op, just keeps compiler quiet */
		best_page_number = 0;	/* ditto */
		for (slotno = 0; slotno < shared->num_slots; slotno++)
		{
			int			this_delta;
			int			this_page_number;

			if (shared->page_status[slotno] == SLRU_PAGE_EMPTY)
				return slotno;
			this_delta = cur_count - shared->page_lru_count[slotno];
			if (this_delta < 0)
			{
				/*
				 * Clean up in case shared updates have caused cur_count
				 * increments to get "lost".  We back off the page counts,
				 * rather than trying to increase cur_count, to avoid any
				 * question of infinite loops or failure in the presence of
				 * wrapped-around counts.
				 */
				shared->page_lru_count[slotno] = cur_count;
				this_delta = 0;
			}
			this_page_number = shared->page_number[slotno];
			if ((this_delta > best_delta ||
				 (this_delta == best_delta &&
				  ctl->PagePrecedes(this_page_number, best_page_number))) &&
				this_page_number != shared->latest_page_number)
			{
				bestslot = slotno;
				best_delta = this_delta;
				best_page_number = this_page_number;
			}
		}

		/*
		 * If the selected page is clean, we're set.
		 */
		if (shared->page_status[bestslot] == SLRU_PAGE_VALID &&
			!shared->page_dirty[bestslot])
			return bestslot;

		/*
		 * We need to wait for I/O.  Normal case is that it's dirty and we
		 * must initiate a write, but it's possible that the page is already
		 * write-busy, or in the worst case still read-busy.  In those cases
		 * we wait for the existing I/O to complete.
		 */
		if (shared->page_status[bestslot] == SLRU_PAGE_VALID)
			SimpleLruWritePage(ctl, bestslot, NULL);
		else
			SimpleLruWaitIO(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.
		 */
	}
}

/*
 * Flush dirty pages to disk during checkpoint or database shutdown
 */
void
SimpleLruFlush(SlruCtl ctl, bool checkpoint)
{
	SlruShared	shared = ctl->shared;
	SlruFlushData fdata;
	int			slotno;
	int			pageno = 0;
	int			i;
	bool		ok;

	/*
	 * Find and write dirty pages
	 */
	fdata.num_files = 0;

	LWLockAcquire(shared->ControlLock, LW_EXCLUSIVE);

	for (slotno = 0; slotno < shared->num_slots; slotno++)
	{
		SimpleLruWritePage(ctl, slotno, &fdata);

		/*
		 * 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_VALID &&
				!shared->page_dirty[slotno]));
	}

	LWLockRelease(shared->ControlLock);

	/*
	 * Now fsync and close any files that were open
	 */
	ok = true;
	for (i = 0; i < fdata.num_files; i++)
	{
		if (ctl->do_fsync && pg_fsync(fdata.fd[i]))
		{
			slru_errcause = SLRU_FSYNC_FAILED;
			slru_errno = errno;
			pageno = fdata.segno[i] * SLRU_PAGES_PER_SEGMENT;
			ok = false;
		}

		if (close(fdata.fd[i]))
		{
			slru_errcause = SLRU_CLOSE_FAILED;
			slru_errno = errno;
			pageno = fdata.segno[i] * SLRU_PAGES_PER_SEGMENT;
			ok = false;
		}
	}
	if (!ok)
		SlruReportIOError(ctl, pageno, InvalidTransactionId);
}

/*
 * Remove all segments before the one holding the passed page number
 */
void
SimpleLruTruncate(SlruCtl ctl, int cutoffPage)
{
	SlruShared	shared = ctl->shared;
	int			slotno;

	/*
	 * The cutoff point is the start of the segment containing cutoffPage.
	 */
	cutoffPage -= cutoffPage % SLRU_PAGES_PER_SEGMENT;

	/*
	 * Scan shared memory and remove any pages preceding the cutoff page, to
	 * ensure we won't rewrite them later.  (Since this is normally called in
	 * or just after a checkpoint, any dirty pages should have been flushed
	 * already ... we're just being extra careful here.)
	 */
	LWLockAcquire(shared->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(shared->ControlLock);
		ereport(LOG,
		  (errmsg("could not truncate directory \"%s\": apparent wraparound",
				  ctl->Dir)));
		return;
	}

	for (slotno = 0; slotno < shared->num_slots; 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_VALID &&
			!shared->page_dirty[slotno])
		{
			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.	(XXX if page is dirty,
		 * wouldn't it be OK to just discard it without writing it?  For now,
		 * keep the logic the same as it was.)
		 */
		if (shared->page_status[slotno] == SLRU_PAGE_VALID)
			SimpleLruWritePage(ctl, slotno, NULL);
		else
			SimpleLruWaitIO(ctl, slotno);
		goto restart;
	}

	LWLockRelease(shared->ControlLock);

	/* Now we can remove the old segment(s) */
	(void) SlruScanDirectory(ctl, cutoffPage, true);
}

/*
 * SimpleLruTruncate 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.
 *
 * This can be called directly from clog.c, for reasons explained there.
 */
bool
SlruScanDirectory(SlruCtl ctl, int cutoffPage, bool doDeletions)
{
	bool		found = false;
	DIR		   *cldir;
	struct dirent *clde;
	int			segno;
	int			segpage;
	char		path[MAXPGPATH];

	/*
	 * The cutoff point is the start of the segment containing cutoffPage.
	 * (This is redundant when called from SimpleLruTruncate, but not when
	 * called directly from clog.c.)
	 */
	cutoffPage -= cutoffPage % SLRU_PAGES_PER_SEGMENT;

	cldir = AllocateDir(ctl->Dir);
	while ((clde = ReadDir(cldir, ctl->Dir)) != 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)
				{
					snprintf(path, MAXPGPATH, "%s/%s", ctl->Dir, clde->d_name);
					ereport(DEBUG2,
							(errmsg("removing file \"%s\"", path)));
					unlink(path);
				}
			}
		}
	}
	FreeDir(cldir);

	return found;
}