buf0rea.c

/*****************************************************************************

Copyright (c) 1995, 2009, Innobase Oy. All Rights Reserved.

This program is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free Software
Foundation; version 2 of the License.

This program is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with
this program; if not, write to the Free Software Foundation, Inc., 59 Temple
Place, Suite 330, Boston, MA 02111-1307 USA

*****************************************************************************/

/******************************************************
The database buffer read

Created 11/5/1995 Heikki Tuuri
*******************************************************/

#include "buf0rea.h"

#include "fil0fil.h"
#include "mtr0mtr.h"

#include "buf0buf.h"
#include "buf0flu.h"
#include "buf0lru.h"
#include "ibuf0ibuf.h"
#include "log0recv.h"
#include "trx0sys.h"
#include "os0file.h"
#include "srv0start.h"

extern ulint srv_read_ahead_rnd;
extern ulint srv_read_ahead_seq;
extern ulint srv_buf_pool_reads;

/* The size in blocks of the area where the random read-ahead algorithm counts
the accessed pages when deciding whether to read-ahead */
#define	BUF_READ_AHEAD_RANDOM_AREA	BUF_READ_AHEAD_AREA

/* There must be at least this many pages in buf_pool in the area to start
a random read-ahead */
#define BUF_READ_AHEAD_RANDOM_THRESHOLD	(5 + buf_read_ahead_random_area / 8)

/* The linear read-ahead area size */
#define	BUF_READ_AHEAD_LINEAR_AREA	BUF_READ_AHEAD_AREA

/* The linear read-ahead threshold */
#define LINEAR_AREA_THRESHOLD_COEF	5 / 8

/* If there are buf_pool->curr_size per the number below pending reads, then
read-ahead is not done: this is to prevent flooding the buffer pool with
i/o-fixed buffer blocks */
#define BUF_READ_AHEAD_PEND_LIMIT	2

/************************************************************************
Low-level function which reads a page asynchronously from a file to the
buffer buf_pool if it is not already there, in which case does nothing.
Sets the io_fix flag and sets an exclusive lock on the buffer frame. The
flag is cleared and the x-lock released by an i/o-handler thread.
@return	1 if a read request was queued, 0 if the page already resided in buf_pool, or if the page is in the doublewrite buffer blocks in which case it is never read into the pool, or if the tablespace does not exist or is being dropped */
static
ulint
buf_read_page_low(
/*==============*/
	ulint*	err,	/*!< out: DB_SUCCESS or DB_TABLESPACE_DELETED if we are
			trying to read from a non-existent tablespace, or a
			tablespace which is just now being dropped */
	ibool	sync,	/*!< in: TRUE if synchronous aio is desired */
	ulint	mode,	/*!< in: BUF_READ_IBUF_PAGES_ONLY, ...,
			ORed to OS_AIO_SIMULATED_WAKE_LATER (see below
			at read-ahead functions) */
	ulint	space,	/*!< in: space id */
	ulint	zip_size,/*!< in: compressed page size, or 0 */
	ibool	unzip,	/*!< in: TRUE=request uncompressed page */
	ib_int64_t tablespace_version, /*!< in: if the space memory object has
			this timestamp different from what we are giving here,
			treat the tablespace as dropped; this is a timestamp we
			use to stop dangling page reads from a tablespace
			which we have DISCARDed + IMPORTed back */
	ulint	offset)	/*!< in: page number */
{
	buf_page_t*	bpage;
	ulint		wake_later;

	*err = DB_SUCCESS;

	wake_later = mode & OS_AIO_SIMULATED_WAKE_LATER;
	mode = mode & ~OS_AIO_SIMULATED_WAKE_LATER;

	if (trx_doublewrite && space == TRX_SYS_SPACE
	    && (   (offset >= trx_doublewrite->block1
		    && offset < trx_doublewrite->block1
		    + TRX_SYS_DOUBLEWRITE_BLOCK_SIZE)
		   || (offset >= trx_doublewrite->block2
		       && offset < trx_doublewrite->block2
		       + TRX_SYS_DOUBLEWRITE_BLOCK_SIZE))) {
		ut_print_timestamp(stderr);
		fprintf(stderr,
			"  InnoDB: Warning: trying to read"
			" doublewrite buffer page %lu\n",
			(ulong) offset);

		return(0);
	}

	if (ibuf_bitmap_page(zip_size, offset)
	    || trx_sys_hdr_page(space, offset)) {

		/* Trx sys header is so low in the latching order that we play
		safe and do not leave the i/o-completion to an asynchronous
		i/o-thread. Ibuf bitmap pages must always be read with
		syncronous i/o, to make sure they do not get involved in
		thread deadlocks. */

		sync = TRUE;
	}

	/* The following call will also check if the tablespace does not exist
	or is being dropped; if we succeed in initing the page in the buffer
	pool for read, then DISCARD cannot proceed until the read has
	completed */
	bpage = buf_page_init_for_read(err, mode, space, zip_size, unzip,
				       tablespace_version, offset);
	if (bpage == NULL) {

		return(0);
	}

#ifdef UNIV_DEBUG
	if (buf_debug_prints) {
		fprintf(stderr,
			"Posting read request for page %lu, sync %lu\n",
			(ulong) offset,
			(ulong) sync);
	}
#endif

	ut_ad(buf_page_in_file(bpage));

	if (zip_size) {
		*err = fil_io(OS_FILE_READ | wake_later,
			      sync, space, zip_size, offset, 0, zip_size,
			      bpage->zip.data, bpage);
	} else {
		ut_a(buf_page_get_state(bpage) == BUF_BLOCK_FILE_PAGE);

		*err = fil_io(OS_FILE_READ | wake_later,
			      sync, space, 0, offset, 0, UNIV_PAGE_SIZE,
			      ((buf_block_t*) bpage)->frame, bpage);
	}
	ut_a(*err == DB_SUCCESS);

	if (sync) {
		/* The i/o is already completed when we arrive from
		fil_read */
		buf_page_io_complete(bpage);
	}

	return(1);
}

/************************************************************************
Applies a random read-ahead in buf_pool if there are at least a threshold
value of accessed pages from the random read-ahead area. Does not read any
page, not even the one at the position (space, offset), if the read-ahead
mechanism is not activated. NOTE 1: the calling thread may own latches on
pages: to avoid deadlocks this function must be written such that it cannot
end up waiting for these latches! NOTE 2: the calling thread must want
access to the page given: this rule is set to prevent unintended read-aheads
performed by ibuf routines, a situation which could result in a deadlock if
the OS does not support asynchronous i/o.
@return	number of page read requests issued; NOTE that if we read ibuf pages, it may happen that the page at the given page number does not get read even if we return a value > 0! */
static
ulint
buf_read_ahead_random(
/*==================*/
	ulint	space,	/*!< in: space id */
	ulint	zip_size,/*!< in: compressed page size in bytes, or 0 */
	ulint	offset)	/*!< in: page number of a page which the current thread
			wants to access */
{
	ib_int64_t	tablespace_version;
	ulint		recent_blocks	= 0;
	ulint		count;
	ulint		LRU_recent_limit;
	ulint		ibuf_mode;
	ulint		low, high;
	ulint		err;
	ulint		i;
	ulint		buf_read_ahead_random_area;

	if (srv_startup_is_before_trx_rollback_phase) {
		/* No read-ahead to avoid thread deadlocks */
		return(0);
	}

	if (ibuf_bitmap_page(zip_size, offset)
	    || trx_sys_hdr_page(space, offset)) {

		/* If it is an ibuf bitmap page or trx sys hdr, we do
		no read-ahead, as that could break the ibuf page access
		order */

		return(0);
	}

	/* Remember the tablespace version before we ask te tablespace size
	below: if DISCARD + IMPORT changes the actual .ibd file meanwhile, we
	do not try to read outside the bounds of the tablespace! */

	tablespace_version = fil_space_get_version(space);

	buf_read_ahead_random_area = BUF_READ_AHEAD_RANDOM_AREA;

	low  = (offset / buf_read_ahead_random_area)
		* buf_read_ahead_random_area;
	high = (offset / buf_read_ahead_random_area + 1)
		* buf_read_ahead_random_area;
	if (high > fil_space_get_size(space)) {

		high = fil_space_get_size(space);
	}

	/* Get the minimum LRU_position field value for an initial segment
	of the LRU list, to determine which blocks have recently been added
	to the start of the list. */

	LRU_recent_limit = buf_LRU_get_recent_limit();

	buf_pool_mutex_enter();

	if (buf_pool->n_pend_reads
	    > buf_pool->curr_size / BUF_READ_AHEAD_PEND_LIMIT) {
		buf_pool_mutex_exit();

		return(0);
	}

	/* Count how many blocks in the area have been recently accessed,
	that is, reside near the start of the LRU list. */

	for (i = low; i < high; i++) {
		const buf_page_t*	bpage = buf_page_hash_get(space, i);

		if (bpage
		    && buf_page_is_accessed(bpage)
		    && (buf_page_get_LRU_position(bpage) > LRU_recent_limit)) {

			recent_blocks++;

			if (recent_blocks >= BUF_READ_AHEAD_RANDOM_THRESHOLD) {

				buf_pool_mutex_exit();
				goto read_ahead;
			}
		}
	}

	buf_pool_mutex_exit();
	/* Do nothing */
	return(0);

read_ahead:
	/* Read all the suitable blocks within the area */

	if (ibuf_inside()) {
		ibuf_mode = BUF_READ_IBUF_PAGES_ONLY;
	} else {
		ibuf_mode = BUF_READ_ANY_PAGE;
	}

	count = 0;

	for (i = low; i < high; i++) {
		/* It is only sensible to do read-ahead in the non-sync aio
		mode: hence FALSE as the first parameter */

		if (!ibuf_bitmap_page(zip_size, i)) {
			count += buf_read_page_low(
				&err, FALSE,
				ibuf_mode | OS_AIO_SIMULATED_WAKE_LATER,
				space, zip_size, FALSE,
				tablespace_version, i);
			if (err == DB_TABLESPACE_DELETED) {
				ut_print_timestamp(stderr);
				fprintf(stderr,
					"  InnoDB: Warning: in random"
					" readahead trying to access\n"
					"InnoDB: tablespace %lu page %lu,\n"
					"InnoDB: but the tablespace does not"
					" exist or is just being dropped.\n",
					(ulong) space, (ulong) i);
			}
		}
	}

	/* In simulated aio we wake the aio handler threads only after
	queuing all aio requests, in native aio the following call does
	nothing: */

	os_aio_simulated_wake_handler_threads();

#ifdef UNIV_DEBUG
	if (buf_debug_prints && (count > 0)) {
		fprintf(stderr,
			"Random read-ahead space %lu offset %lu pages %lu\n",
			(ulong) space, (ulong) offset,
			(ulong) count);
	}
#endif /* UNIV_DEBUG */

	++srv_read_ahead_rnd;
	return(count);
}

/************************************************************************
High-level function which reads a page asynchronously from a file to the
buffer buf_pool if it is not already there. Sets the io_fix flag and sets
an exclusive lock on the buffer frame. The flag is cleared and the x-lock
released by the i/o-handler thread. Does a random read-ahead if it seems
sensible.
@return	number of page read requests issued: this can be > 1 if read-ahead occurred */
UNIV_INTERN
ulint
buf_read_page(
/*==========*/
	ulint	space,	/*!< in: space id */
	ulint	zip_size,/*!< in: compressed page size in bytes, or 0 */
	ulint	offset)	/*!< in: page number */
{
	ib_int64_t	tablespace_version;
	ulint		count;
	ulint		count2;
	ulint		err;

	tablespace_version = fil_space_get_version(space);

	count = buf_read_ahead_random(space, zip_size, offset);

	/* We do the i/o in the synchronous aio mode to save thread
	switches: hence TRUE */

	count2 = buf_read_page_low(&err, TRUE, BUF_READ_ANY_PAGE, space,
				   zip_size, FALSE,
				   tablespace_version, offset);
	srv_buf_pool_reads+= count2;
	if (err == DB_TABLESPACE_DELETED) {
		ut_print_timestamp(stderr);
		fprintf(stderr,
			"  InnoDB: Error: trying to access"
			" tablespace %lu page no. %lu,\n"
			"InnoDB: but the tablespace does not exist"
			" or is just being dropped.\n",
			(ulong) space, (ulong) offset);
	}

	/* Flush pages from the end of the LRU list if necessary */
	buf_flush_free_margin();

	/* Increment number of I/O operations used for LRU policy. */
	buf_LRU_stat_inc_io();

	return(count + count2);
}

/************************************************************************
Applies linear read-ahead if in the buf_pool the page is a border page of
a linear read-ahead area and all the pages in the area have been accessed.
Does not read any page if the read-ahead mechanism is not activated. Note
that the the algorithm looks at the 'natural' adjacent successor and
predecessor of the page, which on the leaf level of a B-tree are the next
and previous page in the chain of leaves. To know these, the page specified
in (space, offset) must already be present in the buf_pool. Thus, the
natural way to use this function is to call it when a page in the buf_pool
is accessed the first time, calling this function just after it has been
bufferfixed.
NOTE 1: as this function looks at the natural predecessor and successor
fields on the page, what happens, if these are not initialized to any
sensible value? No problem, before applying read-ahead we check that the
area to read is within the span of the space, if not, read-ahead is not
applied. An uninitialized value may result in a useless read operation, but
only very improbably.
NOTE 2: the calling thread may own latches on pages: to avoid deadlocks this
function must be written such that it cannot end up waiting for these
latches!
NOTE 3: the calling thread must want access to the page given: this rule is
set to prevent unintended read-aheads performed by ibuf routines, a situation
which could result in a deadlock if the OS does not support asynchronous io.
@return	number of page read requests issued */
UNIV_INTERN
ulint
buf_read_ahead_linear(
/*==================*/
	ulint	space,	/*!< in: space id */
	ulint	zip_size,/*!< in: compressed page size in bytes, or 0 */
	ulint	offset)	/*!< in: page number of a page; NOTE: the current thread
			must want access to this page (see NOTE 3 above) */
{
	ib_int64_t	tablespace_version;
	buf_page_t*	bpage;
	buf_frame_t*	frame;
	buf_page_t*	pred_bpage	= NULL;
	ulint		pred_offset;
	ulint		succ_offset;
	ulint		count;
	int		asc_or_desc;
	ulint		new_offset;
	ulint		fail_count;
	ulint		ibuf_mode;
	ulint		low, high;
	ulint		err;
	ulint		i;
	const ulint	buf_read_ahead_linear_area
		= BUF_READ_AHEAD_LINEAR_AREA;

	if (UNIV_UNLIKELY(srv_startup_is_before_trx_rollback_phase)) {
		/* No read-ahead to avoid thread deadlocks */
		return(0);
	}

	low  = (offset / buf_read_ahead_linear_area)
		* buf_read_ahead_linear_area;
	high = (offset / buf_read_ahead_linear_area + 1)
		* buf_read_ahead_linear_area;

	if ((offset != low) && (offset != high - 1)) {
		/* This is not a border page of the area: return */

		return(0);
	}

	if (ibuf_bitmap_page(zip_size, offset)
	    || trx_sys_hdr_page(space, offset)) {

		/* If it is an ibuf bitmap page or trx sys hdr, we do
		no read-ahead, as that could break the ibuf page access
		order */

		return(0);
	}

	/* Remember the tablespace version before we ask te tablespace size
	below: if DISCARD + IMPORT changes the actual .ibd file meanwhile, we
	do not try to read outside the bounds of the tablespace! */

	tablespace_version = fil_space_get_version(space);

	buf_pool_mutex_enter();

	if (high > fil_space_get_size(space)) {
		buf_pool_mutex_exit();
		/* The area is not whole, return */

		return(0);
	}

	if (buf_pool->n_pend_reads
	    > buf_pool->curr_size / BUF_READ_AHEAD_PEND_LIMIT) {
		buf_pool_mutex_exit();

		return(0);
	}

	/* Check that almost all pages in the area have been accessed; if
	offset == low, the accesses must be in a descending order, otherwise,
	in an ascending order. */

	asc_or_desc = 1;

	if (offset == low) {
		asc_or_desc = -1;
	}

	fail_count = 0;

	for (i = low; i < high; i++) {
		bpage = buf_page_hash_get(space, i);

		if ((bpage == NULL) || !buf_page_is_accessed(bpage)) {
			/* Not accessed */
			fail_count++;

		} else if (pred_bpage
			   && (ut_ulint_cmp(
				       buf_page_get_LRU_position(bpage),
				       buf_page_get_LRU_position(pred_bpage))
			       != asc_or_desc)) {
			/* Accesses not in the right order */

			fail_count++;
			pred_bpage = bpage;
		}
	}

	if (fail_count > buf_read_ahead_linear_area
	    * LINEAR_AREA_THRESHOLD_COEF) {
		/* Too many failures: return */

		buf_pool_mutex_exit();

		return(0);
	}

	/* If we got this far, we know that enough pages in the area have
	been accessed in the right order: linear read-ahead can be sensible */

	bpage = buf_page_hash_get(space, offset);

	if (bpage == NULL) {
		buf_pool_mutex_exit();

		return(0);
	}

	switch (buf_page_get_state(bpage)) {
	case BUF_BLOCK_ZIP_PAGE:
		frame = bpage->zip.data;
		break;
	case BUF_BLOCK_FILE_PAGE:
		frame = ((buf_block_t*) bpage)->frame;
		break;
	default:
		ut_error;
		break;
	}

	/* Read the natural predecessor and successor page addresses from
	the page; NOTE that because the calling thread may have an x-latch
	on the page, we do not acquire an s-latch on the page, this is to
	prevent deadlocks. Even if we read values which are nonsense, the
	algorithm will work. */

	pred_offset = fil_page_get_prev(frame);
	succ_offset = fil_page_get_next(frame);

	buf_pool_mutex_exit();

	if ((offset == low) && (succ_offset == offset + 1)) {

		/* This is ok, we can continue */
		new_offset = pred_offset;

	} else if ((offset == high - 1) && (pred_offset == offset - 1)) {

		/* This is ok, we can continue */
		new_offset = succ_offset;
	} else {
		/* Successor or predecessor not in the right order */

		return(0);
	}

	low  = (new_offset / buf_read_ahead_linear_area)
		* buf_read_ahead_linear_area;
	high = (new_offset / buf_read_ahead_linear_area + 1)
		* buf_read_ahead_linear_area;

	if ((new_offset != low) && (new_offset != high - 1)) {
		/* This is not a border page of the area: return */

		return(0);
	}

	if (high > fil_space_get_size(space)) {
		/* The area is not whole, return */

		return(0);
	}

	/* If we got this far, read-ahead can be sensible: do it */

	if (ibuf_inside()) {
		ibuf_mode = BUF_READ_IBUF_PAGES_ONLY;
	} else {
		ibuf_mode = BUF_READ_ANY_PAGE;
	}

	count = 0;

	/* Since Windows XP seems to schedule the i/o handler thread
	very eagerly, and consequently it does not wait for the
	full read batch to be posted, we use special heuristics here */

	os_aio_simulated_put_read_threads_to_sleep();

	for (i = low; i < high; i++) {
		/* It is only sensible to do read-ahead in the non-sync
		aio mode: hence FALSE as the first parameter */

		if (!ibuf_bitmap_page(zip_size, i)) {
			count += buf_read_page_low(
				&err, FALSE,
				ibuf_mode | OS_AIO_SIMULATED_WAKE_LATER,
				space, zip_size, FALSE, tablespace_version, i);
			if (err == DB_TABLESPACE_DELETED) {
				ut_print_timestamp(stderr);
				fprintf(stderr,
					"  InnoDB: Warning: in"
					" linear readahead trying to access\n"
					"InnoDB: tablespace %lu page %lu,\n"
					"InnoDB: but the tablespace does not"
					" exist or is just being dropped.\n",
					(ulong) space, (ulong) i);
			}
		}
	}

	/* In simulated aio we wake the aio handler threads only after
	queuing all aio requests, in native aio the following call does
	nothing: */

	os_aio_simulated_wake_handler_threads();

	/* Flush pages from the end of the LRU list if necessary */
	buf_flush_free_margin();

#ifdef UNIV_DEBUG
	if (buf_debug_prints && (count > 0)) {
		fprintf(stderr,
			"LINEAR read-ahead space %lu offset %lu pages %lu\n",
			(ulong) space, (ulong) offset, (ulong) count);
	}
#endif /* UNIV_DEBUG */

	/* Read ahead is considered one I/O operation for the purpose of
	LRU policy decision. */
	buf_LRU_stat_inc_io();

	++srv_read_ahead_seq;
	return(count);
}

/************************************************************************
Issues read requests for pages which the ibuf module wants to read in, in
order to contract the insert buffer tree. Technically, this function is like
a read-ahead function. */
UNIV_INTERN
void
buf_read_ibuf_merge_pages(
/*======================*/
	ibool		sync,		/*!< in: TRUE if the caller
					wants this function to wait
					for the highest address page
					to get read in, before this
					function returns */
	const ulint*	space_ids,	/*!< in: array of space ids */
	const ib_int64_t* space_versions,/*!< in: the spaces must have
					this version number
					(timestamp), otherwise we
					discard the read; we use this
					to cancel reads if DISCARD +
					IMPORT may have changed the
					tablespace size */
	const ulint*	page_nos,	/*!< in: array of page numbers
					to read, with the highest page
					number the last in the
					array */
	ulint		n_stored)	/*!< in: number of elements
					in the arrays */
{
	ulint	i;

	ut_ad(!ibuf_inside());
#ifdef UNIV_IBUF_DEBUG
	ut_a(n_stored < UNIV_PAGE_SIZE);
#endif
	while (buf_pool->n_pend_reads
	       > buf_pool->curr_size / BUF_READ_AHEAD_PEND_LIMIT) {
		os_thread_sleep(500000);
	}

	for (i = 0; i < n_stored; i++) {
		ulint	zip_size = fil_space_get_zip_size(space_ids[i]);
		ulint	err;

		if (UNIV_UNLIKELY(zip_size == ULINT_UNDEFINED)) {

			goto tablespace_deleted;
		}

		buf_read_page_low(&err, sync && (i + 1 == n_stored),
				  BUF_READ_ANY_PAGE, space_ids[i],
				  zip_size, TRUE, space_versions[i],
				  page_nos[i]);

		if (UNIV_UNLIKELY(err == DB_TABLESPACE_DELETED)) {
tablespace_deleted:
			/* We have deleted or are deleting the single-table
			tablespace: remove the entries for that page */

			ibuf_merge_or_delete_for_page(NULL, space_ids[i],
						      page_nos[i],
						      zip_size, FALSE);
		}
	}

	os_aio_simulated_wake_handler_threads();

	/* Flush pages from the end of the LRU list if necessary */
	buf_flush_free_margin();

#ifdef UNIV_DEBUG
	if (buf_debug_prints) {
		fprintf(stderr,
			"Ibuf merge read-ahead space %lu pages %lu\n",
			(ulong) space_ids[0], (ulong) n_stored);
	}
#endif /* UNIV_DEBUG */
}

/************************************************************************
Issues read requests for pages which recovery wants to read in. */
UNIV_INTERN
void
buf_read_recv_pages(
/*================*/
	ibool		sync,		/*!< in: TRUE if the caller
					wants this function to wait
					for the highest address page
					to get read in, before this
					function returns */
	ulint		space,		/*!< in: space id */
	ulint		zip_size,	/*!< in: compressed page size in
					bytes, or 0 */
	const ulint*	page_nos,	/*!< in: array of page numbers
					to read, with the highest page
					number the last in the
					array */
	ulint		n_stored)	/*!< in: number of page numbers
					in the array */
{
	ib_int64_t	tablespace_version;
	ulint		count;
	ulint		err;
	ulint		i;

	zip_size = fil_space_get_zip_size(space);
	tablespace_version = fil_space_get_version(space);

	for (i = 0; i < n_stored; i++) {

		count = 0;

		os_aio_print_debug = FALSE;

		while (buf_pool->n_pend_reads >= recv_n_pool_free_frames / 2) {

			os_aio_simulated_wake_handler_threads();
			os_thread_sleep(500000);

			count++;

			if (count > 100) {
				fprintf(stderr,
					"InnoDB: Error: InnoDB has waited for"
					" 50 seconds for pending\n"
					"InnoDB: reads to the buffer pool to"
					" be finished.\n"
					"InnoDB: Number of pending reads %lu,"
					" pending pread calls %lu\n",
					(ulong) buf_pool->n_pend_reads,
					(ulong)os_file_n_pending_preads);

				os_aio_print_debug = TRUE;
			}
		}

		os_aio_print_debug = FALSE;

		if ((i + 1 == n_stored) && sync) {
			buf_read_page_low(&err, TRUE, BUF_READ_ANY_PAGE, space,
					  zip_size, TRUE, tablespace_version,
					  page_nos[i]);
		} else {
			buf_read_page_low(&err, FALSE, BUF_READ_ANY_PAGE
					  | OS_AIO_SIMULATED_WAKE_LATER,
					  space, zip_size, TRUE,
					  tablespace_version, page_nos[i]);
		}
	}

	os_aio_simulated_wake_handler_threads();

	/* Flush pages from the end of the LRU list if necessary */
	buf_flush_free_margin();

#ifdef UNIV_DEBUG
	if (buf_debug_prints) {
		fprintf(stderr,
			"Recovery applies read-ahead pages %lu\n",
			(ulong) n_stored);
	}
#endif /* UNIV_DEBUG */
}