buf0rea.cc

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

Copyright (c) 1995, 2013, Oracle and/or its affiliates. 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.,
51 Franklin Street, Suite 500, Boston, MA 02110-1335 USA

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

/**************************************************//**
@file buf/buf0rea.cc
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 "buf0dblwr.h"
#include "ibuf0ibuf.h"
#include "log0recv.h"
#include "trx0sys.h"
#include "os0file.h"
#include "srv0start.h"
#include "srv0srv.h"
#include "mysql/plugin.h"
#include "mysql/service_thd_wait.h"

/** 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(b)	\
				(5 + BUF_READ_AHEAD_AREA(b) / 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

/********************************************************************//**
Unfixes the pages, unlatches the page,
removes it from page_hash and removes it from LRU. */
static
void
buf_read_page_handle_error(
/*=======================*/
	buf_page_t*	bpage)	/*!< in: pointer to the block */
{
	buf_pool_t*	buf_pool = buf_pool_from_bpage(bpage);
	const bool	uncompressed = (buf_page_get_state(bpage)
					== BUF_BLOCK_FILE_PAGE);

	/* First unfix and release lock on the bpage */
	buf_pool_mutex_enter(buf_pool);
	mutex_enter(buf_page_get_mutex(bpage));
	ut_ad(buf_page_get_io_fix(bpage) == BUF_IO_READ);
	ut_ad(bpage->buf_fix_count == 0);

	/* Set BUF_IO_NONE before we remove the block from LRU list */
	buf_page_set_io_fix(bpage, BUF_IO_NONE);

	if (uncompressed) {
		rw_lock_x_unlock_gen(
			&((buf_block_t*) bpage)->lock,
			BUF_IO_READ);
	}

	mutex_exit(buf_page_get_mutex(bpage));

	/* remove the block from LRU list */
	buf_LRU_free_one_page(bpage);

	ut_ad(buf_pool->n_pend_reads > 0);
	buf_pool->n_pend_reads--;

	buf_pool_mutex_exit(buf_pool);
}

/********************************************************************//**
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
@return 1 if read request is issued. 0 if it is not */
static
ulint
buf_read_page_low(
/*==============*/
	dberr_t*	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 */
	bool	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 */
	trx_t*	trx,
	ibool	should_buffer)	/*!< in: whether to buffer an aio request.
				AIO read ahead uses this. If you plan to
				use this parameter, make sure you remember
				to call os_aio_linux_dispatch_read_array_submit
				when you are read to commit all your requests.*/
{
	buf_page_t*	bpage;
	ulint		wake_later;
	ibool		ignore_nonexistent_pages;

	*err = DB_SUCCESS;

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

	ignore_nonexistent_pages = mode & BUF_READ_IGNORE_NONEXISTENT_PAGES;
	mode &= ~BUF_READ_IGNORE_NONEXISTENT_PAGES;

	if (space == TRX_SYS_SPACE && buf_dblwr_page_inside(offset)) {
		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 %s\n",
			(ulong) offset, sync ? "true" : "false");
	}
#endif

	ut_ad(buf_page_in_file(bpage));

	if (sync) {
		thd_wait_begin(NULL, THD_WAIT_DISKIO);
	}

	if (zip_size) {
		*err = _fil_io(OS_FILE_READ | wake_later
			      | ignore_nonexistent_pages,
			      sync, space, zip_size, offset, 0, zip_size,
			      bpage->zip.data, bpage,
			      trx ? &trx->table_io_perf : NULL,
			      should_buffer);
	} else {
		ut_a(buf_page_get_state(bpage) == BUF_BLOCK_FILE_PAGE);

		*err = _fil_io(OS_FILE_READ | wake_later
			      | ignore_nonexistent_pages,
			      sync, space, 0, offset, 0, UNIV_PAGE_SIZE,
			      ((buf_block_t*) bpage)->frame, bpage,
			      trx ? &trx->table_io_perf : NULL,
			      should_buffer);
	}

	if (sync) {
		thd_wait_end(NULL);
	}

	if (*err != DB_SUCCESS) {
		if (ignore_nonexistent_pages || *err == DB_TABLESPACE_DELETED) {
			buf_read_page_handle_error(bpage);
			return(0);
		}
		/* else */
		ut_error;
	}

	if (sync) {
		/* The i/o is already completed when we arrive from
		fil_read */
		if (!buf_page_io_complete(bpage)) {
			return(0);
		}
	}

	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 positive value!
@return	number of page read requests issued */
UNIV_INTERN
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 */
	ibool	inside_ibuf,	/*!< in: TRUE if we are inside ibuf
				routine */
	trx_t*	trx)
{
	buf_pool_t*	buf_pool = buf_pool_get(space, offset);
	ib_int64_t	tablespace_version;
	ulint		recent_blocks	= 0;
	ulint		ibuf_mode;
	ulint		count;
	ulint		low, high;
	dberr_t		err;
	ulint		i;
	const ulint	buf_read_ahead_random_area
				= BUF_READ_AHEAD_AREA(buf_pool);

	if (!srv_random_read_ahead) {
		/* Disabled by user */
		return(0);
	}

	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);

	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);
	}

	buf_pool_mutex_enter(buf_pool);

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

		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(buf_pool, space, i);

		if (bpage
		    && buf_page_is_accessed(bpage)
		    && buf_page_peek_if_young(bpage)) {

			recent_blocks++;

			if (recent_blocks
			    >= BUF_READ_AHEAD_RANDOM_THRESHOLD(buf_pool)) {

				buf_pool_mutex_exit(buf_pool);
				goto read_ahead;
			}
		}
	}

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

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

	if (inside_ibuf) {
		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, trx, FALSE);
			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 */

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

	buf_pool->stat.n_ra_pages_read_rnd += count;
	srv_stats.buf_pool_reads.add(count);
	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.
@return TRUE if page has been read in, FALSE in case of failure */
UNIV_INTERN
ibool
buf_read_page(
/*==========*/
	ulint	space,	/*!< in: space id */
	ulint	zip_size,/*!< in: compressed page size in bytes, or 0 */
	ulint	offset,	/*!< in: page number */
	trx_t*	trx)
{
	ib_int64_t	tablespace_version;
	ulint		count;
	dberr_t		err;

	tablespace_version = fil_space_get_version(space);

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

	count = buf_read_page_low(&err, true, BUF_READ_ANY_PAGE, space,
				  zip_size, FALSE,
				  tablespace_version, offset, trx, FALSE);
	srv_stats.buf_pool_reads.add(count);
	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);
	}

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

	return(count > 0);
}

/********************************************************************//**
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.
@return TRUE if page has been read in, FALSE in case of failure */
UNIV_INTERN
ibool
buf_read_page_async(
/*================*/
	ulint	space,	/*!< in: space id */
	ulint	offset)	/*!< in: page number */
{
	ulint		zip_size;
	ib_int64_t	tablespace_version;
	ulint		count;
	dberr_t		err;

	zip_size = fil_space_get_zip_size(space);

	if (zip_size == ULINT_UNDEFINED) {
		return(FALSE);
	}

	tablespace_version = fil_space_get_version(space);

	count = buf_read_page_low(&err, true, BUF_READ_ANY_PAGE
				  | OS_AIO_SIMULATED_WAKE_LATER
				  | BUF_READ_IGNORE_NONEXISTENT_PAGES,
				  space, zip_size, FALSE,
				  tablespace_version, offset, NULL, FALSE);
	srv_stats.buf_pool_reads.add(count);

	/* We do not increment number of I/O operations used for LRU policy
	here (buf_LRU_stat_inc_io()). We use this in heuristics to decide
	about evicting uncompressed version of compressed pages from the
	buffer pool. Since this function is called from buffer pool load
	these IOs are deliberate and are not part of normal workload we can
	ignore these in our heuristics. */

	return(count > 0);
}

/********************************************************************//**
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 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; see NOTE 3 above */
	ibool	inside_ibuf,	/*!< in: TRUE if we are inside ibuf routine */
	trx_t*	trx)
{
	buf_pool_t*	buf_pool = buf_pool_get(space, offset);
	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;
	dberr_t		err;
	ulint		i;
	const ulint	buf_read_ahead_linear_area
		= BUF_READ_AHEAD_AREA(buf_pool);
	ulint		threshold;

	/* check if readahead is disabled */
	if (!srv_read_ahead_threshold) {
		return(0);
	}

	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(buf_pool);

	if (high > fil_space_get_size(space)) {
		buf_pool_mutex_exit(buf_pool);
		/* 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(buf_pool);

		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;
	}

	/* How many out of order accessed pages can we ignore
	when working out the access pattern for linear readahead */
	threshold = ut_min((64 - srv_read_ahead_threshold),
			   BUF_READ_AHEAD_AREA(buf_pool));

	fail_count = 0;

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

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

		} else if (pred_bpage) {
			/* Note that buf_page_is_accessed() returns
			the time of the first access.  If some blocks
			of the extent existed in the buffer pool at
			the time of a linear access pattern, the first
			access times may be nonmonotonic, even though
			the latest access times were linear.  The
			threshold (srv_read_ahead_factor) should help
			a little against this. */
			int res = ut_ulint_cmp(
				buf_page_is_accessed(bpage),
				buf_page_is_accessed(pred_bpage));
			/* Accesses not in the right order */
			if (res != 0 && res != asc_or_desc) {
				fail_count++;
			}
		}

		if (fail_count > threshold) {
			/* Too many failures: return */
			buf_pool_mutex_exit(buf_pool);
			return(0);
		}

		if (bpage && buf_page_is_accessed(bpage)) {
			pred_bpage = bpage;
		}
	}

	/* 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(buf_pool, space, offset);

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

		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(buf_pool);

	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 */

	ibuf_mode = inside_ibuf
		? BUF_READ_IBUF_PAGES_ONLY | OS_AIO_SIMULATED_WAKE_LATER
		: BUF_READ_ANY_PAGE | OS_AIO_SIMULATED_WAKE_LATER;

	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,
				space, zip_size, FALSE, tablespace_version, i,
				trx, TRUE);
			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);
			}
		}
	}
#if defined(LINUX_NATIVE_AIO)
	/* Tell aio to submit all buffered requests. */
	ut_a(os_aio_linux_dispatch_read_array_submit());
#endif

	/* 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,
			"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();

	buf_pool->stat.n_ra_pages_read += count;
	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(
/*======================*/
	bool		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;

#ifdef UNIV_IBUF_DEBUG
	ut_a(n_stored < UNIV_PAGE_SIZE);
#endif

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

		buf_pool = buf_pool_get(space_ids[i], page_nos[i]);

		while (buf_pool->n_pend_reads
		       > buf_pool->curr_size / BUF_READ_AHEAD_PEND_LIMIT) {
			os_thread_sleep(500000);
		}

		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], NULL, FALSE);

		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();

#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;
	dberr_t		err;
	ulint		i;

	zip_size = fil_space_get_zip_size(space);

	if (UNIV_UNLIKELY(zip_size == ULINT_UNDEFINED)) {
		/* It is a single table tablespace and the .ibd file is
		missing: do nothing */

		return;
	}

	tablespace_version = fil_space_get_version(space);

	for (i = 0; i < n_stored; i++) {
		buf_pool_t*	buf_pool;

		count = 0;

		os_aio_print_debug = FALSE;
		buf_pool = buf_pool_get(space, page_nos[i]);
		while (buf_pool->n_pend_reads >= recv_n_pool_free_frames / 2) {

			os_aio_simulated_wake_handler_threads();
			os_thread_sleep(10000);

			count++;

			if (count > 1000) {
				fprintf(stderr,
					"InnoDB: Error: InnoDB has waited for"
					" 10 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], NULL, FALSE);
		} 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],
					  NULL, FALSE);
		}
	}

	os_aio_simulated_wake_handler_threads();

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