The GCC __atomic_ functions were removed already in
commit 2b47f8ff03,
and starting with MariaDB Server 10.4, InnoDB relies mostly
on C++11 std::atomic.
Maybe this patch will help catch problems like buffer overflow.
log_t::first_in_use: removed
log_t::buf: this is where mtr_t are supposed to append data
log_t::flush_buf: this is from server writes to a file
Those two buffers are std::swap()ped when some thread is gonna write
to a file
Replace all fsync() with fdatasync() when possible (e.g. On Linux)
InnoDB doesn't care about file timestamps. So, to achieve a better
performance it makes sense to use fdatasync() everywhere.
file_io::flush(): renamed from flush_data_only()
os_file_flush_data(): removed
os_file_sync_posix(): renamed from os_file_fsync_posix(). Now it uses
fdatasync() when it's available.
In main.index_merge_myisam we remove the test that was added in
commit a2d24def8c because
it duplicates the test case that was added in
commit 5af12e4635.
was restored.
Optionally rollback prepared XA's on "mariabackup --prepare".
The fix MUST NOT be ported on 10.5+, as MDEV-742 fix solves the issue for
slaves.
There is no background change buffer merge any more.
Change buffer merge will only take place during a slow shutdown
(a shutdown initiated after SET GLOBAL innodb_fast_shutdown=0).
recv_sys.recovery_on: Replaces recv_recovery_on.
recv_sys_t::apply(): Replaces recv_apply_hashed_log_recs().
recv_sys_var_init(): Remove.
recv_sys_t::recover_low(): Attempt to initialize a page based
on buffered redo log records.
Thanks to MDEV-15058, there is only one InnoDB buffer pool.
Allocating buf_pool statically removes one level of pointer indirection
and makes code more readable, and removes the awkward initialization of
some buf_pool members.
While doing this, we will also declare some buf_pool_t data members
private and replace some functions with member functions. This is
mostly affecting buffer pool resizing.
This is not aiming to be a complete rewrite of buf_pool_t to
a proper class. Most of the buffer pool interface, such as
buf_page_get_gen(), will remain in the C programming style
for now.
buf_pool_t::withdrawing: Replaces buf_pool_withdrawing.
buf_pool_t::withdraw_clock_: Replaces buf_withdraw_clock.
buf_pool_t::create(): Repalces buf_pool_init().
buf_pool_t::close(): Replaces buf_pool_free().
buf_bool_t::will_be_withdrawn(): Replaces buf_block_will_be_withdrawn(),
buf_frame_will_be_withdrawn().
buf_pool_t::clear_hash_index(): Replaces buf_pool_clear_hash_index().
buf_pool_t::get_n_pages(): Replaces buf_pool_get_n_pages().
buf_pool_t::validate(): Replaces buf_validate().
buf_pool_t::print(): Replaces buf_print().
buf_pool_t::block_from_ahi(): Replaces buf_block_from_ahi().
buf_pool_t::is_block_field(): Replaces buf_pointer_is_block_field().
buf_pool_t::is_block_mutex(): Replaces buf_pool_is_block_mutex().
buf_pool_t::is_block_lock(): Replaces buf_pool_is_block_lock().
buf_pool_t::is_obsolete(): Replaces buf_pool_is_obsolete().
buf_pool_t::io_buf: Make default-constructible.
buf_pool_t::io_buf::create(): Delayed 'constructor'
buf_pool_t::io_buf::close(): Early 'destructor'
HazardPointer: Make default-constructible. Define all member functions
inline, also for derived classes.
Write log header just ones when file is created, instead of
writing to it on every log file wrap around.
log_t::file::write_header_durable(): this one writes to log header
log_write_buf(): this one stops writing to log header
The -Wconversion in GCC seems to be stricter than in clang.
GCC at least since version 4.4.7 issues truncation warnings for
assignments to bitfields, while clang 10 appears to only issue
warnings when the sizes in bytes rounded to the nearest integer
powers of 2 are different.
Before GCC 10.0.0, -Wconversion required more casts and would not
allow some operations, such as x<<=1 or x+=1 on a data type that
is narrower than int.
GCC 5 (but not GCC 4, GCC 6, or any later version) is complaining
about x|=y even when x and y are compatible types that are narrower
than int. Hence, we must rewrite some x|=y as
x=static_cast<byte>(x|y) or similar, or we must disable -Wconversion.
In GCC 6 and later, the warning for assigning wider to bitfields
that are narrower than 8, 16, or 32 bits can be suppressed by
applying a bitwise & with the exact bitmask of the bitfield.
For older GCC, we must disable -Wconversion for GCC 4 or 5 in such
cases.
The bitwise negation operator appears to promote short integers
to a wider type, and hence we must add explicit truncation casts
around them. Microsoft Visual C does not allow a static_cast to
truncate a constant, such as static_cast<byte>(1) truncating int.
Hence, we will use the constructor-style cast byte(~1) for such cases.
This has been tested at least with GCC 4.8.5, 5.4.0, 7.4.0, 9.2.1, 10.0.0,
clang 9.0.1, 10.0.0, and MSVC 14.22.27905 (Microsoft Visual Studio 2019)
on 64-bit and 32-bit targets (IA-32, AMD64, POWER 8, POWER 9, ARMv8).
The following parameters are deprecated:
innodb-background-scrub-data-uncompressed
innodb-background-scrub-data-compressed
innodb-background-scrub-data-interval
innodb-background-scrub-data-check-interval
Removed scrubbing code completely(btr0scrub.h, btr0scrub.cc)
Removed information_schema.innodb_tablespaces_scrubbing tables
Removed the scrubbing logic from fil_crypt_thread()
create_log_file(): Delete all old redo log files where they used to be
deleted, after the crash injection point innodb_log_abort_6,
before commit 9ef2d29ff4
deprecated and ignored the setting innodb_log_files_in_group.
Some fields were protected by log_sys.mutex, which adds quite some
overhead for readers. Some readers were submitting dirty reads.
log_t::lsn: Declare private and atomic. Add wrappers get_lsn()
and set_lsn() that will use relaxed memory access. Many accesses
to log_sys.lsn are still protected by log_sys.mutex; we avoid the
mutex for some readers.
log_t::flushed_to_disk_lsn: Declare private and atomic, and move
to the same cache line with log_t::lsn.
log_t::buf_free: Declare as size_t, and move to the same cache line
with log_t::lsn.
log_t::check_flush_or_checkpoint_: Declare private and atomic,
and move to the same cache line with log_t::lsn.
log_get_lsn(): Define as an alias of log_sys.get_lsn().
log_get_lsn_nowait(), log_peek_lsn(): Remove.
log_get_flush_lsn(): Define as an alias of log_sys.get_flush_lsn().
log_t::initiate_write(): Replaces log_buffer_sync_in_background().
The configuration parameter innodb_scrub_log never really worked, as
reported in MDEV-13019 and MDEV-18370.
Because MDEV-14425 is changing the redo log format, the innodb_scrub_log
feature would have to be adjusted for it. Due to the known problems,
it is easier to remove the feature for now, and to ignore and deprecate
the parameters.
If old log contents should be kept secret, then enabling innodb_encrypt_log
or setting a smaller innodb_log_file_size could help.
Compute MONITOR_LSN_CHECKPOINT_AGE on demand in
srv_mon_process_existing_counter().
This allows us to remove the overhead of MONITOR_SET
calls for the counter.
Introduce special synchronization primitive group_commit_lock
for more efficient synchronization of redo log writing and flushing.
The goal is to reduce CPU consumption on log_write_up_to, to reduce
the spurious wakeups, and improve the throughput in write-intensive
benchmarks.
There is no reason for the dummy index object dict_ind_redundant
to exist any more. It was only being passed to btr_create().
btr_create(): If !index, assume that a ROW_FORMAT=REDUNDANT
table is being created.
We could pass ibuf.index, dict_sys.sys_tables->indexes.start
and so on, if those objects had been initialized before the
function btr_create() is called.
Now there can be only one log file instead of several which
logically work as a single file.
Possible names of redo log files: ib_logfile0,
ib_logfile101 (for just created one)
innodb_log_fiels_in_group: value of this variable is not used
by InnoDB. Possible values are still 1..100, to not break upgrade
LOG_FILE_NAME: add constant of value "ib_logfile0"
LOG_FILE_NAME_PREFIX: add constant of value "ib_logfile"
get_log_file_path(): convenience function that returns full
path of a redo log file
SRV_N_LOG_FILES_MAX: removed
srv_n_log_files: we can't remove this for compatibility reasons,
but now server doesn't use this variable
log_sys_t::file::fd: now just one, not std::vector
log_sys_t::log_capacity: removed word 'group'
find_and_check_log_file(): part of logic from huge srv_start()
moved here
recv_sys_t::files: file descriptors of redo log files.
There can be several of those in case we're upgrading
from older MariaDB version.
recv_sys_t::remove_extra_log_files: whether to remove
ib_logfile{1,2,3...} after successfull upgrade.
recv_sys_t::read(): open if needed and read from one
of several log files
recv_sys_t::files_size(): open if needed and return files count
redo_file_sizes_are_correct(): check that redo log files
sizes are equal. Just to log an error for a user.
Corresponding check was moved from srv0start.cc
namespace deprecated: put all deprecated variables here to
prevent usage of it by us, developers
log_t::FORMAT_10_5: physical redo log format tag
log_phys_t: Buffered records in the physical format.
The log record bytes will follow the last data field,
making use of alignment padding that would otherwise be wasted.
If there are multiple records for the same page, also those
may be appended to an existing log_phys_t object if the memory
is available.
In the physical format, the first byte of a record identifies the
record and its length (up to 15 bytes). For longer records, the
immediately following bytes will encode the remaining length
in a variable-length encoding. Usually, a variable-length-encoded
page identifier will follow, followed by optional payload, whose
length is included in the initially encoded total record length.
When a mini-transaction is updating multiple fields in a page,
it can avoid repeating the tablespace identifier and page number
by setting the same_page flag (most significant bit) in the first
byte of the log record. The byte offset of the record will be
relative to where the previous record for that page ended.
Until MDEV-14425 introduces a separate file-level log for
redo log checkpoints and file operations, we will write the
file-level records in the page-level redo log file.
The record FILE_CHECKPOINT (which replaces MLOG_CHECKPOINT)
will be removed in MDEV-14425, and one sequential scan of the
page recovery log will suffice.
Compared to MLOG_FILE_CREATE2, FILE_CREATE will not include any flags.
If the information is needed, it can be parsed from WRITE records that
modify FSP_SPACE_FLAGS.
MLOG_ZIP_WRITE_STRING: Remove. The record was only introduced temporarily
as part of this work, before being replaced with WRITE (along with
MLOG_WRITE_STRING, MLOG_1BYTE, MLOG_nBYTES).
mtr_buf_t::empty(): Check if the buffer is empty.
mtr_t::m_n_log_recs: Remove. It suffices to check if m_log is empty.
mtr_t::m_last, mtr_t::m_last_offset: End of the latest m_log record,
for the same_page encoding.
page_recv_t::last_offset: Reflects mtr_t::m_last_offset.
Valid values for last_offset during recovery should be 0 or above 8.
(The first 8 bytes of a page are the checksum and the page number,
and neither are ever updated directly by log records.)
Internally, the special value 1 indicates that the same_page form
will not be allowed for the subsequent record.
mtr_t::page_create(): Take the block descriptor as parameter,
so that it can be compared to mtr_t::m_last. The INIT_INDEX_PAGE
record will always followed by a subtype byte, because same_page
records must be longer than 1 byte.
trx_undo_page_init(): Combine the writes in WRITE record.
trx_undo_header_create(): Write 4 bytes using a special MEMSET
record that includes 1 bytes of length and 2 bytes of payload.
flst_write_addr(): Define as a static function. Combine the writes.
flst_zero_both(): Replaces two flst_zero_addr() calls.
flst_init(): Do not inline the function.
fsp_free_seg_inode(): Zerofill the whole inode.
fsp_apply_init_file_page(): Initialize FIL_PAGE_PREV,FIL_PAGE_NEXT
to FIL_NULL when using the physical format.
btr_create(): Assert !page_has_siblings() because fsp_apply_init_file_page()
must have been invoked.
fil_ibd_create(): Do not write FILE_MODIFY after FILE_CREATE.
fil_names_dirty_and_write(): Remove the parameter mtr.
Write the records using a separate mini-transaction object,
because any FILE_ records must be at the start of a mini-transaction log.
recv_recover_page(): Add a fil_space_t* parameter.
After applying log to the a ROW_FORMAT=COMPRESSED page,
invoke buf_zip_decompress() to restore the uncompressed page.
buf_page_io_complete(): Remove the temporary hack to discard the
uncompressed page of a ROW_FORMAT=COMPRESSED page.
page_zip_write_header(): Remove. Use mtr_t::write() or
mtr_t::memset() instead, and update the compressed page frame
separately.
trx_undo_header_add_space_for_xid(): Remove.
trx_undo_seg_create(): Perform the changes that were previously
made by trx_undo_header_add_space_for_xid().
btr_reset_instant(): New function: Reset the table to MariaDB 10.2
or 10.3 format when rolling back an instant ALTER TABLE operation.
page_rec_find_owner_rec(): Merge with the only callers.
page_cur_insert_rec_low(): Combine writes by using a local buffer.
MEMMOVE data from the preceding record whenever feasible
(copying at least 3 bytes).
page_cur_insert_rec_zip(): Combine writes to page header fields.
PageBulk::insertPage(): Issue MEMMOVE records to copy a matching
part from the preceding record.
PageBulk::finishPage(): Combine the writes to the page header
and to the sparse page directory slots.
mtr_t::write(): Only log the least significant (last) bytes
of multi-byte fields that actually differ.
For updating FSP_SIZE, we must always write all 4 bytes to the
redo log, so that the fil_space_set_recv_size() logic in
recv_sys_t::parse() will work.
mtr_t::memcpy(), mtr_t::zmemcpy(): Take a pointer argument
instead of a numeric offset to the page frame. Only log the
last bytes of multi-byte fields that actually differ.
In fil_space_crypt_t::write_page0(), we must log also any
unchanged bytes, so that recovery will recognize the record
and invoke fil_crypt_parse().
Future work:
MDEV-21724 Optimize page_cur_insert_rec_low() redo logging
MDEV-21725 Optimize btr_page_reorganize_low() redo logging
MDEV-21727 Optimize redo logging for ROW_FORMAT=COMPRESSED
mtr_t::log_write_low(): Replaces mlog_write_initial_log_record_low().
mtr_t::log_file_op(): Replaces fil_op_write_log().
mtr_t::free(): Write MLOG_INIT_FREE_PAGE.
mtr_t::init(): Write MLOG_INIT_FILE_PAGE2.
mtr_t::page_create(): Write record about the partial initialization
of an index page.
mlog_catenate_ulint(), mlog_catenate_string(),
mlog_open(), mlog_close(): Remove.
Our benchmarking efforts indicate that the reasons for splitting the
buf_pool in commit c18084f71b
have mostly gone away, possibly as a result of
mysql/mysql-server@ce6109ebfd
or similar work.
Only in one write-heavy benchmark where the working set size is
ten times the buffer pool size, the buf_pool->mutex would be
less contended with 4 buffer pool instances than with 1 instance,
in buf_page_io_complete(). That contention could be alleviated
further by making more use of std::atomic and by splitting
buf_pool_t::mutex further (MDEV-15053).
We will deprecate and ignore the following parameters:
innodb_buffer_pool_instances
innodb_page_cleaners
There will be only one buffer pool and one page cleaner task.
In a number of INFORMATION_SCHEMA views, columns that indicated
the buffer pool instance will be removed:
information_schema.innodb_buffer_page.pool_id
information_schema.innodb_buffer_page_lru.pool_id
information_schema.innodb_buffer_pool_stats.pool_id
information_schema.innodb_cmpmem.buffer_pool_instance
information_schema.innodb_cmpmem_reset.buffer_pool_instance
During native table rebuild or index creation, InnoDB used to skip
redo logging and write MLOG_INDEX_LOAD records to inform crash recovery
and Mariabackup of the gaps in redo log. This is fragile and prohibits
some optimizations, such as skipping the doublewrite buffer for
newly (re)initialized pages (MDEV-19738).
row_merge_write_redo(): Remove. We do not write MLOG_INDEX_LOAD
records any more. Instead, we write full redo log.
FlushObserver: Remove.
fseg_free_page_func(): Remove the parameter log. Redo logging
cannot be disabled.
fil_space_t::redo_skipped_count: Remove.
We cannot remove buf_block_t::skip_flush_check, because PageBulk
will temporarily generate invalid B-tree pages in the buffer pool.
class log_file_t: more or less sane RAII wrapper around redo log file
descriptor and its path.
This change is motivated by the need of using that log_file_t somewhere else.
os_file_flush_data_func(): fix builds on POSIX OSs where fdatasync()
is not avaiable
log_t::files::flush_data_only(): rename from fdatasync()
log_t::files::fsync(): removed and replaced with flush_data_only().
It will flush everything we need for using redo log files.
I found that memcpy_aligned was used incorrectly at redo log and decided to put
assertions in aligned functions. And found even more incorrect cases.
Given the amount discovered of bugs, I left assertions to prevent future bugs.
my_assume_aligned(): instead of MY_ASSUME_ALIGNED macro
Redo log subsystem was decoupled from tablespace subsystem. It now manages file
descriptors for redo log files by itself.
FIL_TYPE_LOG: removed, code in various places was simplified
SRV_LOG_SPACE_FIRST_ID: renamed to SRV_SPACE_ID_UPPER_BOUND
to better match its purpose. Code in various places was simplified
fil_n_log_flushes: replaced with log_sys::flushes
fil_n_pending_log_flushes: replaced with log_sys::pending_flushes
log_t::files::files: redo log file descriptors
log_t::files::file_names: redo log file names
log_t::files::set_file_names(): set file names without opening them
log_t::files::open_files(): opens redo log files
log_t::files::read(): treats several files as one big
log_t::files::write(): treats several files as one big
log_t::files::fsync(): flushes page cache to disk
log_t::files::close_files(): closes redo log files
fil_open_log_and_system_tablespace_files(): renamed to
fil_open_system_tablespace_files()
and obviously it now doesn't open redo log files
global files[1000]: removed. Why it was needed at all?
The debug instrumentation with the MLOG_LSN pseudo-record has not been
used for debugging for years. Let us remove this code now.
It would have to be removed as part of MDEV-12353 or MDEV-14425 anyway,
when implementing a new redo log file format.
The function fsp_header_get_space_id() returns ulint instead of
uint32_t, only to be able to complain that the two adjacent
tablespace ID fields in the page differ. Remove the function,
and merge the check to the callers.
Also, make some more use of aligned_malloc().
InnoDB startup was discovering undo tablespaces in a dirty way.
It was reading a possibly stale copy of the TRX_SYS page before
processing any redo log records.
srv_start(): Do not call buf_pool_invalidate(). Invoke
trx_rseg_get_n_undo_tablespaces() after the recovery has been initiated.
recv_recovery_from_checkpoint_start(): Assert that the buffer pool is
empty. This used to be guaranteed by the buf_pool_invalidate() call.
trx_rseg_get_n_undo_tablespaces(): Move to the calling compilation unit,
and reimplement in a simpler way.
srv_undo_tablespace_create(): Remove the constant parameter
size=SRV_UNDO_TABLESPACE_SIZE_IN_PAGES.
srv_undo_tablespace_open(): Reimplement in a cleaner way, with
more robust error handling.
srv_all_undo_tablespaces_open(): Split from srv_undo_tablespaces_init().
srv_undo_tablespaces_init(): Read all "undo001","undo002" tablespace
files directly, without consulting the TRX_SYS page via calling
trx_rseg_get_n_undo_tablespaces().
This is joint work with Thirunarayanan Balathandayuthapani.
mtr_t::write(): Replaces mlog_write_ulint(), mlog_write_ull().
Optimize away writes if the page contents does not change,
except when a dummy write has been explicitly requested.
Because the member function template takes a block descriptor as a
parameter, it is possible to introduce better consistency checks.
Due to this, the code for handling file-based lists, undo logs
and user transactions was refactored to pass around buf_block_t.
Before commit c0f47a4a58 (MDEV-12026)
introduced the innodb_checksum_algorithm=full_crc32 format,
it was impossible to tell if InnoDB data files contained garbage in
the FIL_PAGE_TYPE header field (and possibly other fields).
This is because before commit 3926673ce7
in MySQL 5.1.48, InnoDB would write uninitialized data to some fields,
and because there was no way to tell with which InnoDB version a data
file was created.
If fil_space_t::full_crc32() holds, the data file cannot contain
uninitialized garbage or invalid FIL_PAGE_TYPE, and thus
fil_block_check_type() should not be invoked to correct anything.
Almost all threads have gone
- the "ticking" threads, that sleep a while then do some work)
(srv_monitor_thread, srv_error_monitor_thread, srv_master_thread)
were replaced with timers. Some timers are periodic,
e.g the "master" timer.
- The btr_defragment_thread is also replaced by a timer , which
reschedules it self when current defragment "item" needs throttling
- the buf_resize_thread and buf_dump_threads are substitutes with tasks
Ditto with page cleaner workers.
- purge workers threads are not tasks as well, and purge cleaner
coordinator is a combination of a task and timer.
- All AIO is outsourced to tpool, Innodb just calls thread_pool::submit_io()
and provides the callback.
- The srv_slot_t was removed, and innodb_debug_sync used in purge
is currently not working, and needs reimplementation.
We will remove the InnoDB background operation of merging buffered
changes to secondary index leaf pages. Changes will only be merged as a
result of an operation that accesses a secondary index leaf page,
such as a SQL statement that performs a lookup via that index,
or is modifying the index. Also ROLLBACK and some background operations,
such as purging the history of committed transactions, or computing
index cardinality statistics, can cause change buffer merge.
Encryption key rotation will not perform change buffer merge.
The motivation of this change is to simplify the I/O logic and to
allow crash recovery to happen in the background (MDEV-14481).
We also hope that this will reduce the number of "mystery" crashes
due to corrupted data. Because change buffer merge will typically
take place as a result of executing SQL statements, there should be
a clearer connection between the crash and the SQL statements that
were executed when the server crashed.
In many cases, a slight performance improvement was observed.
This is joint work with Thirunarayanan Balathandayuthapani
and was tested by Axel Schwenke and Matthias Leich.
The InnoDB monitor counter innodb_ibuf_merge_usec will be removed.
On slow shutdown (innodb_fast_shutdown=0), we will continue to
merge all buffered changes (and purge all undo log history).
Two InnoDB configuration parameters will be changed as follows:
innodb_disable_background_merge: Removed.
This parameter existed only in debug builds.
All change buffer merges will use synchronous reads.
innodb_force_recovery will be changed as follows:
* innodb_force_recovery=4 will be the same as innodb_force_recovery=3
(the change buffer merge cannot be disabled; it can only happen as
a result of an operation that accesses a secondary index leaf page).
The option used to be capable of corrupting secondary index leaf pages.
Now that capability is removed, and innodb_force_recovery=4 becomes 'safe'.
* innodb_force_recovery=5 (which essentially hard-wires
SET GLOBAL TRANSACTION ISOLATION LEVEL READ UNCOMMITTED)
becomes safe to use. Bogus data can be returned to SQL, but
persistent InnoDB data files will not be corrupted further.
* innodb_force_recovery=6 (ignore the redo log files)
will be the only option that can potentially cause
persistent corruption of InnoDB data files.
Code changes:
buf_page_t::ibuf_exist: New flag, to indicate whether buffered
changes exist for a buffer pool page. Pages with pending changes
can be returned by buf_page_get_gen(). Previously, the changes
were always merged inside buf_page_get_gen() if needed.
ibuf_page_exists(const buf_page_t&): Check if a buffered changes
exist for an X-latched or read-fixed page.
buf_page_get_gen(): Add the parameter allow_ibuf_merge=false.
All callers that know that they may be accessing a secondary index
leaf page must pass this parameter as allow_ibuf_merge=true,
unless it does not matter for that caller whether all buffered
changes have been applied. Assert that whenever allow_ibuf_merge
holds, the page actually is a leaf page. Attempt change buffer
merge only to secondary B-tree index leaf pages.
btr_block_get(): Add parameter 'bool merge'.
All callers of btr_block_get() should know whether the page could be
a secondary index leaf page. If it is not, we should avoid consulting
the change buffer bitmap to even consider a merge. This is the main
interface to requesting index pages from the buffer pool.
ibuf_merge_or_delete_for_page(), recv_recover_page(): Replace
buf_page_get_known_nowait() with much simpler logic, because
it is now guaranteed that that the block is x-latched or read-fixed.
mlog_init_t::mark_ibuf_exist(): Renamed from mlog_init_t::ibuf_merge().
On crash recovery, we will no longer merge any buffered changes
for the pages that we read into the buffer pool during the last batch
of applying log records.
buf_page_get_gen_known_nowait(), BUF_MAKE_YOUNG, BUF_KEEP_OLD: Remove.
btr_search_guess_on_hash(): Merge buf_page_get_gen_known_nowait()
to its only remaining caller.
buf_page_make_young_if_needed(): Define as an inline function.
Add the parameter buf_pool.
buf_page_peek_if_young(), buf_page_peek_if_too_old(): Add the
parameter buf_pool.
fil_space_validate_for_mtr_commit(): Remove a bogus comment
about background merge of the change buffer.
btr_cur_open_at_rnd_pos_func(), btr_cur_search_to_nth_level_func(),
btr_cur_open_at_index_side_func(): Use narrower data types and scopes.
ibuf_read_merge_pages(): Replaces buf_read_ibuf_merge_pages().
Merge the change buffer by invoking buf_page_get_gen().
