page_cur_dtuple_cmp(): Take DESC index fields into account also when
comparing a NULL value in a ROW_FORMAT≠REDUNDANT record.
Thanks to Elena Stepanova for finding this bug.
page_cur_search_with_match(): Remove rec_get_offsets(), and instead
determine the start and end of each field while comparing.
page_dir_slot_get_rec(), page_dir_slot_get_rec_validate():
Add a parameter to avoid invoking page_align().
page_cur_dtuple_cmp(): Replaces cmp_dtuple_rec_leaf() for both
leaf and non-leaf pages. In SPATIAL INDEX, non-leaf records are
special, because the child page number may be part of the comparison.
Reviewed by: Vladislav Lesin
For some reason, page_cur_search_with_match_bytes(), which can speed
up append operations (PAGE_CUR_LE used by INSERT), was only enabled
if innodb_adaptive_hash_index=ON even though it has nothing to do with
the adaptive hash index.
Furthermore, mysql/mysql-server@c9bbc83d11
a.k.a. commit c9bbc83d11 reduced a limit
from 3 to 2 but forgot to adjust the PAGE_N_DIRECTION limit accordingly.
We are adjusting that as well.
Reviewed by: Vladislav Lesin
During a workload, an adaptive hash index had been built on
UNIQUE INDEX(ID) on SYS_TABLES, and during a DROP TABLE
operation the adaptive hash index would be widened to cover
also the PRIMARY KEY(NAME) field that the index includes: (ID,NAME).
Such an adaptive hash index is unlikely to satisfy (m)any queries.
Let us limit the AHI prefix to the unique fields.
Reviewed by: Vladislav Lesin
btr_search_drop_page_hash_index(): Replace the Boolean parameter
with const dict_index_t *not_garbage. If buf_block_t::index points
to that, there is no need to acquire btr_sea::partition::latch.
The old parameter bool garbage_collect=false is equivalent to the
parameter not_garbage=nullptr. The parameter garbage_collect=true
will be replaced either with the actual index that is associated
with the buffer page, or with a bogus pointer not_garbage=-1 to
indicate that any lazily entries for a freed index need to be removed.
buf_page_get_low(), buf_page_get_gen(), mtr_t::page_lock(),
mtr_t::upgrade_buffer_fix(): Do not invoke
btr_search_drop_page_hash_index(). Our caller will have to do it
when appropriate.
buf_page_create_low(): Keep invoking btr_search_drop_page_hash_index().
This is the normal way of lazily dropping the adaptive hash index
after a DDL operation such as DROP INDEX operation.
btr_block_get(), btr_root_block_get(), btr_root_adjust_on_import(),
btr_read_autoinc_with_fallback(), btr_cur_instant_init_low(),
btr_cur_t::search_leaf(), btr_cur_t::pessimistic_search_leaf(),
btr_pcur_optimistic_latch_leaves(), dict_stats_analyze_index_below_cur():
Invoke btr_search_drop_page_hash_index(block, index) for pages that
may be leaf pages. No adaptive hash index may have been created on
anything else than a B-tree leaf page.
btr_cur_search_to_nth_level(): Do not invoke
btr_search_drop_page_hash_index(), because we are only accessing
non-leaf pages and the adaptive hash index may only have been created
on leaf pages.
btr_page_alloc_for_ibuf() and many other callers of buf_page_get_gen()
or similar functions do not invoke btr_search_drop_page_hash_index(),
because the adaptive hash index is never created on such pages.
If a page in the tablespace was freed as part of a DDL operation and
reused for something else, then buf_page_create_low() will take care
of dropping the adaptive hash index before the freed page will be
modified.
It is notable that while the flst_ functions may access pages that are
related to allocating B-tree index pages (the BTR_SEG_TOP and BTR_SEG_LEAF
linked from the index root page), those pages themselves can never be
stored in the adaptive hash index. Therefore, it is not necessary to
invoke btr_search_drop_page_hash_index() on them.
Reviewed by: Vladislav Lesin
btr_search_info_update_hash(): Do nothing if the record is positioned
on the page supremum or infimum pseudo-record. The adaptive hash index
can only include user records. This deficiency would cause the
adaptive hash index parameters to change between hashing a prefix of
1 field or a prefix of 1 byte.
Reviewed by: Vladislav Lesin
btr_search_guess_on_hash(): Only set BTR_CUR_HASH_FAIL on actual mismatch.
If the page latch cannot be acquired, the hash search might very well
have succeeded. Do not count that as a failure, that is, do not
unnecessarily invoke btr_search_update_hash_ref() after a normal search.
Set cursor->flag=BTR_CUR_HASH_ABORT if the current parameters of the
adaptive hash index are not suitable for the search and a call to
btr_cur_t::search_info_update() might help.
btr_cur_t::search_leaf(): Do not invoke search_info_update()
if btr_search_guess_on_hash() failed due to contention.
btr_cur_t::pessimistic_search_leaf(): Do not invoke search_info_update()
on the change buffer tree. Preivously, this condition was being checked
inside search_info_update().
btr_cur_t::search_leaf(): Do not attempt to use the adaptive
hash index for PAGE_CUR_G or PAGE_CUR_L, because those modes
expect an inequal result, and the adaptive hash index can only
deliver equal results.
btr_cur_t::check_mismatch(): Only handle PAGE_CUR_LE and PAGE_CUR_GE.
For PAGE_CUR_LE (bool ge=false), qualify a full match for the last
record of a page that is not at the end of the index. Previously,
an adaptive hash index lookup would fail when the record is at the end
of an index page but not at the end of the index. This would lead to
unnecessary rebuild of the adaptive hash index in read-only workloads.
Reviewed by: Vladislav Lesin
Now that ut_fold_ulint_pair() and ut_fold_binary() are no longer needed
for anything else than compatibility with old InnoDB data files that may
use innodb_checksum_algorithm=innodb, let us move the code to a single
compilation unit.
Reviewed by: Vladislav Lesin
For the adaptive hash index, dtuple_fold() and rec_fold() were employing
a slow rolling hash algorithm, computing hash values ("fold") for one
field and one byte at a time, while depending on calls to
rec_get_offsets().
We already have optimized implementations of CRC-32C and have been
successfully using that function in some other InnoDB tables, but not
yet in the adaptive hash index.
Any linear function such as any CRC will fail the avalanche test that
any cryptographically secure hash function is expected to pass:
any single-bit change in the input key should affect on average half
the bits in the output.
But we always were happy with less than cryptographically secure:
in fact, ut_fold_ulint_pair() or ut_fold_binary() are just about as
linear as any CRC, using a combination of multiplication and addition,
partly carry-less. It is worth noting that exclusive-or corresponds to
carry-less subtraction or addition in a binary Galois field, or GF(2).
We only need some way of reducing key prefixes into hash values.
The CRC-32C should be better than a Rabin–Karp rolling hash algorithm.
Compared to the old hash algorithm, it has the drawback that there will
be only 32 bits of entropy before we choose the hash table cell by a
modulus operation. The size of each adaptive hash index array is
(innodb_buffer_pool_size / 512) / innodb_adaptive_hash_index_parts.
With the maximum number of partitions (512), we would not exceed 1<<32
elements per array until the buffer pool size exceeds 1<<50 bytes (1 PiB).
We would hit other limits before that: the virtual address space on many
contemporary 64-bit processor implementations is only 48 bits (256 TiB).
So, we can simply go for the SIMD accelerated CRC-32C.
rec_fold(): Take a combined parameter n_bytes_fields. Determine the
length of each field on the fly, and compute CRC-32C over a single
contiguous range of bytes, from the start of the record payload area
to the end of the last full or partial field. For secondary index records
in ROW_FORMAT=REDUNDANT, also the data area that is reserved for NULL
values (to facilitate in-place updates between NULL and NOT NULL values)
will be included in the count. Luckily, InnoDB always zero-initialized
such unused area; refer to data_write_sql_null() in
rec_convert_dtuple_to_rec_old(). For other than ROW_FORMAT=REDUNDANT,
no space is allocated for NULL values, and therefore the CRC-32C will
only cover the actual payload of the key prefix.
dtuple_fold(): For ROW_FORMAT=REDUNDANT, include the dummy NULL values
in the CRC-32C, so that the values will be comparable with rec_fold().
innodb_ahi-t: A unit test for rec_fold() and dtuple_fold().
btr_search_build_page_hash_index(), btr_search_drop_page_hash_index():
Use a fixed-size stack buffer for computing the fold values, to avoid
dynamic memory allocation.
btr_search_drop_page_hash_index(): Do not release part.latch if we
need to invoke multiple batches of rec_fold().
dtuple_t: Allocate fewer bits for the fields. The maximum number of
data fields is about 1023, so uint16_t will be fine for them. The
info_bits is stored in less than 1 byte.
ut_pair_min(), ut_pair_cmp(): Remove. We can actually combine and compare
int(n_fields << 16 | n_bytes).
PAGE_CUR_LE_OR_EXTENDS, PAGE_CUR_DBG: Remove. These were never defined,
because they would only work with latin1_swedish_ci if at all.
btr_cur_t::check_mismatch(): Replaces !btr_search_check_guess().
cmp_dtuple_rec_bytes(): Replaces cmp_dtuple_rec_with_match_bytes().
Determine the offsets of fields on the fly.
page_cur_try_search_shortcut_bytes(): This caller of
cmp_dtuple_rec_bytes() will not be invoked on the change buffer tree.
cmp_dtuple_rec_leaf(): Replaces cmp_dtuple_rec_with_match()
for comparing leaf-page records.
buf_block_t::ahi_left_bytes_fields: Consolidated Atomic_relaxed<uint32_t>
of curr_left_side << 31 | curr_n_bytes << 16 | curr_n_fields.
The other set of parameters (n_fields, n_bytes, left_side) was removed
as redundant.
btr_search_update_hash_node_on_insert(): Merged to
btr_search_update_hash_on_insert().
btr_search_build_page_hash_index(): Take combined left_bytes_fields
instead of n_fields, n_bytes, left_side.
btr_search_update_block_hash_info(), btr_search_update_hash_ref():
Merged to btr_search_info_update_hash().
btr_cur_t::n_bytes_fields: Replaces n_bytes << 16 | n_fields.
We also remove many redundant checks of btr_search.enabled.
If we are holding any btr_sea::partition::latch, then a nonnull pointer
in buf_block_t::index must imply that the adaptive hash index is enabled.
Reviewed by: Vladislav Lesin
Let us use implement a simple fixed-size allocator for the adaptive hash
index, insted of complicating mem_heap_t or mem_block_info_t.
MEM_HEAP_BTR_SEARCH: Remove.
mem_block_info_t::free_block(), mem_heap_free_block_free(): Remove.
mem_heap_free_top(), mem_heap_get_top(): Remove.
btr_sea::partition::spare: Replaces mem_block_info_t::free_block.
This keeps one spare block per adaptive hash index partition, to
process an insert.
We must not wait for buf_pool.mutex while holding
any btr_sea::partition::latch. That is why we cache one block for
future allocations. This is protected by a new
btr_sea::partition::blocks_mutex in order to relieve pressure on
btr_sea::partition::latch.
btr_sea::partition::prepare_insert(): Replaces
btr_search_check_free_space_in_heap().
btr_sea::partition::erase(): Replaces ha_search_and_delete_if_found().
btr_sea::partition::cleanup_after_erase(): Replaces the most part of
ha_delete_hash_node(). Unlike the previous implementation, we will
retain a spare block for prepare_insert().
This should reduce some contention on buf_pool.mutex.
btr_search.n_parts: Replaces btr_ahi_parts.
btr_search.enabled: Replaces btr_search_enabled. This must hold
whenever buf_block_t::index is set while a thread is holding a
btr_sea::partition::latch.
dict_index_t::search_info: Remove pointer indirection, and use
Atomic_relaxed or Atomic_counter for most fields.
btr_search_guess_on_hash(): Let the caller ensure that latch_mode is
BTR_MODIFY_LEAF or BTR_SEARCH_LEAF. Release btr_sea::partition::latch
before buffer-fixing the block. The page latch that we already acquired
is preventing buffer pool eviction. We must validate both
block->index and block->page.state while holding part.latch
in order to avoid race conditions with buffer page relocation
or buf_pool_t::resize().
btr_search_check_guess(): Remove the constant parameter
can_only_compare_to_cursor_rec=false.
ahi_node: Replaces ha_node_t.
This has been tested by running the regression test suite
with the adaptive hash index enabled:
./mtr --mysqld=--loose-innodb-adaptive-hash-index=ON
Reviewed by: Vladislav Lesin
- InnoDB fails to recover the full crc32 encrypted page from
doublewrite buffer. The reason is that buf_dblwr_t::recover()
fails to identify the space id from the page because the page has
been encrypted from FIL_PAGE_FILE_FLUSH_LSN_OR_KEY_VERSION bytes.
Fix:
===
buf_dblwr_t::recover(): preserve any pages whose space_id
does not match a known tablespace. These could be encrypted pages
of tablespaces that had been created with
innodb_checksum_algorithm=full_crc32.
buf_page_t::read_complete(): If the page looks corrupted and the
tablespace is encrypted and in full_crc32 format, try to
restore the page from doublewrite buffer.
recv_dblwr_t::recover_encrypted_page(): Find the page which
has the same page number and try to decrypt the page using
space->crypt_data. After decryption, compare the space id.
Write the recovered page back to the file.
Heap tables are allocated blocks to store rows according to
my_default_record_cache (mapped to the server global variable
read_buffer_size).
This causes performance issues when the record length is big
(> 1000 bytes) and the my_default_record_cache is small.
Changed to instead split the default heap allocation to 1/16 of the
allowed space and not use my_default_record_cache anymore when creating
the heap. The allocation is also aligned to be just under a power of 2.
For some test that I have been running, which was using record length=633,
the speed of the query doubled thanks to this change.
Other things:
- Fixed calculation of max_records passed to hp_create() to take
into account padding between records.
- Updated calculation of memory needed by heap tables. Before we
did not take into account internal structures needed to access rows.
- Changed block sized for memory_table from 1 to 16384 to get less
fragmentation. This also avoids a problem where we need 1K
to manage index and row storage which was not counted for before.
- Moved heap memory usage to a separate test for 32 bit.
- Allocate all data blocks in heap in powers of 2. Change reported
memory usage for heap to reflect this.
Reviewed-by: Sergei Golubchik <serg@mariadb.org>
Note: Changes to the test innodb.stats_persistent
in commit e5c4c0842d (MDEV-35443)
are not merged, because the test scenario is impossible
due to commit e66928ab28 (MDEV-33462).
Problem:
=======
- insert..select statement on partition table fails to use
bulk insert for the transaction.
Solution:
========
- Enable the bulk insert operation for insert..select
statement for partition table.
Make Item_func_eq of the following forms sargable by updating the relevant range
analysis methods:
1. substr(col, 1, n) = str
2. str = substr(col, 1, n)
3. left(col, n) = str
4. str = left(col, n)
where col is a indexed column and str is a const and inexpensive item
of length n.
We do this by factoring out Item_func_like::get_mm_leaf() and apply it
to a string obtained from escaping str and then appending a wildcard
"%" to it.
The addition of the two Functype enums, LEFT_FUNC and SUBSTR_FUNC,
requires changes in the spider group by handler to continue handling
LEFT and SUBSTR correctly.
Co-authored-by: Yuchen Pei <ycp@mariadb.com>
Co-authored-by: Sergei Petrunia <sergey@mariadb.com>
opt_calc_index_goodness(): Correct an inaccurate condition.
We can very well use a clustered index of a table that is subject
to online rebuild. But we must not choose an index that has not been
committed (it is a secondary index that was not fully created)
or that is corrupted or not a normal B-tree index.
opt_search_plan_for_table(): Remove some redundant code, now that
opt_calc_index_goodness() checks against corrupted indexes.
The test case allows this code to be exercised. The main observation
in the following:
./mtr --rr innodb.stats_persistent
rr replay var/log/mysqld.1.rr/latest-trace
should be that when opt_search_plan_for_table() is being invoked by
dict_stats_update_persistent() on the being-altered statistics table
in the 2nd call after ha_innobase::inplace_alter_table(),
and the fix in opt_calc_index_goodness() is absent,
it would choose the code path if (n_fields == 0), that is, a full
table scan, instead of searching for the record. The GDB commands to
execute in "rr replay" would be as follows:
break ha_innobase::inplace_alter_table
continue
break opt_search_plan_for_table
continue
continue
next
next
…
Reviewed by: Vladislav Lesin
The assertion fails during wsrep recovery step, in function
innobase_rollback_by_xid(). The transaction's xid is normally
cleared as part of lookup by xid, unless the transaction has
a wsrep specific xid.
This is a regression from MDEV-24035 (commit ddd7d5d8e3)
which removed the part clears xid before rollback for transaction
with a wsrep specific xid.
In function buf_page_create_low(), remove duplicate code that
over-write the ibuf_exist variable incorrectly when only compressed
page is loaded in buffer pool. This would help removing any old change
buffer record immediately before re-using the page.
The limit of socket length on unix according to libc is 108, see
sockaddr_un::sun_path, but in the table it is a string of max length
64, which results in truncation of socket and failure to connect by
plugins using servers such as spider.
If InnoDB is killed in such a way that there had been no writes
to a newly resized ib_logfile101 after it replaced ib_logfile0
in log_t::write_checkpoint(), it is possible that recovery will
accidentally interpret some garbage at the end of the log as valid.
log_t::write_buf(): To prevent the corruption, write an extra NUL byte
at the end of log_sys.resize_buf, like we always did for the main
log_sys.buf. To remove some conditional branches from a time critical
code path, we instantiate a separate template for the rare case that the
log is being resized. Define as __attribute__((always_inline)) so that
this will be inlined also in the rare case the log is being resized.
log_t::writer: Pointer to the current implementation of
log_t::write_buf(). For quick access, this is located in the
same cache line with log_sys.latch, which protects it.
log_t::writer_update(): Update log_sys.writer.
log_t::resize_write_buf(): Remove ATTRIBUTE_NOINLINE ATTRIBUTE_COLD.
Now that log_t::write_buf() will be instantiated separately for the
rare case of log resizing being in progress, there is no need to forbid
this code from being inlined.
Thanks to Thirunarayanan Balathandayuthapani for finding the
root cause of this bug and suggesting the fix of writing an extra
NUL byte.
Reviewed by: Debarun Banerjee
This regression is introduced in 10.6 by following commit.
commit 35d477dd1d
MDEV-34453 Trying to read 16384 bytes at 70368744161280
The page state could change after being buffer-fixed and needs to be
read again after locking the page.
trx_sys_t::find_same_or_older_in_purge(): Correct a mistake that
was made in commit 19acb0257e
(MDEV-35508) and make the caching logic correspond to the one in
trx_sys_t::find_same_or_older(). In the more common code path
for 64-bit systems, the condition !hot was inadvertently inverted,
making us wrongly skip calls to find_same_or_older_low() when the
transaction may still be active.
Furthermore, the call should have been to find_same_or_older_low()
and not the wrapper find_same_or_older().
Under unknown circumstances, the SQL layer may wrongly disregard an
invocation of thd_mark_transaction_to_rollback() when an InnoDB
transaction had been aborted (rolled back) due to one of the following errors:
* HA_ERR_LOCK_DEADLOCK
* HA_ERR_RECORD_CHANGED (if innodb_snapshot_isolation=ON)
* HA_ERR_LOCK_WAIT_TIMEOUT (if innodb_rollback_on_timeout=ON)
Such an error used to cause a crash of InnoDB during transaction commit.
These changes aim to catch and report the error earlier, so that not only
this crash can be avoided but also the original root cause be found and
fixed more easily later.
The idea of this fix is from Michael 'Monty' Widenius.
HA_ERR_ROLLBACK: A new error code that will be translated into
ER_ROLLBACK_ONLY, signalling that the current transaction
has been aborted and the only allowed action is ROLLBACK.
trx_t::state: Add TRX_STATE_ABORTED that is like
TRX_STATE_NOT_STARTED, but noting that the transaction had been
rolled back and aborted.
trx_t::is_started(): Replaces trx_is_started().
ha_innobase: Check the transaction state in various places.
Simplify the logic around SAVEPOINT.
ha_innobase::is_valid_trx(): Replaces ha_innobase::is_read_only().
The InnoDB logic around transaction savepoints, commit, and rollback
was unnecessarily complex and might have contributed to this
inconsistency. So, we are simplifying that logic as well.
trx_savept_t: Replace with const undo_no_t*. When we rollback to
a savepoint, all we need to know is the number of undo log records
that must survive.
trx_named_savept_t, DB_NO_SAVEPOINT: Remove. We can store undo_no_t
directly in the space allocated at innobase_hton->savepoint_offset.
fts_trx_create(): Do not copy previous savepoints.
fts_savepoint_rollback(): If a savepoint was not found, roll back
everything after the default savepoint of fts_trx_create().
The test innodb_fts.savepoint is extended to cover this code.
Reviewed by: Vladislav Lesin
Tested by: Matthias Leich
buf_dblwr_t::recover(): Correct a debug assertion failure that had
been added in commit bb47e575de (MDEV-34830).
The server may have been killed while a log write was in progress, and
therefore recv_sys.scanned_lsn may be up to RECV_PARSING_BUF_SIZE bytes
ahead of recv_sys.recovered_lsn.
Thanks to Matthias Leich for providing "rr replay" traces and
testing this.
fil_space_t::create(): Instead of invoking the default fil_space_t
constructor on a zero-filled buffer, allocate an uninitialized buffer
and invoke an explicitly defined constructor on it. Also, specify
initializer expressions for all constant data members, so that all of them
will be initialized in the constructor.
fil_space_t::being_imported: Replaces part of fil_space_t::purpose.
fil_space_t::is_being_imported(), fil_space_t::is_temporary():
Replaces fil_space_t::purpose.
fil_space_t:🆔 Changed the type from ulint to uint32_t to reduce
incompatibility with later branches that include
commit ca501ffb04 (MDEV-26195).
fil_space_t::try_to_close(): Do not attempt to close files that are
in an I/O bound phase of ALTER TABLE…IMPORT TABLESPACE.
log_file_op, first_page_init: recv_spaces_t:
Use uint32_t for the tablespace id.
Reviewed by: Debarun Banerjee
os_innodb_umask was of the incorrect type resulting in warnings
in clang-19. The correct type is mode_t.
As os_innodb_umask was set during innnodb_init from my_umask,
corrected the type there along with its companion my_umask_dir.
Because of this, the defaults mask values in innodb never
had an effect.
The resulting change allow found signed differences in
my_create{,_nosymlink}, open_nosymlinks:
mysys/my_create.c:47:20: error: operand of ?: changes signedness from ‘int’ to ‘mode_t’ {aka ‘unsigned int’} due to unsignedness of other operand [-Werror=sign-compare]
47 | CreateFlags ? CreateFlags : my_umask);
Ref: clang-19 warnings:
[55/123] Building CXX object storage/innobase/CMakeFiles/innobase.dir/os/os0file.cc.o
storage/innobase/os/os0file.cc:1075:46: warning: implicit conversion loses integer precision: 'ulint' (aka 'unsigned long') to 'mode_t' (aka 'unsigned int') [-Wshorten-64-to-32]
1075 | file = open(name, create_flag | O_CLOEXEC, os_innodb_umask);
| ~~~~ ^~~~~~~~~~~~~~~
storage/innobase/os/os0file.cc:1249:46: warning: implicit conversion loses integer precision: 'ulint' (aka 'unsigned long') to 'mode_t' (aka 'unsigned int') [-Wshorten-64-to-32]
1249 | file = open(name, create_flag | O_CLOEXEC, os_innodb_umask);
| ~~~~ ^~~~~~~~~~~~~~~
storage/innobase/os/os0file.cc:1381:45: warning: implicit conversion loses integer precision: 'ulint' (aka 'unsigned long') to 'mode_t' (aka 'unsigned int') [-Wshorten-64-to-32]
1381 | file = open(name, create_flag | O_CLOEXEC, os_innodb_umask);
| ~~~~ ^~~~~~~~~~~~~~~
Threads can normally exit without a explicit pthread_exit call.
There seem to date to old glibc bugs, many around 2.2.5.
The semi related bug was https://bugs.mysql.com/bug.php?id=82886.
To improve safety in the signal handlers DBUG_* code was removed.
These where also needed to avoid some MSAN unresolved stack issues.
This is effectively a backport of 2719cc4925.
Problem:
=======
InnoDB wrongly stores the primary key field in externally
stored off page during bulk insert operation. This leads
to assert failure.
Solution:
========
row_merge_buf_blob(): Should store the primary key fields
inline. Store the variable length field data externally
based on the row format of the table.
row_merge_buf_write(): check whether the record size exceeds
the maximum record size.
row_merge_copy_blob_from_file(): Construct the tuple based on
the variable length field
In commit 6acada713a the
logic for treating the file system of /dev/shm
as if it were persistent memory was broken.
Let us restore the original logic, so that we will have
some more CI coverage of the memory-mapped redo log interface.
