MySQL Innodb Engine--修改数据时先写Buffer Pool还是先写Redo Log

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问题描述

在做DML操作时，是先修改buffer pool数据，还是先记录redo log呢？

答案：先修改buffer pool数据，然后再记录redo log。

代码学习

在page0cur.cc文件有page_cur_insert_rec_low方法，该方法用于插入新记录，其方法中有如下注解：

/* 1. Get the size of the physical record in the page */

/* 2. Try to find suitable space from page memory management */

/* 3. Create the record */

/* 4. Insert the record in the linked list of records */

/* 5. Set the n_owned field in the inserted record to zero,
	and set the heap_no field */

/* 6. Update the last insertion info in page header */

/* 7. It remains to update the owner record. */

/* 8. Now we have incremented the n_owned field of the owner
	record. If the number exceeds PAGE_DIR_SLOT_MAX_N_OWNED,
	we have to split the corresponding directory slot in two. */

/* 9. Write log record of the insert */
if (UNIV_LIKELY(mtr != NULL)) {
	page_cur_insert_rec_write_log(insert_rec, rec_size,
					      current_rec, index, mtr);
}

在第9步中会调用函数page_cur_insert_rec_write_log来记录redo log。

page_cur_insert_rec_low函数

/***********************************************************//**
Inserts a record next to page cursor on an uncompressed page.
Returns pointer to inserted record if succeed, i.e., enough
space available, NULL otherwise. The cursor stays at the same position.
@return pointer to record if succeed, NULL otherwise */
rec_t*
page_cur_insert_rec_low(
/*====================*/
	rec_t*		current_rec,/*!< in: pointer to current record after
				which the new record is inserted */
	dict_index_t*	index,	/*!< in: record descriptor */
	const rec_t*	rec,	/*!< in: pointer to a physical record */
	ulint*		offsets,/*!< in/out: rec_get_offsets(rec, index) */
	mtr_t*		mtr)	/*!< in: mini-transaction handle, or NULL */
{
	byte*		insert_buf;
	ulint		rec_size;
	page_t*		page;		/*!< the relevant page */
	rec_t*		last_insert;	/*!< cursor position at previous
					insert */
	rec_t*		free_rec;	/*!< a free record that was reused,
					or NULL */
	rec_t*		insert_rec;	/*!< inserted record */
	ulint		heap_no;	/*!< heap number of the inserted
					record */

	ut_ad(rec_offs_validate(rec, index, offsets));

	page = page_align(current_rec);
	ut_ad(dict_table_is_comp(index->table)
	      == (ibool) !!page_is_comp(page));
	ut_ad(fil_page_index_page_check(page));
	ut_ad(mach_read_from_8(page + PAGE_HEADER + PAGE_INDEX_ID) == index->id
	      || recv_recovery_is_on()
	      || (mtr ? mtr->is_inside_ibuf() : dict_index_is_ibuf(index)));

	ut_ad(!page_rec_is_supremum(current_rec));

	/* 1. Get the size of the physical record in the page */
	rec_size = rec_offs_size(offsets);

#ifdef UNIV_DEBUG_VALGRIND
	{
		const void*	rec_start
			= rec - rec_offs_extra_size(offsets);
		ulint		extra_size
			= rec_offs_extra_size(offsets)
			- (rec_offs_comp(offsets)
			   ? REC_N_NEW_EXTRA_BYTES
			   : REC_N_OLD_EXTRA_BYTES);

		/* All data bytes of the record must be valid. */
		UNIV_MEM_ASSERT_RW(rec, rec_offs_data_size(offsets));
		/* The variable-length header must be valid. */
		UNIV_MEM_ASSERT_RW(rec_start, extra_size);
	}
#endif /* UNIV_DEBUG_VALGRIND */

	/* 2. Try to find suitable space from page memory management */

	free_rec = page_header_get_ptr(page, PAGE_FREE);
	if (UNIV_LIKELY_NULL(free_rec)) {
		/* Try to allocate from the head of the free list. */
		ulint		foffsets_[REC_OFFS_NORMAL_SIZE];
		ulint*		foffsets	= foffsets_;
		mem_heap_t*	heap		= NULL;

		rec_offs_init(foffsets_);

		foffsets = rec_get_offsets(
			free_rec, index, foffsets, ULINT_UNDEFINED, &heap);
		if (rec_offs_size(foffsets) < rec_size) {
			if (UNIV_LIKELY_NULL(heap)) {
				mem_heap_free(heap);
			}

			goto use_heap;
		}

		insert_buf = free_rec - rec_offs_extra_size(foffsets);

		if (page_is_comp(page)) {
			heap_no = rec_get_heap_no_new(free_rec);
			page_mem_alloc_free(page, NULL,
					rec_get_next_ptr(free_rec, TRUE),
					rec_size);
		} else {
			heap_no = rec_get_heap_no_old(free_rec);
			page_mem_alloc_free(page, NULL,
					rec_get_next_ptr(free_rec, FALSE),
					rec_size);
		}

		if (UNIV_LIKELY_NULL(heap)) {
			mem_heap_free(heap);
		}
	} else {
use_heap:
		free_rec = NULL;
		insert_buf = page_mem_alloc_heap(page, NULL,
						 rec_size, &heap_no);

		if (UNIV_UNLIKELY(insert_buf == NULL)) {
			return(NULL);
		}
	}

	/* 3. Create the record */
	insert_rec = rec_copy(insert_buf, rec, offsets);
	rec_offs_make_valid(insert_rec, index, offsets);

	/* 4. Insert the record in the linked list of records */
	ut_ad(current_rec != insert_rec);

	{
		/* next record after current before the insertion */
		rec_t*	next_rec = page_rec_get_next(current_rec);
#ifdef UNIV_DEBUG
		if (page_is_comp(page)) {
			ut_ad(rec_get_status(current_rec)
				<= REC_STATUS_INFIMUM);
			ut_ad(rec_get_status(insert_rec) < REC_STATUS_INFIMUM);
			ut_ad(rec_get_status(next_rec) != REC_STATUS_INFIMUM);
		}
#endif
		page_rec_set_next(insert_rec, next_rec);
		page_rec_set_next(current_rec, insert_rec);
	}

	page_header_set_field(page, NULL, PAGE_N_RECS,
			      1 + page_get_n_recs(page));

	/* 5. Set the n_owned field in the inserted record to zero,
	and set the heap_no field */
	if (page_is_comp(page)) {
		rec_set_n_owned_new(insert_rec, NULL, 0);
		rec_set_heap_no_new(insert_rec, heap_no);
	} else {
		rec_set_n_owned_old(insert_rec, 0);
		rec_set_heap_no_old(insert_rec, heap_no);
	}

	UNIV_MEM_ASSERT_RW(rec_get_start(insert_rec, offsets),
			   rec_offs_size(offsets));
	/* 6. Update the last insertion info in page header */

	last_insert = page_header_get_ptr(page, PAGE_LAST_INSERT);
	ut_ad(!last_insert || !page_is_comp(page)
	      || rec_get_node_ptr_flag(last_insert)
	      == rec_get_node_ptr_flag(insert_rec));

	if (!dict_index_is_spatial(index)) {
		if (UNIV_UNLIKELY(last_insert == NULL)) {
			page_header_set_field(page, NULL, PAGE_DIRECTION,
					      PAGE_NO_DIRECTION);
			page_header_set_field(page, NULL, PAGE_N_DIRECTION, 0);

		} else if ((last_insert == current_rec)
			   && (page_header_get_field(page, PAGE_DIRECTION)
			       != PAGE_LEFT)) {

			page_header_set_field(page, NULL, PAGE_DIRECTION,
					      PAGE_RIGHT);
			page_header_set_field(page, NULL, PAGE_N_DIRECTION,
					      page_header_get_field(
						page, PAGE_N_DIRECTION) + 1);

		} else if ((page_rec_get_next(insert_rec) == last_insert)
			   && (page_header_get_field(page, PAGE_DIRECTION)
			       != PAGE_RIGHT)) {

			page_header_set_field(page, NULL, PAGE_DIRECTION,
					      PAGE_LEFT);
			page_header_set_field(page, NULL, PAGE_N_DIRECTION,
					      page_header_get_field(
						page, PAGE_N_DIRECTION) + 1);
		} else {
			page_header_set_field(page, NULL, PAGE_DIRECTION,
					      PAGE_NO_DIRECTION);
			page_header_set_field(page, NULL, PAGE_N_DIRECTION, 0);
		}
	}

	page_header_set_ptr(page, NULL, PAGE_LAST_INSERT, insert_rec);

	/* 7. It remains to update the owner record. */
	{
		rec_t*	owner_rec	= page_rec_find_owner_rec(insert_rec);
		ulint	n_owned;
		if (page_is_comp(page)) {
			n_owned = rec_get_n_owned_new(owner_rec);
			rec_set_n_owned_new(owner_rec, NULL, n_owned + 1);
		} else {
			n_owned = rec_get_n_owned_old(owner_rec);
			rec_set_n_owned_old(owner_rec, n_owned + 1);
		}

		/* 8. Now we have incremented the n_owned field of the owner
		record. If the number exceeds PAGE_DIR_SLOT_MAX_N_OWNED,
		we have to split the corresponding directory slot in two. */

		if (UNIV_UNLIKELY(n_owned == PAGE_DIR_SLOT_MAX_N_OWNED)) {
			page_dir_split_slot(
				page, NULL,
				page_dir_find_owner_slot(owner_rec));
		}
	}

	/* 9. Write log record of the insert */
	if (UNIV_LIKELY(mtr != NULL)) {
		page_cur_insert_rec_write_log(insert_rec, rec_size,
					      current_rec, index, mtr);
	}

	return(insert_rec);
}

page_cur_insert_rec_write_log函数

***********************************************************//**
Writes the log record of a record insert on a page. */
static
void
page_cur_insert_rec_write_log(
/*==========================*/
	rec_t*		insert_rec,	/*!< in: inserted physical record */
	ulint		rec_size,	/*!< in: insert_rec size */
	rec_t*		cursor_rec,	/*!< in: record the
					cursor is pointing to */
	dict_index_t*	index,		/*!< in: record descriptor */
	mtr_t*		mtr)		/*!< in: mini-transaction handle */
{
	ulint	cur_rec_size;
	ulint	extra_size;
	ulint	cur_extra_size;
	const byte* ins_ptr;
	const byte* log_end;
	ulint	i;

	/* Avoid REDO logging to save on costly IO because
	temporary tables are not recovered during crash recovery. */
	if (dict_table_is_temporary(index->table)) {
		byte*	log_ptr = mlog_open(mtr, 0);
		if (log_ptr == NULL) {
			return;
		}
		mlog_close(mtr, log_ptr);
		log_ptr = NULL;
	}

	ut_a(rec_size < UNIV_PAGE_SIZE);
	ut_ad(mtr->is_named_space(index->space));
	ut_ad(page_align(insert_rec) == page_align(cursor_rec));
	ut_ad(!page_rec_is_comp(insert_rec)
	      == !dict_table_is_comp(index->table));

	{
		mem_heap_t*	heap		= NULL;
		ulint		cur_offs_[REC_OFFS_NORMAL_SIZE];
		ulint		ins_offs_[REC_OFFS_NORMAL_SIZE];

		ulint*		cur_offs;
		ulint*		ins_offs;

		rec_offs_init(cur_offs_);
		rec_offs_init(ins_offs_);

		cur_offs = rec_get_offsets(cursor_rec, index, cur_offs_,
					   ULINT_UNDEFINED, &heap);
		ins_offs = rec_get_offsets(insert_rec, index, ins_offs_,
					   ULINT_UNDEFINED, &heap);

		extra_size = rec_offs_extra_size(ins_offs);
		cur_extra_size = rec_offs_extra_size(cur_offs);
		ut_ad(rec_size == rec_offs_size(ins_offs));
		cur_rec_size = rec_offs_size(cur_offs);

		if (UNIV_LIKELY_NULL(heap)) {
			mem_heap_free(heap);
		}
	}

	ins_ptr = insert_rec - extra_size;

	i = 0;

	if (cur_extra_size == extra_size) {
		ulint		min_rec_size = ut_min(cur_rec_size, rec_size);

		const byte*	cur_ptr = cursor_rec - cur_extra_size;

		/* Find out the first byte in insert_rec which differs from
		cursor_rec; skip the bytes in the record info */

		do {
			if (*ins_ptr == *cur_ptr) {
				i++;
				ins_ptr++;
				cur_ptr++;
			} else if ((i < extra_size)
				   && (i >= extra_size
				       - page_rec_get_base_extra_size
				       (insert_rec))) {
				i = extra_size;
				ins_ptr = insert_rec;
				cur_ptr = cursor_rec;
			} else {
				break;
			}
		} while (i < min_rec_size);
	}

	byte*	log_ptr;

	if (mtr_get_log_mode(mtr) != MTR_LOG_SHORT_INSERTS) {

		if (page_rec_is_comp(insert_rec)) {
			log_ptr = mlog_open_and_write_index(
				mtr, insert_rec, index, MLOG_COMP_REC_INSERT,
				2 + 5 + 1 + 5 + 5 + MLOG_BUF_MARGIN);
			if (UNIV_UNLIKELY(!log_ptr)) {
				/* Logging in mtr is switched off
				during crash recovery: in that case
				mlog_open returns NULL */
				return;
			}
		} else {
			log_ptr = mlog_open(mtr, 11
					    + 2 + 5 + 1 + 5 + 5
					    + MLOG_BUF_MARGIN);
			if (UNIV_UNLIKELY(!log_ptr)) {
				/* Logging in mtr is switched off
				during crash recovery: in that case
				mlog_open returns NULL */
				return;
			}

			log_ptr = mlog_write_initial_log_record_fast(
				insert_rec, MLOG_REC_INSERT, log_ptr, mtr);
		}

		log_end = &log_ptr[2 + 5 + 1 + 5 + 5 + MLOG_BUF_MARGIN];
		/* Write the cursor rec offset as a 2-byte ulint */
		mach_write_to_2(log_ptr, page_offset(cursor_rec));
		log_ptr += 2;
	} else {
		log_ptr = mlog_open(mtr, 5 + 1 + 5 + 5 + MLOG_BUF_MARGIN);
		if (!log_ptr) {
			/* Logging in mtr is switched off during crash
			recovery: in that case mlog_open returns NULL */
			return;
		}
		log_end = &log_ptr[5 + 1 + 5 + 5 + MLOG_BUF_MARGIN];
	}

	if (page_rec_is_comp(insert_rec)) {
		if (UNIV_UNLIKELY
		    (rec_get_info_and_status_bits(insert_rec, TRUE)
		     != rec_get_info_and_status_bits(cursor_rec, TRUE))) {

			goto need_extra_info;
		}
	} else {
		if (UNIV_UNLIKELY
		    (rec_get_info_and_status_bits(insert_rec, FALSE)
		     != rec_get_info_and_status_bits(cursor_rec, FALSE))) {

			goto need_extra_info;
		}
	}

	if (extra_size != cur_extra_size || rec_size != cur_rec_size) {
need_extra_info:
		/* Write the record end segment length
		and the extra info storage flag */
		log_ptr += mach_write_compressed(log_ptr,
						 2 * (rec_size - i) + 1);

		/* Write the info bits */
		mach_write_to_1(log_ptr,
				rec_get_info_and_status_bits(
					insert_rec,
					page_rec_is_comp(insert_rec)));
		log_ptr++;

		/* Write the record origin offset */
		log_ptr += mach_write_compressed(log_ptr, extra_size);

		/* Write the mismatch index */
		log_ptr += mach_write_compressed(log_ptr, i);

		ut_a(i < UNIV_PAGE_SIZE);
		ut_a(extra_size < UNIV_PAGE_SIZE);
	} else {
		/* Write the record end segment length
		and the extra info storage flag */
		log_ptr += mach_write_compressed(log_ptr, 2 * (rec_size - i));
	}

	/* Write to the log the inserted index record end segment which
	differs from the cursor record */

	rec_size -= i;

	if (log_ptr + rec_size <= log_end) {
		memcpy(log_ptr, ins_ptr, rec_size);
		mlog_close(mtr, log_ptr + rec_size);
	} else {
		mlog_close(mtr, log_ptr);
		ut_a(rec_size < UNIV_PAGE_SIZE);
		mlog_catenate_string(mtr, ins_ptr, rec_size);
	}
}
#else /* !UNIV_HOTBACKUP */
# define page_cur_insert_rec_write_log(ins_rec,size,cur,index,mtr) ((void) 0)
#endif /* !UNIV_HOTBACKUP */

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