InnoDB implements a checkpoint mechanism
known as “fuzzy” checkpointing.
InnoDB flushes modified database pages from
the buffer pool in small batches. There is no need to flush the
buffer pool in one single batch, which would in practice stop
processing of user SQL statements during the checkpointing
During crash recovery,
InnoDB looks for a
checkpoint label written to the log files. It knows that all
modifications to the database before the label are present in
the disk image of the database. Then
scans the log files forward from the checkpoint, applying the
logged modifications to the database.
InnoDB writes to its log files on a rotating
basis. It also writes checkpoint information to the first log
file at each checkpoint. All committed modifications that make
the database pages in the buffer pool different from the images
on disk must be available in the log files in case
InnoDB has to do a recovery. This means that
InnoDB starts to reuse a log file, it
has to make sure that the database page images on disk contain
the modifications logged in the log file that
InnoDB is going to reuse. In other words,
InnoDB must create a checkpoint and this
often involves flushing of modified database pages to disk.
The preceding description explains why making your log files very large may reduce disk I/O in checkpointing. It often makes sense to set the total size of the log files as large as the buffer pool or even larger. The disadvantage of using large log files is that crash recovery can take longer because there is more logged information to apply to the database.
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