InnoDB uses simulated asynchronous disk I/O:
InnoDB creates a number of threads to take care
of I/O operations, such as read-ahead.
There are two read-ahead heuristics in
In sequential read-ahead, if
that the access pattern to a segment in the tablespace is
sequential, it posts in advance a batch of reads of database
pages to the I/O system.
In random read-ahead, if
that some area in a tablespace seems to be in the process of
being fully read into the buffer pool, it posts the remaining
reads to the I/O system.
InnoDB uses a novel file flush technique called
doublewrite. It adds safety to recovery
following an operating system crash or a power outage, and
improves performance on most varieties of Unix by reducing the
Doublewrite means that before writing pages to a data file,
InnoDB first writes them to a contiguous
tablespace area called the doublewrite buffer. Only after the
write and the flush to the doublewrite buffer has completed does
InnoDB write the pages to their proper
positions in the data file. If the operating system crashes in the
middle of a page write,
InnoDB can later find a
good copy of the page from the doublewrite buffer during recovery.
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