InnoDB is a multi-versioned storage
engine, it must keep information about old versions of rows in the
tablespace. This information is stored in a data structure called
a rollback segment (after an analogous data
structure in Oracle).
InnoDB adds three fields to each
row stored in the database. A 6-byte
field indicates the transaction identifier for the last
transaction that inserted or updated the row. Also, a deletion is
treated internally as an update where a special bit in the row is
set to mark it as deleted. Each row also contains a 7-byte
DB_ROLL_PTR field called the roll pointer. The
roll pointer points to an undo log record written to the rollback
segment. If the row was updated, the undo log record contains the
information necessary to rebuild the content of the row before it
was updated. A 6-byte
DB_ROW_ID field contains
a row ID that increases monotonically as new rows are inserted. If
InnoDB generates a clustered index
automatically, the index contains row ID values. Otherwise, the
DB_ROW_ID column does not appear in any index.
InnoDB uses the information in the rollback
segment to perform the undo operations needed in a transaction
rollback. It also uses the information to build earlier versions
of a row for a consistent read.
Undo logs in the rollback segment are divided into insert and
update undo logs. Insert undo logs are needed only in transaction
rollback and can be discarded as soon as the transaction commits.
Update undo logs are used also in consistent reads, but they can
be discarded only after there is no transaction present for which
InnoDB has assigned a snapshot that in a
consistent read could need the information in the update undo log
to build an earlier version of a database row.
You must remember to commit your transactions regularly, including
those transactions that issue only consistent reads. Otherwise,
InnoDB cannot discard data from the update undo
logs, and the rollback segment may grow too big, filling up your
The physical size of an undo log record in the rollback segment is typically smaller than the corresponding inserted or updated row. You can use this information to calculate the space need for your rollback segment.
InnoDB multi-versioning scheme, a row is
not physically removed from the database immediately when you
delete it with an SQL statement. Only when
InnoDB can discard the update undo log record
written for the deletion can it also physically remove the
corresponding row and its index records from the database. This
removal operation is called a purge, and it is quite fast, usually
taking the same order of time as the SQL statement that did the
In a scenario where the user inserts and deletes rows in smallish
batches at about the same rate in the table, it is possible that
the purge thread starts to lag behind, and the table grows bigger
and bigger, making everything disk-bound and very slow. Even if
the table would carry just 10MB of useful data, it may grow to
occupy 10GB with all the dead rows. In such a case, it would be
good to throttle new row operations and allocate more resources
for the purge thread. Starting with MySQL 4.0.22 and 4.1.6, the
variable exists for exactly this purpose. See
Section 13.2.4, “
InnoDB Startup Options and System Variables”, for more information.