Oracle 5.0 Reference Manual page 1597

Effective Version
Restart Type: N
The UNDO index buffer, whose size is set by this parameter, is used during local checkpoints. The
storage engine uses a recovery scheme based on checkpoint consistency in conjunction with
NDB
an operational REDO log. To produce a consistent checkpoint without blocking the entire system for
writes, UNDO logging is done while performing the local checkpoint. UNDO logging is activated on
a single table fragment at a time. This optimization is possible because tables are stored entirely in
main memory.
The UNDO index buffer is used for the updates on the primary key hash index. Inserts and deletes
rearrange the hash index; the NDB storage engine writes UNDO log records that map all physical
changes to an index page so that they can be undone at system restart. It also logs all active insert
operations for each fragment at the start of a local checkpoint.
Reads and updates set lock bits and update a header in the hash index entry. These changes are
handled by the page-writing algorithm to ensure that these operations need no UNDO logging.
This buffer is 2MB by default. The minimum value is 1MB, which is sufficient for most applications.
For applications doing extremely large or numerous inserts and deletes together with large
transactions and large primary keys, it may be necessary to increase the size of this buffer. If this
buffer is too small, the NDB storage engine issues internal error code 677
overloaded).
Important
It is not safe to decrease the value of this parameter during a rolling restart.
•
UndoDataBuffer
Effective Version
MySQL 5.0.0
Restart Type: N
This parameter sets the size of the UNDO data buffer, which performs a function similar to that of
the UNDO index buffer, except the UNDO data buffer is used with regard to data memory rather
than index memory. This buffer is used during the local checkpoint phase of a fragment for inserts,
deletes, and updates.
Because UNDO log entries tend to grow larger as more operations are logged, this buffer is also
larger than its index memory counterpart, with a default value of 16MB.
This amount of memory may be unnecessarily large for some applications. In such cases, it is
possible to decrease this size to a minimum of 1MB.
It is rarely necessary to increase the size of this buffer. If there is such a need, it is a good idea to
check whether the disks can actually handle the load caused by database update activity. A lack of
sufficient disk space cannot be overcome by increasing the size of this buffer.
If this buffer is too small and gets congested, the NDB storage engine issues internal error code 891
(Data UNDO buffers
Important
It is not safe to decrease the value of this parameter during a rolling restart.
•
MySQL Cluster Configuration Files
Type/Units
Type/Units
unsigned
overloaded).
1577
Default
Default
16M
Range/Values
(Index UNDO buffers
Range/Values
1M - 4G