Thursday, May 29, 2008

Some more details about DiskSyncSize

The parameter DiskSyncSize is a MySQL Cluster parameter and was added in MySQL 5.1.23.

After the amount of stored bytes of data per file, the data node will fsync (flush) the LCP file to disk, even if a fsync is not needed for consistency.
This is done because the OS will otherwise buffer all the writes, and when a fsync is really needed, it can take a lot of time...

Originally this parameter was hard coded. Now it defaults to 4 Mbyte.

The parameter DiskSyncSize is related to the parameters NoOfDiskPagesToDiskAfterRestartTUP and NoOfDiskPagesToDiskAfterRestartACC which are deprecated right now. It does NOT replace the parameter TimeBetweenLocalCheckpoint.

This parameter should not be changed on any OS (with reasonable settings). With ODIRECT it is not used at all.

Thanks to Jonas for the help.


Johan Andersson said...

I guess it is related to DiskCheckpointSpeed nowadays (>= 5.1 ).

johan (

Jon Stephens said...

According to the information I have, it was added in 5.1.12, not 5.1.23.

In any case, it is available in all recent MySQL Cluster NDB 6.2 and 6.3 releases.