For writing the transaction log to disk Derby uses a RandomAccessFile. If it is supported by the JVM, the log files are opened in "rws" mode making the file system take care of syncing writes to disk. "rws" mode will ensure that both the data and the file meta-data is updated for every write to the file. On most operating system this leads to two write operation to the disk for every write issued by Derby. This is limiting the throughput of update intensive applications.
I have run some simple tests where I have changed mode from "rws" to "rwd" for the Derby log file. When running a small numbers of concurrent client threads the throughput is almost doubled and the response time is almost halved. I am enclosing two graphs that show this when running a given number of concurrent "tpc-b" clients. The graphs show the throughput when running with "rws" and "rwd" mode when the disk's write cache has been enabled and disabled. This change should also have a positive impact on the Derby startup time (DERBY-1664) and derbyall. With this change the time for running derbyall goes down by about 10-15 minutes (approximately 10%) :-) Is there anyone that is aware of any issues by not updating the file meta-data for every write? Is there any recovery scenarios where this can make recovery fail? Derby seems to preallocates the log file before starting using the file, so I think this should not influence the ability to fine the last data written to the file after a power failure. Any comments? Thanks, Olav
