Estimating concurrent I/O requests
The number of concurrent I/O requests is important when you calculate the number of data paths for your DB2® subsystem.
About this task
DB2 has a multi-tasking structure in which each user's request runs under a different task control block (TCB). In addition, the DB2 system itself has its own TCBs and SRBs for logging and database writes.
Procedure
To estimate the maximum number of concurrent I/O requests when your system is loaded with data:
Use the following formula: 
MAX USERS + 600 prefetches + 600 asynchronous writes (A
maximum of 300 deferred write engines and 300 castout engines asynchronous writes are supported)
MAX USERS + 600 prefetches + 600 asynchronous writes (A
maximum of 300 deferred write engines and 300 castout engines asynchronous writes are supported)