Async io & dbwr

Question

my box has Solaris 5.8,veritas FS, ODM,Oracle9206, 56CPU,120G RAM, DSS kind of application but high dataload
1.db_writer_process(7)
2.disk_async_io(async)
3.filesystem_io_option(async)
4.parallel_automatic_tuning(true)
5.parallel_min_Servers(16)
6.parallel_max_servers(160)
settings. If 2 is async what could be the value of 1 and
filesystem_io_option(async). my knowledge is multiple writer process will help in no-async environment. Is there any negative impact if 1# is > 1. I have PIOT with buffer busy waits,defile sequential read, To reduce the buffer busy waits and all, shall we do the frequent checkpointing or increase the db_writer process.
Also I am getting ora-1555 even after increasing undo from 65 to 230G. and retention 6 to 25hr. Most of the queries are parallel queries/sub queries. what could be the proper settings
thanks

Answer 1

If you are really using ASYNC IO then you should only
need one dbwr process.
HTH -- Mark D Powell --Not quite true
http://download-west.oracle.com/docs/cd/B19306_01/server.102/b14211/instance_tune.htm#sthref909
10.3.9.3.3 Choosing Between Multiple DBWR Processes and I/O Slaves
Configuring multiple DBWR processes benefits performance when a single DBWR process is unable to keep up with the required workload. However, before configuring multiple DBWR processes, check whether asynchronous I/O is available and configured on the system. If the system supports asynchronous I/O but it is not currently used, then enable asynchronous I/O to see if this alleviates the problem. If the system does not support asynchronous I/O, or if asynchronous I/O is already configured and there is still a DBWR bottleneck, then configure multiple DBWR processes.
Note:
If asynchronous I/O is not available on your platform, then asynchronous I/O can be disabled by setting the DISK_ASYNCH_IO initialization parameter to FALSE.
Using multiple DBWRs parallelizes the gathering and writing of buffers. Therefore, multiple DBWn processes should deliver more throughput than one DBWR process with the same number of I/O slaves. For this reason, the use of I/O slaves has been deprecated in favor of multiple DBWR processes. I/O slaves should only be used if multiple DBWR processes cannot be configured.