Re: What is better asychronous IO or multiple db_writers?

From: Steve Adams <steveadams_at_acslink.net.au>
Date: 1998/01/19
Message-ID: <34c3d97c.74764416@newsserver.trl.oz.au>#1/1

Hello Neil, Peter, Matt and others,

Here are some quotes from the "Oracle8 Server Installation Guide for HP 9000" under the heading "Using Asynchronous I/O": "The asynchronous I/O pseudo-driver on HP-UX allows the Oracle Server to perform I/O to raw disk partitions using an asynchronous method, resulting in less I/O overhead and higher throughput." "Oracle Corporation recommends asynchronous I/O on Series 800 systems if the database files are created with raw disk partitions." This is followed by a detailed explanation of how to set it up.

There is an alternate asynchronous I/O mechanism in HP-UX (fs_async) which applies to filesystems and does not affect raw I/O. As you have stated, Neil, this is NOT a good idea. It can corrupt your database and your filesystems as well. However, your conculsion does not follow. Using async I/O on raw datafiles provides much higher write bandwidth and scalability than multiple DB writers on either filesystem based or raw datafiles.

In HP-UX 11, I believe that there will be a new asynchronous I/O mechanism for filesystem based datafiles, which uses the same technology as Solaris do presently -- namely, simulating multiple DBWR's in the single process using threads. Nevertheless, true asynchronous I/O on raw devices should still outperform this significantly.

Regards,
Steve Adams
steveadams_at_acslink.net.au

---

On 19 Jan 1998 15:54:41 GMT, oracle_at_tchp2.tcamuk.stratus.com (Neil Chandler) wrote:

>As far a I remember, if you switch on Async-IO on HP-UX (Kernel Parameter)
>and have you datafiles in filesystems (ADFS or BSD 4.2),
>you can corrupt your database.
>
>Use multiple DB Writers - (about 2 x No-of-disks works well for me)
>
>>As far as I know, on HP-UX async IO is only possible if you use raw
>>devices. If you use file system based files then you have to simulate async
>>IO thru multiple db writers.
>>
>>> We have several 7.3.3 databases running on hp 10.20. What are the pluses
>>> and minuses of both asychronous IO and multiple DB_WRITERS? Also, has
>>> anyone had problems with either of these on HP?
Received on Mon Jan 19 1998 - 00:00:00 CST