Kevin Closson's Blog: Platforms, Databases and Storage

Some of the topics in this post are also covered in the Julian Dyke/Steve Shaw RAC book that came out last summer. I enjoyed being one of the technical reviewers of the book. It is a good book.

Monitoring DBWR Specifically
I have received several emails from existing PolyServe customers asking me why I didn’t just use the Oracle Disk Manager (ODM) I/O monitoring package that is included in the PolyServe Database Utility for Oracle to show the multi-block DBWR writes I blogged about in this post. After all, there is very little left to the imagination when monitoring Oracle using this unique feature of our implementation of the Oracle Disk Manager library specification.

This URL will get you a copy of the I/O Monitoring feature of our Oracle Disk Manager library. It is quite a good feature.

I didn’t use ODM for the first part of that thread because I wanted to discuss using strace(1) for such purposes. Yes, I could have just used the mxodmstat(1) command that comes with that package and I would have seen that the average I/O size was not exactly db_block_size as one would expect. For instance, the following screen shot is an example of cluster wide monitoring of DBWR processes. The first invocation of the command is used to monitor DBWR only followed by another execution of the command to monitor LGWR. The average size of the async writes for DBWR are not precisely 8KB (the db_block_size for this database) as they would be if this was Oracle9i:

NOTE: You may have to right click->view the screen shot

As an aside, the system was pretty busy as the following screen shot will show. This is a non-RAC database on an HP Proliant DL-585 where database writes are peaking at roughly 140MB/s. You can also see that the service times (Ave ms) for the writes are averaging a bit high (as high as 30ms). Looks like I/O subsystem saturation.

Oh, here’s a quick peek at one nice feature of mxodmstat(1). You can dump out all the active files, clusterwide, for any number of database/instances and nodes using the –lf options:

I hope you take peek at the User Guide for this feature. It has a lot of examples of what the tool can do. You might find it interesting—perhaps something you should push your vendor to implement?