IBM DS8000 manual Boot device support, AIX on IBM iSeries, Direct I/O, Concurrent I/O

Tip: If the number of async I/O (AIO) requests is high, then the recommendation is to increase maxservers to approximately the number of simultaneous I/Os there might be. In most cases, it is better to leave the minservers parameter to the default value since the AIO kernel extension will generate additional servers if needed. By looking at the CPU utilization of the AIO servers, if the utilization is even across all of them, that means that they’re all being used; you may want to try increasing their number in this case. Running pstat -awill allow you to see the AIO servers by name, and running ps -kwill show them to you as the name kproc.

Direct I/O

An alternative I/O technique called Direct I/O bypasses the Virtual Memory Manager (VMM) altogether and transfers data directly from the user’s buffer to the disk and vice versa. The concept behind this is similar to raw I/O in the sense that they both bypass caching at the file system level. This reduces CPU overhead and makes more memory available to the database instance, which can make more efficient use of it for its own purposes.

Direct I/O is provided as a file system option in JFS2. It can be used either by mounting the corresponding file system with the mount –o dio option, or by opening a file with the O_DIRECT flag specified in the open() system call. When a file system is mounted with the –o dio option, all files in the file system use Direct I/O by default.

Direct I/O benefits applications that have their own caching algorithms by eliminating the overhead of copying data twice, first between the disk and the OS buffer cache, and then from the buffer cache to the application’s memory.

For applications that benefit from the operating system cache, Direct I/O should not be used, because all I/O operations would be synchronous. Direct I/O also bypasses the JFS2 read-ahead. Read-ahead can provide a significant performance boost for sequentially accessed files.

Concurrent I/O

In 2003, IBM introduced a new file system feature called Concurrent I/O (CIO) for JFS2. It includes all the advantages of Direct I/O and also relieves the serialization of write accesses. It improves performance for many environments, particularly commercial relational databases. In many cases, the database performance achieved using Concurrent I/O with JFS2 is comparable to that obtained by using raw logical volumes.

A method for enabling the concurrent I/O mode is to use the mount -o cio option when mounting a file system.

Boot device support

The DS8100 and DS8300 are supported as boot devices on RS/6000 and pSeries that support Fibre Channel boot capability. This support is not currently available for the IBM eServer BladeCenter. Refer to DS8000 Host Systems Attachment Guide, SC26-7628, for additional information.

AIX on IBM iSeries

With the announcement of the IBM iSeries i5, it is now possible to run AIX in a partition on the i5. This can be either AIX 5L V5.2 or V5.3. All supported functions of these operating system levels are supported on i5, including HACMP for high availability and external boot from Fibre Channel devices.

Appendix A. Open systems operating systems specifics 353