I/O Express is installed on my cluster, but no caching is taking place

Why isn't I/O Express caching all of my disks?

There are a number of reasons why I/O Express may not be caching.

1.Was I/O Express started using /NOAUTOMATIC?

If so, you will have to either not use the /NOAUTOMATIC switch, or else manually tell I/O Express to cache each disk.

2. Is I/O Express started on every node of the cluster?

If you have not started I/O Express on each node in the cluster, the device driver will not be loaded on all nodes. I/O Express will not start caching on any node if the device driver is not loaded on all nodes. To check this, type IOX SHOW CACHE. At the bottom of the display will be a list of all devices I/O Express is currently executing on.

3. Are there sufficient CDTEs on each node?

I/O Express requires one Cluster Descriptor Table Entry (CDTE) per node to establish its communication between nodes in a cluster. The default value for CDTEs is 40. However, with I/O Express on the system, there must be 6 CDTEs per node and an additional 12 free per node in order to allow two more nodes to join the cluster. To determine the number of CDTEs that each node in the cluster needs, multiply the number of nodes in the cluster by 8. Then add 12 to that number to find out how many CDTEs there should be on each node. CDTEs are represented in SYSGEN by the parameter SCSCONNCNT. Refer to the FAQ Some nodes say I/O Express is not loaded on all nodes for more details.

4. Was there a demo or an earlier version of I/O Express on this system?

If there was a demo of I/O Express, or a new version of I/O Express has just been installed, the old driver may still be installed on one or more nodes in the system. Whenever you upgrade versions of I/O Express, you must reboot the system without starting I/O Express. This will remove the old device driver. Then, once the system is up and running, start I/O Express on each node. This will load the new device driver.

5. Is there enough activity to maintain a cache?

I/O Express evaluates three criteria in determining whether or not to begin or continue caching on a disk. First, there must be 60 I/Os a minute averaged over a ten minute period. Second, at least 25% of the I/O activity on the disk must be reads. Third, a 10% hit rate must be maintained for caching to continue. If these three criteria are not met, then I/O Express will not be caching that disk.

6. Is there enough available memory to maintain a cache?

There may not be enough free memory available on the system for I/O Express to begin or continue caching. On each node, run IOX MONITOR DISK for any disk and note the "Cache Blocks In Use" and "Cache Blocks Available" values. If these are both zero, then there is no memory available to build a cache. If "Cache Blocks Available" is zero, then all available memory is in use; there is nothing left to cache another disk.

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