Sybase Device I/O Test

This test monitors the I/O activity on each device on the Sybase server, and proactively alerts administrators to contention for device I/O semaphores. To ensure that this test runs smoothly, enable the enable monitoring configuration parameter on the Sybase server.

Target of the test : A Sybase ASE server on which the MDA tables have been installed

Agent deploying the test : An internal agent

Outputs of the test : One set of results for every Sybase ASE server being monitored.

Configurable parameters for the test
Parameter Description

Test Period

How often should the test be executed.


Refers to the IP address of the Sybase server.


The port number at which the Sybase server listens to.


A Sybase user who has the “sa_role”.


The password corresponding to the above user.

Confirm Password

Confirm the password by retyping it here.

Measurements made by the test
Measurement Description Measurement Unit Interpretation


The number of reads that occurred on the device during the last measurement period.



APF reads

Indicates the number of APF reads that occurred on this device during the last measurement period. 


Asynchronous prefetch (APF) issues I/O requests for pages before the query needs them so that most pages are in cache by the time query processing needs to access the page. High APF activity can hence improve performance for:

  • Sequential scans, such as table scans, clustered index scans, and covered nonclustered index scans
  • Access via nonclustered indexes
  • Some dbcc checks and update statistics
  • Recovery


Indicates the number of writes that occurred on this device during the last measurement period.




The number of requests from this device during the last measurement period.



IO waits

The number of I/O requests to this device that were waiting during the last measurement period.



Total io time

The time taken to read from or write to this device during the last measurement period.


A high value of this measure indicates that the device is taking too much time to process I/O requests. This could be owing to any of the following reasons:

  • An I/O overload on the device;
  • Disk fragmentation on the device;
  • Table fragmentation

Percentage io granted

The percentage of IO granted.


Ideally, this value should be high.

When Adaptive Server needs to perform a disk I/O, it gives the task to the semaphore for that device in order to acquire a block I/O structure. On SMP systems, multiple engines can try to post I/Os to the same device simultaneously. This creates contention for that semaphore, especially if there are hot devices or if the data is not well distributed across devices.

A low value for this measure indicates that many I/O requests were waiting for the semaphore – in other words, it indicates a contention for the semaphore.  One solution might be to redistribute the data on the physical devices.