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Measures reported by SybDeviceTest
The SybDeviceTest,
executed by an internal agent, tracks statistics pertaining to the devices
of the Sybase adaptive server. The measures
made by this test are as
follows:
| Measurement |
Description |
Measurement
Unit |
Interpretation |
| IO_requests
|
The average number of IO requests for a device |
IOs/Sec
|
|
| Phy_read_rate
|
The average number of reads performed on a device |
Reads/Sec
|
|
| Phy_write_rate
|
The average number of writes performed on a device |
Writes/Sec
|
|
| IO_errors
|
The percentage of times there was an error in IO requests |
Percent
|
|
| IO_waits
|
The percentage of times the semaphore was busy and the IO operation had to wait for the semaphore to be released |
Percent
|
When Adaptive Server needs to perform a disk I/O, it gives the task 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 large value could indicate a semaphore contention issue. One solution might be to redistribute the data on the physical devices.
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