The use of the dsync setting with database device files incurs the following performance trade-offs:Adaptive Server does not support asynchronous I/O on operating system files for HP-UX and Digital UNIX.
If database device files on these platforms use the dsync option, then the Adaptive Server engine writing to the device file will block until the write operation completes. This can cause poor performance during update operations.
When dsync is on, write operations to database device files may be slower compared to previous versions of Adaptive Server (where dsync is not supported). This is because Adaptive Server must write data to disk instead of simply copying cached data to the UNIX file system buffer.
In cases where highest write performance is required (but data integrity after a system failure is not required) turning dsync off yields device file performance similar to earlier Adaptive Server versions. For example, you may consider storing tempdb on a dedicated device file with dsync disabled, if performance is not acceptable while using dsync.
Response time for read operations is generally better for devices stored on UNIX operating system files as compared to devices stored on raw partitions. Data from device files can benefit from the UNIX file system cache as well as the Adaptive Server cache, and more reads may take place without requiring physical disk access.
The disk init command takes longer to complete with previous Adaptive Server versions, because the required disk space is allocated during device initialization.
