HCM and Multiple-drive Write Operations
When using InveStore, if you are not seeing the expected physical drive activity during operation, keep in mind that drive activity varies based on your configured write mode.
· When writing in LAZY WRITE mode, all writes are first cached to memory and then flushed in the background in a contiguous fashion that optimizes throughput and reduces file system overhead on disk. This write process uses only one drive. Therefore you won't necessarily see activity on any more than one drive as InveStore is managing the write process in the background. Please note that the user process continues normally and does not have to wait for the data to be "flushed" while in LAZY WRITE mode.
· For file reads in LAZY WRITE mode, if a set of data remains in cache there is no need to access the storage media itself and therefore you won't see any drive activity. If the data is not in the RAM cache and resides only on storage then one or more drives will be accessed to retrieve and deliver the requested files.
· Writing in the WRITE THROUGH mode will use multiple drives IF data is being written to multiple disks at once. The same applies to FLUSH ON CLOSE but with less frequency than with WRITE THROUGH. This does not necessarily equate to better file system performance. LAZY WRITE optimizes overall file system write performance, even though it gives the appearance of single threaded activity.
· Reading in WRITE THROUGH and FLUSH ON CLOSE is the same as LAZY WRITE.
· If the host adapter is improperly set then Multi-threaded I/O will be impossible. ALL devices on the bus MUST be set to disconnect and asynchronous I/O. If this is not done our I/O will appear to be single threaded
· Using the larger storage libraries in LUN mode will also prevent multi-threaded writes. The adapter card will NOT send multiple requests to the same target ID, but queues each until completion. (This process is not controlled by InveStore). Your library MUST be in target mode to obtain multi-threaded writes.
To help performance, try increasing both the LOCK TIME and HOLD TIME settings in the Performance Mangers display, accessed under the Administrator Tools option in the main console screen. If you're concerned about the lack of overall drive activity, set your system to WRITE_THROUGH and test performance.
If you are writing a large quantity of data to a single volume you may want to set the bias for one of the drives to WRITE ONLY (this is set under the drive details via the Server view, under the Library Drive Settings option). This will allow the individual drive to handle write requests only.