Jens, While working on one issue of less IOPs for sequential READ/WRITE, I found interesting stuffs which was causing performance drop for sequential IO. I did some reverse engineering on Block layer code to understand how to get benefit from any sysfs parameters settings, but could not find anything useful to solve this issue.
I have described problem statement and root cause of this issue in this mail thread. Problem statement - "Cannot achieve Sequential read/write performance, because of back merge is not happening frequently" Here is my understanding of back merge done in elevator. Linux block layer is responsible for merging/sorting of the IO with the help of Elevator hook in OS + IO scheduler. IO scheduler does not have any role in merging sequential IO. It is done in Elevator hook, so choosing any IO scheduler in Linux will not help (OR you can consider that behavior will be unchanged irrespective of IO scheduler). Any sequential IO will be merge at Elevator code path. 1. When IO comes from upper layer, it will be queued at Elevator/IO scheduler level. It will also add IO into hash look up which will be used for merge and other purpose. 2. Elevator code will search any outstanding IO (in the queue of the same layer). If there is any chance to merge it, it will perform (BACK MERGE) merge 3. If there is no merge possible, IO will be queued to the next level (which is IO scheduler). 4. In IO completion Path, IO scheduler will post IO to the Driver queue, if at all there is any outstanding IO. (There are many other condition, but this is very common code path) To merge more command, #2 should find more outstanding in hash table look up. This is possible if flow control start either at block layer/Driver level. It means, driver/block layer forcefully delay IO submission to next level and give more chance at elevator code to merge more IO via accumulating more IO from user space. If I manually change queue depth of the device to lower value (between 1-8), which is doing only Sequential IO.. I am able to see maximum IO coming to the driver after merge and it eventually increase the IOPs. Is there any way to increase possibility of merged IO coming from block layer to the Low level driver ? Thanks, Kashyap -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
