At the moment scst_vdisk handler for FILEIO uses regular synchronous read/write() calls - and makes deep queue depth by using multiple threads. This is not too high performance - model of operations. It would be much better to use asynchronous I/O with not blocking - I/O calls.
- -In the user space native AIO is available for many years, but only very recently ability to - use it was added in the kernel. Changing FILEIO to use the new interface should significantly - (up to multiple times) increase performance of FILEIO devices.
- -At the moment, scst_vdisk handler doesn't support O_DIRECT option and possibility to set it - was disabled. This limitation caused by Linux kernel expectation that memory supplied to - read() and write() functions with O_DIRECT flag is mapped to some user space application. - Having O_DIRECT together with above asynchronous FILEIO would be another significant - performance boost for modern solid state devices. For instance, in fio utility - direct AIO long ago proven to be the fastest way to benchmark storage.
- -It is relatively easy to remove that limitation. Function dio_refill_pages() - should be modified to check before calling get_user_pages() if current->mm is not NULL. - If it is NULL, then, instead of calling get_user_pages(), dio->pages should be filled - by pages, taken directly from dio->curr_user_address. Each such page should be referenced - by page_cache_get(). That's all.
-In the pass-through mode (i.e. using the pass-through device handlers like