On 06/11/2015 12:29 PM, Christoph Hellwig wrote: > On Wed, Jun 10, 2015 at 08:11:42PM +0200, Matias Bjorling wrote: >> 1. A get/put flash block API, that user-space applications can use. >> That will enable application-driven FTLs. E.g. RocksDB can be integrated >> tightly with the SSD. Allowing data placement and garbage collection to >> be strictly controlled. Data placement will reduce the need for >> over-provisioning, as data that age at the same time are placed in the >> same flash block, and garbage collection can be scheduled to not >> interfere with user requests. Together, it will remove I/O outliers >> significantly. >> >> 2. Large drive arrays with global FTL. The stacking block device model >> enables this. It allows an FTL to span multiple devices, and thus >> perform data placement and garbage collection over tens to hundred of >> devices. That'll greatly improve wear-leveling, as there is a much >> higher probability of a fully inactive block with more flash. >> Additionally, as the parallelism grows within the storage array, we can >> slice and dice the devices using the get/put flash block API and enable >> applications to get predictable performance, while using large arrays >> that have a single address space. >> >> If it too much for now to get upstream, I can live with (2) removed and >> then I make the changes you proposed. > > In this case your driver API really isn't the Linux block API > anymore. I think the right API is a simple asynchronous submit with > callback into the driver, with the block device only provided by > the lightnvm layer. Agree. A group is working on a RocksDB prototype at the moment. When that is done, such an interface would be polished and submitted for review. The first patches here are to lay the groundwork for block I/O FTLs and generic flash block interface. > > Note that for NVMe it might still make sense to implement this using > blk-mq and a struct request, but those should be internal similar to > how NVMe implements admin commands. How about handling I/O merges? In the case where a block API is exposed with a global FTL, filesystems relies on I/O merges for improving performance. If using internal commands, merging has to implemented in the lightnvm stack itself, I rather want to use blk-mq and not duplicate the effort. I've kept the stacking model, so that I/Os go through the queue I/O path and then picked up in the device driver. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
