Messages in this thread | | | Date | Sat, 13 Jun 2015 18:17:11 +0200 | From | Matias Bjorling <> | Subject | Re: [PATCH v4 0/8] Support for Open-Channel SSDs |
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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.
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