On Thu, Jun 11, 2015 at 1:11 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > From: Vishal Verma <vishal.l.verma@xxxxxxxxxxxxxxx> > > BTT stands for Block Translation Table, and is a way to provide power > fail sector atomicity semantics for block devices that have the ability > to perform byte granularity IO. It relies on the capability of libnvdimm > namespace devices to do byte aligned IO. > > The BTT works as a stacked blocked device, and reserves a chunk of space > from the backing device for its accounting metadata. It is a bio-based > driver because all IO is done synchronously, and there is no queuing or > asynchronous completions at either the device or the driver level. > > The BTT uses 'lanes' to index into various 'on-disk' data structures, > and lanes also act as a synchronization mechanism in case there are more > CPUs than available lanes. We did a comparison between two lane lock > strategies - first where we kept an atomic counter around that tracked > which was the last lane that was used, and 'our' lane was determined by > atomically incrementing that. That way, for the nr_cpus > nr_lanes case, > theoretically, no CPU would be blocked waiting for a lane. The other > strategy was to use the cpu number we're scheduled on to and hash it to > a lane number. Theoretically, this could block an IO that could've > otherwise run using a different, free lane. But some fio workloads > showed that the direct cpu -> lane hash performed faster than tracking > 'last lane' - my reasoning is the cache thrash caused by moving the > atomic variable made that approach slower than simply waiting out the > in-progress IO. This supports the conclusion that the driver can be a > very simple bio-based one that does synchronous IOs instead of queuing. > Copy / paste error... following 2 paragraphs were from the previous version of this patch and will be deleted when pushing upstream. > BTT stands for Block Translation Table, and is a way to provide power > fail sector atomicity semantics for block devices that have the ability > to perform byte granularity IO. It relies on the ->rw_bytes() capability > of libnd namespace devices. > > The BTT works as a stacked blocked device, and reserves a chunk of space > from the backing device for its accounting metadata. -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
