On Wed, Jun 17, 2015 at 4:54 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > Why do we need a new method in block_device_operations? > > The capacities of persistent memory make it too large to map as RAM (no > struct page coverage by default), so Linux arranges for it to be managed > as a block device. The bio interface to a block device enforces sector > at a time i/o and has infrastructure for asynchronous completion of i/o. > The ->rw_page() interface is closely tied to the page cache and also > carries asynchronous i/o completion assumptions. NVDIMM devices are > fast enough to complete i/o's synchronously (memcpy) and some in kernel > applications can take advantage of the byte-aligned (as opposed to > sector-aligned) nature of the media. The ->rw_bytes() operation is > added to fill this role that does not fit into any existing access > method. > > It could be argued that a ->rw_bytes() method makes a struct > block_device not a *block* device. However, the applications for > persistent memory as storage devices makes them more "block" devices > than "character" devices. > > The first consumer of the ->rw_bytes() capability is a stacked > block_device driver (BTT - block translation table) that adds atomic > sector update semantics on top of an nvdimm storage device. > > Why enable drivers like BTT on top of a new globally visibly > block_device_operations op rather than an internal detail of nvdimm > drivers? > > 1/ We want ->rw_bytes() consumers to be enabled on either a per-disk or > per-partition basis. Consider the case of enabling DAX+XFS on a single > persistent memory disk whereby the metadata needs atomic sector update > guarantees, but the data would like to be DAX capable. Solution is to > create two partitions and enable BTT on the "metadata/XFS-logdev" > partition. > > 2/ We want this configuration topology to be visible to the sysfs device > model, and not an internal detail of nvdimm drivers requiring special > tooling. For example if you ever wanted to "fsck" BTT metadata that > could be carried out on the raw nvdimm device directly rather than > require custom tooling / mechanisms to access the raw media. > > 3/ It becomes trivial to add new BTT like drivers without touching the > nvdimm drivers to add is_btt_mode(), is_foo_mode(), etc... checks in the > fast path. > Acked-by: Jens Axboe <axboe@xxxxxx> ...off list. Christoph, I assume that satisfies your primary concern with the BTT infrastructure and implementation? -- 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
