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Re: [PATCH 01/15] block: introduce an ->rw_bytes() block device operation

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On Wed, Jun 17, 2015 at 4:54 PM, Dan Williams <dan.j.williams@intel.com> 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@fb.com> ...off list.

Christoph, I assume that satisfies your primary concern with the BTT
infrastructure and implementation?
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[PATCH 00/15] libnvdimm: ->rw_bytes(), BLK-mode, unit tests,  and misc features Dan Williams <dan.j.williams@intel.com> - 2015-06-18 02:00 +0200
  [PATCH 12/15] libnvdimm: enable iostat Dan Williams <dan.j.williams@intel.com> - 2015-06-18 02:00 +0200
    Re: [PATCH 12/15] libnvdimm: enable iostat Christoph Hellwig <hch@lst.de> - 2015-06-19 10:40 +0200
      Re: [PATCH 12/15] libnvdimm: enable iostat Dan Williams <dan.j.williams@intel.com> - 2015-06-19 11:10 +0200
        Re: [PATCH 12/15] libnvdimm: enable iostat Christoph Hellwig <hch@lst.de> - 2015-06-21 12:20 +0200
  [PATCH 07/15] fs/block_dev.c: skip rw_page if bdev has integrity Dan Williams <dan.j.williams@intel.com> - 2015-06-18 02:00 +0200
  [PATCH 13/15] libnvdimm: flag libnvdimm block devices as  non-rotational Dan Williams <dan.j.williams@intel.com> - 2015-06-18 02:00 +0200
  [PATCH 01/15] block: introduce an ->rw_bytes() block device  operation Dan Williams <dan.j.williams@intel.com> - 2015-06-18 02:00 +0200
    Re: [PATCH 01/15] block: introduce an ->rw_bytes() block device operation Dan Williams <dan.j.williams@intel.com> - 2015-06-18 21:30 +0200
  [PATCH 09/15] libnvdimm, blk: add support for blk integrity Dan Williams <dan.j.williams@intel.com> - 2015-06-18 02:00 +0200
  [PATCH 15/15] libnvdimm, nfit: handle acpi_nfit_memory_map flags Dan Williams <dan.j.williams@intel.com> - 2015-06-18 02:00 +0200
  [PATCH 11/15] libnvdimm: pmem, blk, and btt make_request cleanups Dan Williams <dan.j.williams@intel.com> - 2015-06-18 02:10 +0200
    Re: [PATCH 11/15] libnvdimm: pmem, blk, and btt make_request  cleanups Christoph Hellwig <hch@lst.de> - 2015-06-21 12:20 +0200
  [PATCH 08/15] libnvdimm, btt: add support for blk integrity Dan Williams <dan.j.williams@intel.com> - 2015-06-18 02:10 +0200
  [PATCH 02/15] libnvdimm: infrastructure for btt devices Dan Williams <dan.j.williams@intel.com> - 2015-06-18 02:10 +0200
  [PATCH 10/15] libnvdimm: fix up max_hw_sectors Dan Williams <dan.j.williams@intel.com> - 2015-06-18 02:10 +0200
    Re: [PATCH 10/15] libnvdimm: fix up max_hw_sectors Christoph Hellwig <hch@lst.de> - 2015-06-21 12:10 +0200
  Re: [PATCH 03/15] nd_btt: atomic sector updates Christoph Hellwig <hch@lst.de> - 2015-06-21 12:10 +0200
    Re: [PATCH 03/15] nd_btt: atomic sector updates Dan Williams <dan.j.williams@intel.com> - 2015-06-21 18:40 +0200
  Re: [PATCH 04/15] libnvdimm, nfit, nd_blk: driver for BLK-mode  access persistent memory Christoph Hellwig <hch@lst.de> - 2015-06-21 12:10 +0200
    Re: [PATCH 04/15] libnvdimm, nfit, nd_blk: driver for BLK-mode access  persistent memory Dan Williams <dan.j.williams@intel.com> - 2015-06-21 16:50 +0200

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