Re: [PATCH 14/15] libnvdimm: support read-only btt backing devices

[Christoph Hellwig <hch@lst.de>]

On Sun, Jun 21, 2015 at 08:11:25AM -0700, Dan Williams wrote:
> The labels only allow allocation of persistent media between pmem and
> blk.  For a given dimm you may access in either mode and the label
> records the decision.  We can have a btt on either the pmem or
> blk-mode disk type, or partition thereof.

Sounds like the spec should allow a btt type as well insteaad of
requiring the OS to work around it, as that seems to be one of the few
useful things to do with a run-time label.

Either way, partitions are trivial things and we could add them to the
nvdimm layer.

> Yes, it's this hybrid thing that mostly fits into the existing block
> device model save for two new block_device_operations
> ->direct_access() and ->rw_bytes().  We then use property of a
> block_device that allows it to be claimed for exclusive ownership by a
> filesystem or another block_device to layer storage semantics on top
> be it files+directories, raid, caching, or atomic sectors.  NVDIMM
> devices don't present the same complexity as MTD devices.  The only
> complexity they present is byte-address-ability, not erase-block-size,
> wear-leveling, etc...

I didn't say they show the same complexities, but the same layering.

> Good to hear that we don't need BTT for XFS v5, can we make the
> guarantee for all filesystems that may want to support DAX?  I still
> think stacking is a natural fit for this problem.

I can't make any guarantees, especially not without verification.  But
if correctly implemented any filesystems that does out of place metadata
writes (and that includes a traditional log) and uses checksum to ensure
the integrity of these updates it should be fine.  You'd still have
the issue of sector atomicy of file I/O though.
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