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

[Christoph Hellwig]

On Mon, Jun 22, 2015 at 09:36:50AM -0700, Dan Williams wrote:
> In that case "don't stack" is too coarse of a hammer.  I see this as a
> request to hide the subordinate ULD which is a new capability that DM
> and MD might benefit from as well.  We already have the case in MD
> where it internally holds a reference to bdev that has been hot
> removed, it seems not much of a stretch to have stacking drivers be
> able to hide device nodes for bdevs that they are holding.

I don't see why you're comparing with MD and DM here.  MD and DM
sit cleanly ontop of any block device.  If btt was independent of
libnvdimm and just used ->rw_bytes we could see it as this.

But it's all a giant entangled mess, where btt for example is probed
by libnvdimm.  At the same time pmem.c isn't really a true block
driver, it's really just a trivial shim between the block API
and pmem-style memcpy.  Especially with the proper pmem API btt
would become cleaner just calling that directly.  

> Yes, if they want to use DAX they should do it consciously and audit
> their application to be sure it is safe to abandon atomic sector
> guarantees.  With the current flexibility to do BTT on a partition
> they can do this conversion piecemeal and, for example, keep metadata
> on BTT and data on DAX.

By that logic you'd want to attach BTT by default and allow opt-out
at some level.  This could be a libnvmdimm-level partitioning scheme,
which would also allow storing the bit if BTT is used or not persistently.
Or it could be on fine grained boundaries which might be more useful.
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