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

[Christoph Hellwig <hch@lst.de>]

On Sun, Jun 21, 2015 at 06:21:50AM -0700, Dan Williams wrote:
> This question has come up before.  Making btt an internal property of
> a device makes some things cleaner and others more messy.  We lose the
> ability to place a btt instance on top of a partition, rather than a
> whole disk.

I thought the addition of nfit labels avoids the need for a partition
table now?

> If we ever need to access the raw device we no longer
> have a direct block device to reference.  Linux has been doing stacked
> configurations to change the personality of block devices since
> forever (md, dm, bcache...), why invent something new to handle the
> btt-personality of ->rw_bytes() devices?

Because the underlying abstraction really isn't a block device
anymore, it's a byte addressable device.  This is more similar to
for example how the mtd subsystem is structured.

> BTT precludes DAX, if you want both modes on one pmem disk placing BTT
> on a partition of the disk for fs metadata and DAX-capable data on the
> rest is our proposed solution.  We chose this architecture after a
> conversation with Dave Chinner about XFS's need to have atomic sector
> guarantees for its metadata and wanting to simultaneously enable
> XFS-DAX.

I can't see why a v5 XFS filesystem with CRCs on all metadata would need
sector atomic updates any more.  But even in a case where it would it
seem like whatever label you use for partioning should sit above the
block layer.
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