On Mon, Dec 19, 2016 at 5:09 PM, Darrick J. Wong
[off-list ref] wrote:

On Mon, Dec 19, 2016 at 02:11:49PM -0700, Ross Zwisler wrote:

quoted

On Fri, Sep 16, 2016 at 03:54:05PM +1000, Nicholas Piggin wrote:
<>

quoted

Definitely the first step would be your simple preallocated per
inode approach until it is shown to be insufficient.

Reviving this thread a few months later...

Dave, we're interested in taking a serious look at what it would take to get
PMEM_IMMUTABLE working.  Do you still hold the opinion that this is (or could
become, with some amount of work) a workable solution?

We're happy to do the grunt work for this feature, but we will probably need
guidance from someone with more XFS experience.  With you out on extended leave
the first half of 2017, who would be the best person to ask for this guidance?
Darrick?

Yes, probably. :)

I think where we left off with this (on the XFS side) is some sort of
fallocate mode that would allocate blocks, zero them, and then set the
DAX and PMEM_IMMUTABLE on-disk inode flags.  After that, you'd mmap the
file and thereby gain the ability to control write persistents behavior
without having to worry about fs metadata updates.  As an added plus, I
think zeroing the pmem also clears media errors, or something like that.

<shrug> Is that a reasonable starting point?  My memory is a little foggy.

Hmm, I see Dan just posted something about blockdev fallocate.  I'll go
read that.

That's for device-dax, which is basically a poor man's PMEM_IMMUTABLE
via a character device interface. It's useful for cases where you want
an entire nvdimm namespace/volume in "no fs-metadata to worry about"
mode.  But, for sub-allocations of a namespace and support for
existing tooling, PMEM_IMMUTABLE is much more usable.

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