Re: XFS_IOC_FSEMAP requirements

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Dec 20, 2016 at 11:29:35AM +0100, Carlos Maiolino wrote:
> Hi folks,
> 
> I've been working on the implementation of FSEMAP ioctl as we've been
> discussed previously, and, the first discussion about this, was about
> using the same fiemap structures to retrieve free extents from the
> btrees.
> 
> By our last chat about it (it's been a while, I know, I got busy with
> more important stuff :), Dave suggested another uses for FSEMAP that
> were not in my mind, so, I think it deserves its own implementation,
> independent of fiemap from where the same idea came from.
> 
> So, I'd like to know, what else might FSEMAP be used for, beyond
> iterating free space extents, so I can think of a new struct to be
> exchanged between user<-> kernel. FSEMAP is supposed to complement
> GETFSMAPX, discussed in LSF this year, but I don't know if is there
> any plan to keep with GETFSMAPX or not, or even if FSEMAP is still a
> valuable idea :)
> 
> Any comments, suggestions about to what direction should FSEMAP go?

GETFSMAP reports free space extents along with the other space mappings.
If there is no rmapbt, the ioctl reports free space extents from the
bnobt and reports the non-free space as being owned by "unknown".
I was planning to send out the whole GETFSMAP + online scrub series for
review (for 4.11) after the 4.10 merge window closes.  Internally, the
online scrub kernel code cross-references space metadata against the
rmapbt if it's available.

For xfsprogs 4.11, the userspace online scrub tool uses the fsmap data
to figure out where to do media read testing after having the kernel
perform online checking of the metadata.  I also forward-ported spaceman
to current xfsprogs and getfsmap, so I'll be sending that out for review
for the 4.11 release too.

As for project ideas, I can think of a handful of them -- reworking the
in-core extent tree not to require large contiguous memory allocations,
sorting out reflink+dax, and stomping out the rest of the buffer head
usage, and all the rest of the ongoing cleanups and fix branches.
There's probably more, but let's see if Dave will chime in. :)

(I intend to track all this via google spreadsheet or something to keep
my head on straight.)

--D

> 
> Cheers
> -- 
> Carlos
> --
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