fallocate mode flag for "unshare blocks"?

[Christoph Hellwig]

On Wed, Mar 30, 2016 at 02:58:38PM -0400, Austin S. Hemmelgarn wrote:
> Nothing that I can find in the man-pages or API documentation for Linux's
> fallocate explicitly says that it will be fast.  There are bits that say it
> should be efficient, but that is not itself well defined (given context, I
> would assume it to mean that it doesn't use as much I/O as writing out that
> many bytes of zero data, not necessarily that it will return quickly).

And that's pretty much as narrow as an defintion we get.  But apparently
gfs2 already breaks that expectation :(

> >delalloc system is careful enough to check that there are enough free
> >blocks to handle both the allocation and the metadata updates.  The
> >only gap in this scheme that I can see is if we fallocate, crash, and
> >upon restart the program then tries to write without retrying the
> >fallocate.  Can we trade some performance for the added requirement
> >that we must fallocate -> write -> fsync, and retry the trio if we
> >crash before the fsync returns?  I think that's already an implicit
> >requirement, so we might be ok here.
> Most of the software I've seen that doesn't use fallocate like this is
> either doing odd things otherwise, or is just making sure it has space for
> temporary files, so I think it is probably safe to require this.

posix_fallocate gurantees you that you don't get ENOSPC from the write,
and there is plenty of software relying on that or crashing / cause data
integrity problems that way.