Re: [Bug-tar] [patch] O_NOATIME support

[Paul Eggert]

Ian Turner <address@hidden> writes:

> Because of these limitations, and because of the very different
> side-effects, I have added a new option --no-atime,

Thanks for bringing this to our attention.  It would be very nice to
add this sort of thing to GNU tar.

> rather than extend the existing --preserve-atime option. The option
> exists (as a no-op), even if configure did not find required system
> headers.

Does that sound wise?  If 'tar' can't implement --no-atime correctly,
wouldn't it be better to reject the option?

> --no-atime does not at present interact with --preserve-atime
> (though you can use them both); it might be useful to do the "best
> thing" if the user specifies both,

Could you elaborate on this a bit?  It will confuse users to have both
options.  Can't we just implement --preserve-atime with O_NOATIME if
O_NOATIME works, and document it that way?

> but at present it is impossible to know if O_NOATIME is actually
> working, at least under certain circumstances.

Can't tar fstat the file and see whether its last access time is
changed, after reading the file?  If it hasn't changed, O_NOATIME is
probably working.

Presumably O_NOATIME is something that works (or doesn't) on a
per-filesystem basis.  (Am I right?)  Is there some way this can be
queried, e.g., with stat or fpathconf?  If not, perhaps there should
be.  And at any rate, the O_NOATIME-works info could be cached by tar
(regardless of whether it can be queried directly).

> +AC_CHECK_DECL(O_NOATIME,

Is the Autoconf stuff necessary?  Why not just use "#ifdef O_NOATIME"?

Also, would you be willing to assign the copyright to your changes to
the Free Software Foundation, so that we could install it in tar?