Michael Kerrisk (man-pages) wrote:
> >> 7. On Linux (and probably many other modern systems), the only
> >> call that has any real use is msync(MS_SYNC), meaning
> >> "flush the buffers *now*, and I want to wait for that to
> >> complete, so that I can then continue safe in the knowledge
> >> that my data has landed on a device". That's useful if we
> >> want insurance for our data in the event of a system crash.
> >
> > Right. It's basically another way to call fsync, which is used to
> > implement it underneath. It actually should be a ranged-fdatasync
> > but right it's it's implemented horribly inefficiently in that it
> > does a fsync call for each vma that it encounters in the range
> > specified.
A ranged-fdatasync, for databases with little logs inside the big data
file, would be nice. AIX, NetBSD and FreeBSD all have one :) Any
likelihood of that ever appearing in Linux? sync_file_range() comes
with its Warning in the man page which basically means "don't trust me
unless you know the filesystem exactly".
> Thanks for checking all of this over and thanks also
> for confirming that I learned my lessens well in the
> "Jamie Lokier school of tough technical reviewing" ;-).
Hi! That was a long time ago :)
> >> 9. The mmap() man page says this:
> >>
> >> MAP_SHARED
> >> Share this mapping. Updates to the mapping are vis???
> >> ible to other processes that map this file, and are
> >> carried through to the underlying file. The file
> >> may not actually be updated until msync(2) or mun???
> >> map() is called.
> >>
> >> I believe the piece "or munmap()" is misleading. It implies
> >> that munmap() must trigger a sync action. I don't think this
> >> is true. All that it is required to do is remove some range
> >> of pages from the process's virtual address space. I'm
> >> inclined to remove those words, but I'd like to see if any
> >> FS person has a correction to my understanding first.
> >
> > I would expect non-coherent systems to update their caches on munmap,
> > Posix does not seem to require this, and I can't find any language
> > towards that in the HP-UX man page, which was a system that I remember
> > as non-coherent until the end.
>
> Yes, that's how I read it too. POSIX seems to have no requirements here,
> so I assume it was catering to to the lowest common denominator.
According to this:
http://h30499.www3.hp.com/t5/System-Administration/2-second-delays-in-fsync-msync-munmap/td-p/3092785/page/2#.U1WBw8dSI1-
and the conclusion of the following page:
- munmap() does _something_ on HP-UX, but it might be just a poorly
implemented artifact rather than equivalent to msync.
- While we're there, the lowest common denominator for HP-UX was
that pwrite() followed by mmap() does not provide the data
recently written, even with fsync() between. The thread ended
there, but I would guess either it's a bug _or_ perhaps
write+mmap+msync(MS_INVALIDATE) are needed in that order despite
the write being before the mmap, perhaps if the shared segment
was maintained by another process.
- To keep it exciting, if you look at the HP-UX man page, 32-bit
and 64-bit processes have separate mmap caches - writing to
shared memory in one of them won't be seen immediately by the other.
Then there's this, about Linux NFS incoherency with msync() and O_DIRECT:
- https://groups.google.com/d/msg/comp.os.linux.development.apps/B49Rej6KV24/xEouZOVXs9gJ
I don't know if any of the above are _true_ though :)
Best,
-- Jamie
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