[Libguestfs] [PATCH nbdkit] file: Implement cache=none and fadvise=normal|random|sequential.

[Richard W.M. Jones]

On Fri, Aug 07, 2020 at 04:43:12PM +0300, Nir Soffer wrote:
> On Fri, Aug 7, 2020, 16:16 Richard W.M. Jones <rjones at redhat.com> wrote:
> > I'm not sure if or even how we could ever do a robust O_DIRECT
> >
> 
> We can let the plugin an filter deal with that. The simplest solution is to
> drop it on the user and require aligned requests.

I mean this is very error prone.  It requires the end user to know
about the basically unknowable restrictions of O_DIRECT and isn't even
possible in one common case - if the size of the file isn't an exact
multiple of the filesystem block size.

> Maybe a filter can handle alignment?
>
> > implementation, but my idea was that it might be an alternate
> > implementation of cache=none.  But if we thought we might use O_DIRECT
> > as a separate mode, then maybe we should rename cache=none.
> > cache=advise?  cache=dontneed?  I can't think of a good name!
> >
> 
> Yes, don't call it none if you use the cache.
> 
> How about advise=?
> 
> I would keep cache semantics similar to qemu.

qemu uses cache=none as a synonym for O_DIRECT, but AFAIK it has
nothing that tries to use posix_fadvise(DONTNEED) with or without
Linus's double buffering technique.  qemu does use
posix_fadvise(DONTNEED) in one place but AFAICT it is only used for
live migration.

...
> We already tried this with dd and the results were not good.

These ones?
https://www.redhat.com/archives/libguestfs/2020-August/msg00078.html

Rich.

-- 
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