Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT)

[Avi Kivity]

Andrea Arcangeli wrote:
> On Wed, May 21, 2008 at 07:48:24PM +0300, Avi Kivity wrote:
>   
> > Hmm, if remap_file_pages() supports MAP_PRIVATE, that solves all the 
> > problems neatly.
> >
> > Andrea, do you know if that combination works?
> >     
>
> No, it only supports MAP_SHARED. Why would anyone want MAP_PRIVATE for
> real I/O operations? MAP_PRIVATE is primarily for binaries and things
> that are readonly on disk and that we may want to update (like for
> dynamic linking w/o hardlinking).
>
> Or is that for -snapshot support, to keep the modifications in
> anonymous ram?
>
> I imagine you want remap_file_pages for zerocopy I/O without using
> O_DIRECT. If you use O_DIRECT you don't need mmap. However O_DIRECT
> work best with kernel asyncio for small seeking blocks so all I/O can
> be submitted at the same time. writev also works better than write for
> O_DIRECT.
>   

Yes, that's the reason.  Here zerocopy is not the motivation; instead, 
we have host-cached pages that are used directly in the guest.  So we 
get both reduced memory footprint, and host caching.  O_DIRECT reduces 
the memory footprint but kills host caching.

The scenario is desktop/laptop use.  For server use O_DIRECT is clearly 
preferred due to much reduced overhead.

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