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

[Anthony Liguori <anthony@codemonkey.ws>]

Andrea Arcangeli wrote:
>> As has been pointed out, this is probably not ideal since it would cause 
>> heavy vma fragmentation.  We may be able to simulate this using the slots 
>> API although slots are quite similar to vma's in that we optimize for a 
>> small number of them.
>>     
>
> I'm quite sure remap_file_pages can be extended to work on
> MAP_PRIVATE. But I don't see the big benefit in sharing the ram
> between host and guest, when having it in the guest is enough and this
> only works for read anyway and it can only share ram among different
> guests with -snapshot.
>   

Or if multiple guests are using the same backing file (imagine each 
guest has it's own qcow file backed to a common one).  Or if we had a 
more advanced storage system that did something like content addressable 
storage.

Regards,

Anthony Liguori

> So while it sounds a clever trick, I doubt it's a worthwhile
> optimization, it has downsides, and the worst is that I don't see how
> we could extend this logic to work for writes because the pagecache of
> the guest can't be written on disk before the dma is explicitly
> started on the guest.
>