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

[Jamie Lokier <jamie@shareable.org>]

Avi Kivity wrote:
> Here's a summary of the use cases I saw so far:
> 
> - casual use, no critical data: write back cache
> 
> - backing file shared among many guests: read-only, cached
> 
> - desktop system, but don't lose my data: O_SYNC
> (significant resources on the host)
> 
> - dedicated virtualization engine: O_DIRECT
> (most host resources assigned to guests)

Sounds alright, but on _my_ desktop system (a laptop), I would use O_DIRECT.

There isn't enough RAM in my system to be happy duplicating data in
guests and hosts at the same time.  VMs are quite demanding on RAM.

However, if you find a way to map host cached pages into the guest
without copying - so sharing the RAM - that would be excellent.  It
can be done in principle, by remapping pages to satisfy IDE/SCSI DMA
requests.  I don't know if it would be fast enough.  Perhaps it would
work better in KVM than QEMU.

-- Jamie