[Qemu-devel] Re: [RFC][PATCH] make sure disk writes actually hit disk

[Anthony Liguori]

On Fri, 28 Jul 2006 15:54:30 -0400, Rik van Riel wrote:

> This is the simple approach to making sure that disk writes actually hit
> disk before we tell the guest OS that IO has completed.  Thanks to
> DMA_MULTI_THREAD the performance still seems to be adequate.

Hi Rik,

Right now Fabrice is working on rewriting the block API to be
asynchronous.  There's been quite a lot of discussion about why using
threads isn't a good idea for this (I wish Xen wouldn't use this patch but
that's another conversation :-)).

The async block API will allow the use of different kinds of async
"backends".  The default (on Linux) will be posix-aio.  I'm currently
working on an HTTP backend and will also write a linux-aio (which, of
course, will be using O_DIRECT).

> A fancier solution would be to make the sync/non-sync behaviour of the
> qemu disk backing store tunable from the guest OS, by tuning the IDE disk
> write cache on/off with hdparm, and having hw/ide.c call ->fsync functions
> in the block backends.

With a proper async API, is there any reason why we would want this to be
tunable?  I don't think there's much of a benefit of prematurely claiming
a write is complete especially once the SCSI emulation can support
multiple simultaneous requests.

I was hoping to just make linux-aio the default if it was available...

Regards,

Anthony Liguori

> I'm willing to code up the fancy solution if people prefer that.