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

[Paul Brook <paul@codesourcery.com>]

On Saturday 29 July 2006 15:59, Rik van Riel wrote:
> Fabrice Bellard wrote:
> > Hi,
> >
> > Using O_SYNC for disk image access is not acceptable: QEMU relies on the
> > host OS to ensure that the data is written correctly.
>
> This means that write ordering is not preserved, and on a power
> failure any data written by qemu (or Xen fully virt) guests may
> not be preserved.

I might be willing to accept this (or similar) patch if you made it 
conditional on the guest having disabled write caching. I agree with Fabrice 
that the performance impact it too severe to consider turning it on by 
default. 

The same problems occurs with many hardware RAID controllers, and even many 
harddrives: fsync() only guarantees that the data has been passed to the 
controller (in this case the host OS). If you need absolute reliability you 
either need more flusing in your guest OS, disable the write cache, or 
battery backup to make sure the IDE hardware (ie. host OS) doesn't die 
unexpectedly.

Paul