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

[Rik van Riel <riel@redhat.com>]

Paul Brook wrote:
> 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. 

Easy to do with the fsync infrastructure, but probably not worth
doing since people are working on the AIO I/O backend, which would
allow multiple outstanding writes from a guest.  That, in turn,
means I/O completion in the guest can be done when the data really
hits disk, but without a performance impact.

-- 
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan