Re: [Xen-devel] Shouldn't backend devices for VMX domain disks be opened with O_DIRECT?

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Stephen Tweedie wrote:

> Hi,
>
> On Thu, 2006-02-02 at 18:09 -0600, Anthony Liguori wrote:
>
>  
>
> > Referring to the original question, which has been quoted away, 
> > journaling doesn't require that data be written to disk per-say but that 
> > writes occur in a particular order.  A journal is always recoverable 
> > given that writes occur in the expected order.
> >    
> Sure... it's *internally* consistent, maybe.  But you need more than
> that.  You need guarantees that things are on disk, else external
> consistency guarantees will be broken.
>  
Ok, this is certainly correct (but not the original point).

> Consider things like sendmail fsync()ing a spool file before telling the
> sender that the email has been accepted.  After that acknowledgement,
> the sender can delete the mail from its queues knowing that the
> recipient MTA definitely has the data, and even if it crashes, the mail
> won't be lost.  Databases frequently have similar consistency
> requirements.  If a power failure loses writes that you have told the
> domU have completed --- even if you maintain write ordering --- then you
> *are* putting application correctness at risk, there's no doubt about
> it.
>  
Ok, this is a good argument for using O_SYNC.

> Fortunately, that's just what blkback is doing --- it's using submit_bio
> to submit the write IOs without waiting for completion, and is using the
> bio's bi_end_io callback to process the IO completion once it is hard on
> disk.  
>  
Yup, the question here is with the device model which doesn't use the 
block frontend/backend.  Would O_DIRECT be helpful over O_SYNC?
Regards,

Anthony Liguori

> --Stephen
>
>
>
>  

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