Re: Shouldn't backend devices for VMX domain disks be opened with O_DIRECT?

[Anthony Liguori <aliguori@us.ibm.com>]

Stephen Tweedie wrote:

>Hi,
>
>On Thu, 2006-02-02 at 18:09 -0600, Anthony Liguori wrote:
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>>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.
>>    
>>
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>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.
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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.
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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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