Re: [PATCH v2 10/11] iomap: Add support for zone append writes

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On Wed, Mar 25, 2020 at 09:59:19AM +0000, Damien Le Moal wrote:
> On 2020/03/25 18:48, hch@xxxxxxxxxxxxx wrote:
> > On Wed, Mar 25, 2020 at 09:45:39AM +0000, Johannes Thumshirn wrote:
> >>
> >> Can you please elaborate on that? Why doesn't this hold true for a 
> >> normal file system? If we split the DIO write into multiple BIOs with 
> >> zone-append, there is nothing which guarantees the order of the written 
> >> data (at least as far as I can see).
> > 
> > Of course nothing gurantees the order.  But the whole point is that the
> > order does not matter.  
> > 
> 
> The order does not matter at the DIO level since iomap dio end callback will
> allow the FS to add an extent mapping the written data using the drive indicated
> write location. But that callback is for the entire DIO, not per BIO fragment of
> the DIO. So if the BIO fragments of a large DIO get reordered, as Johannes said,
> we will get data corruption in the FS extent. No ?

I thought of recording the location in ->iomap_end (and in fact
had a prototype for that), but that is not going to work for AIO of
course.  So yes, we'll need some way to have per-extent completion
callbacks.



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