On Wed, Mar 25, 2020 at 09:59:19AM +0000, Damien Le Moal wrote:

On 2020/03/25 18:48, hch@infradead.org wrote:

quoted

On Wed, Mar 25, 2020 at 09:45:39AM +0000, Johannes Thumshirn wrote:

quoted

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.