Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

[Nikita Danilov <nikita@clusterfs.com>]

Neil Brown writes:
 > 

[...]

 > Thus the general sequence might be:
 > 
 >   a/ issue all "preceding writes".
 >   b/ issue the commit write with BIO_RW_BARRIER
 >   c/ wait for the commit to complete.
 >          If it was successful - done.
 >          If it failed other than with EOPNOTSUPP, abort
 >          else continue
 >   d/ wait for all 'preceding writes' to complete
 >   e/ call blkdev_issue_flush
 >   f/ issue commit write without BIO_RW_BARRIER
 >   g/ wait for commit write to complete
 >        if it failed, abort
 >   h/ call blkdev_issue
 >   DONE
 > 
 > steps b and c can be left out if it is known that the device does not
 > support barriers.  The only way to discover this to try and see if it
 > fails.
 > 
 > I don't think any filesystem follows all these steps.

It seems that steps b/ -- h/ are quite generic, and can be implemented
once in a generic code (with some synchronization mechanism like
wait-queue at d/).

Nikita.

[...]

 > 
 > Thank you for your attention.
 > 
 > NeilBrown
 > 

Nikita.
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