Re: [Xen-devel] [patch 0/6] xenblk: Add O_DIRECT and O_SYNC support.

[Joe Jin <joe.jin@xxxxxxxxxx>]

> > To direct-io answer is yes for request have marked if write request with
> > O_DIRECT flag, but to O_SYNC, filesystem implentation it at filesystem,
> > layer, I think do not touch filesystem codes to implement O_SYNC support
> > is impossible.
> 
> O_DIRECT has no meaning really across the xenblk interface. The intention is
> that blkback talks at a raw block device below the buffer cache anyway.
> 
> For O_SYNC it can be implemented by pushing out all I/Os into the ring
> buffer and then wait for all required responses. Why do you think that
> doesn't work? Indeed it should work already if such logic exists in the
> generic Linux block layer, since blkfront notifies that layer as responses
> come in from blkback.
> 
Do you think when xenblk backend make_reponse() means the request have
commited to disk? 
We have did a sample testing and found without BI_RW_SYNC flag setted,
either O_DIRECT or O_SYNC flag, when vm crashed/power outage, lots of 
data lost, sometimes more than 1M data lost, that means vm under 
high data lost risk. With BI_RW_SYNC flag when call submit_bio, open
file with O_DIRECT or O_SYNC flag, could sync data very well.

Thanks,
Joe



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