Re: [PATCH 1/3] nbd: support FLUSH requests

[Alex Bligh]

Paolo,

> Add support for a new flag that the server can pass.  If the flag is
> enabled, we translate REQ_FLUSH requests into the NBD_CMD_FLUSH
> command.
> 
> Cc: <nbd-general@xxxxxxxxxxxx>
> Cc: Paul Clements <Paul.Clements@xxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Alex Bligh <alex@xxxxxxxxxxx>
> [ Removed FUA support for reasons similar to those outlined in
>  https://lkml.org/lkml/2010/8/17/234 for virtio - Paolo ]
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>


For my education, why remove the FUA stuff? The link you posted
was for virtio drivers, and says:

> > Hmmm... the underlying storage could be md/dm RAIDs in which case FUA
> > should be cheaper than FLUSH.
> 
> If someone ever wrote a virtio-blk backend that sits directly ontop
> of the Linux block layer that would be true.

In this case we don't know what the backend is sitting on top of
a-priori. It might be the current nbd server code, but it might
not be.

-- 
Alex Bligh




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