Re: [PATCH 1/7] Make DISCARD_BARRIER and DISCARD_NOBARRIER writes instead of reads

David Woodhouse
2009-09-03T17:52:20Z

On Sat, 2009-08-29 at 19:03 -0400, Christoph Hellwig wrote:
>
> The commands are conceptually writes, and in the case of IDE and SCSI
> commands actually are writes. They were only reads because we thought
> that would interact better with the elevators. Now the elevators know
> about discard requests, that advantage no longer exists.

Can you drop the final sentence of that? It isn't true, and I never said
it.

s/. Now.*/, but that isn't necessary, and making them writes makes it
easier for the low-level IDE and SCSI code to cope with the fact that
the command has to be sent with a payload./

The elevators _still_ don't know about discards, and will still let
reads and writes (and discards, which are just a special case of writes)
to the same sector all cross each other on the queue unless there's some
external factor to prevent it.

--
David Woodhouse Open Source Technology Centre
David.Woodhouse@intel.com Intel Corporation