[linux-lvm] do not disable ext4 discards on first discard failure? [was: Re: dm snapshot: ignore discards issued to the snapshot-origin target]

[Christoph Hellwig]

On Mon, May 02, 2011 at 09:47:16AM -0500, Eric Sandeen wrote:
> I still think we need to stick with the simple rule: "EOPNOTSUPP returned for a particular bio means that it is not supported for that particular bio" - I don't know what else we can do, without creating an ambiguity...
> 
> This does, however, suck for the layer calling in to a complex device.
> 
> What is the overhead for sending discard bios down to a device that does not support it?

For a DM-like device there is very little overhead.  The bio gets
dispatched to make_request and failed in there almost immediately.