[dm-devel] REQ_OP for zeroing, was Re: [PATCH 1/4] brd: handle misaligned discard

[Mikulas Patocka]

On Fri, 28 Oct 2016, Christoph Hellwig wrote:

> [adding Chaitanya to Cc]
> 
> On Fri, Oct 28, 2016 at 07:43:41AM -0400, Mikulas Patocka wrote:
> > We could detect if the REQ_OP_WRITE_SAME command contains all zeroes and 
> > if it does, turn it into "Write Zeroes" or TRIM command (if the device 
> > guarantees zeroing on trim). If it doesn't contain all zeroes and the 
> > device doesn't support non-zero WRITE SAME, then reject it.
> 
> I don't like this because it's very inefficient - we have to allocate
> a payload first and then compare the whole payload for very operation.
> 
> > Or maybe we could add a new command REQ_OP_WRITE_ZEROES - I'm not sure 
> > which of these two possibilities is better.
> 
> Chaitanya actually did an initial prototype implementation of this for
> NVMe that he shared with me.  I liked it a lot and I think he'll be
> ready to post it in a few days.  Now that we have the REQ_OP* values
> instead of mapping different command types to flags it's actually
> surprisingly easy to add new block layer operations.

OK - when it is in the kernel, let me know, so that I can write device 
mapper support for that.

We should remove the flag "discard_zeroes_data" afterwards, because it is 
unreliable and impossible to support correctly in the device mapper.

Mikulas