Re: Questions on block drivers, REQ_FLUSH and REQ_FUA

[Tejun Heo <tj@kernel.org>]

Hey, Vivek.

On Tue, May 24, 2011 at 06:32:20PM -0400, Vivek Goyal wrote:
> I think documentation is fine. It specifically talks about completed
> requests. The requests which have been sent to drive (and may be in
> controller's cache). 
> 
> So in above example, if driver holds back WRITE1 and never signals
> the completion of request, then I think it is fine to complete
> the WRITE3+FLUSH ahead of WRITE1.

Yeap, that's correct.  Ordering between flush and other writes are now
completely the responsibility of filesystems.  Block layer just
doesn't care.

> I think issue will arise only if you signaled that WRITE1 has completed
> and cached it in driver (as you seem to indicating) and never sent to the
> drive and then you received WRITE3 + FLUSH requests. In that case you shall
> have to make sure that by the time WRITE3 + FLUSH completion is signaled,
> WRITE1 is on the disk.

A FLUSH command means "flush out all data from writes upto this
point".  If a driver has indicated completion of a write and then
received a FLUSH, the data from the write should be written to disk.

> > Again this does not appear to be illegal, as the FLUSH operation is
> > not defined as a barrier, meaning it should in theory be possible
> > to handle (and write to disk) requests received after the
> > FLUSH request before the FLUSH request finishes, provided that the
> > commands received before the FLUSH request itself complete before
> > the FLUSH request is replied to. I really don't know what the answer
> > is to this one. It makes a big difference to me as I can write multiple
> > blocks in parallel, and would really rather not slow up future write
> > requests until everything is flushed unless I need to.
> 
> IIUC, you are right. You can finish WRITE4 before completing FLUSH+WRITE3
> here.
> 
> We just need to make sure that any request completed by the driver
> is on disk by the time FLUSH+WRITE3 completes.

Yeap, again, block layer just doesn't care and the only thing block
driver should pay attention to regarding FLUSH is implementing FLUSH
command properly.

Thanks.

-- 
tejun