geli and BIO_FLUSH and/or BIO_ORDERED issue?

[John-Mark Gurney]

Pawel Jakub Dawidek wrote this message on Sat, Sep 22, 2012 at 18:20 +0200:
> On Tue, Sep 18, 2012 at 09:04:30PM -0700, John-Mark Gurney wrote:
> > I was looking at geli and I'm not sure if it's implementing BIO_FLUSH
> > and/or BIO_ORDERED properly...
> > 
> > >From my understanding is the BIO_ORDERED is suppose to wait for the
> > previous _WRITES to complete before returning so that you can ensure
> > that data is on disk, i.e. _ORDERED is set on a BIO_FLUSH...
> > 
> > BIO_ORDERED is handled by diskq_* code such that when you add an _ORDERED
> > command, all commands are put after it, but there doesn't appear to
> > be any code to ensure that an _ORDERED command waits for prevoius
> > pending commands to complete..
> > 
> > This is extra obvious in eli in that a _FLUSH is immediately dispatched,
> > even when there may be _WRITEs that haven't been finished encrypting and
> > sent down to the disk to get _FLUSHed...
> > 
> > Any comments about this?
> 
> Hmm, BIO_ORDERED was introduced pretty recently and GEOM classes were
> not updated to honour it, but it also seems to be to complex to handle
> in GEOM classes. I wonder if we could hold off new writes and wait for
> the in-progress writes in GEOM if we spot BIO_ORDERED request without
> the need to implement this logic in GEOM classes.

Yeh.  When I was looking at it, it definately seems like it should be
something that we provide a generic method of handling (as part of
bioq_*), since all the geom classes need to handle it...

It'll be a bit difficult since we'd need to introduce some syncronization
between the up/down threads to start the new writes when the previous
writes finish...  And with a class like geli, you can get better latency
if we move handling into the class though...

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."