Re: Proposal: B_ARRIER (addresses wapbl performance?)

[Thor Lancelot Simon <tls%rek.tjls.com@localhost>]

On Wed, Oct 29, 2008 at 08:57:16PM -0500, David Young wrote:
> On Wed, Oct 29, 2008 at 05:51:09PM -0400, Thor Lancelot Simon wrote:
> > I'd like to propose a new buffer flag, B_ARRIER.  This flag would be
> > defined to cause a write barrier at whatever I/O request carries it.
> 
> Thor,
> 
> I have two questions about this.
> 
> How can a disk driver re-order B_BARRIER blocks with respect to other
> blocks on its queue?  I.e., can a driver ever postpone writing a B_BARRIER
> block until after it writes some !B_BARRIER blocks?

The answer has to be "if they were submitted first, sure" -- doesn't
it?  In practice, I think disksort has to partition its sort (of
writes, at least) by these bufs.  I am not entirely sure how to do
that, unless the sort algorithm is "fifo".

It can enqueue more !B_BARRIER blocks after a B_BARRIER block, too; the
point is to tell the disk there is a barrier there (even if some disks
are so stupid you have to implement this by completion waiting).

Thor