Re: Proposal: B_ARRIER (addresses wapbl performance?)

[David Young <dyoung%pobox.com@localhost>]

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?

> In practice, this would mean using an ordered tag with SCSI disks
> (because ordered tags do not complete until all prior simple-tagged
> commands complete) and causing a cache flush for IDE disks (at least
> until we get tagged queueing support for them).  I don't know how FUA
> works for IDE disks but it's been suggested to me that FUA commands
> cause all pending I/O to finish before they do -- that would suffice
> too.

Are there IDE disks that support neither FUA nor cache flush?
It seems that IDE disks on a USB-IDE adapter may fit this description.

Dave

-- 
David Young             OJC Technologies
dyoung%ojctech.com@localhost      Urbana, IL * (217) 278-3933 ext 24