Re: write barrier patches for 2.4.21

[Mike Fedyk <mfedyk@matchmail.com>]

On Thu, Aug 28, 2003 at 08:07:30PM -0400, Tom Vier wrote:
> On Thu, Aug 28, 2003 at 10:07:25AM -0700, Mike Fedyk wrote:
> > On Wed, Aug 27, 2003 at 08:37:40PM -0400, Chris Mason wrote:
> > > scsi drives don't really need them because most scsi drives don't have
> > > write back caching on by default, and most actually listen when you turn
> > > the cache off.  The scsi tag queuing makes good performance possible
> > > even without writeback caching.
> > 
> > So, in essence, TCQ is exposing what is happening in the cache, and allowing
> > control/showing state compared to IDE's non-TCQ write cache.  Do I have that
> > right?
> 
> i'm not sure what you mean. chris is just saying tcq makes performance w/o
> write cache better. i WOULD like to see wb's used w/ scsi. it would be nice
> to be able to safely enable write cache on my drives.

I'm just trying to clairify that it looks like TCQ shows exactly what it
happening in the drive's cache, and that's why with tcq the wb's aren't
explicitly needed.

With ide without tcq and with write caching, it accepts the write request
and says it's complete when it's actually still in the disks cache, not on
the platters yet.

Contrast that to TCQ (IDE or SCSI), where the write request is sent to the
drive, and once it hits the platter, the tcq command completes and notifies
the host OS (Linux in this case) that it's done.  And SCSI with no TCQ is
like IDE with no write cache.

Do I have that right?