On Tue, 11 Sep 2012, Jim Ramsay wrote:
>
> On Sat, Sep 08, 2012 at 12:35:22PM -0400, Mikulas Patocka wrote:
> > Your changes are ok, it could be included in the kernel.
> >
> > Regarding flushes ... could you please explain how write-back caching
> > works in your design?
>
> We are not actually using write-back caching in our design at all, but
> assumed that for a general-purpose DM device like dm-switch, some
> underlying storage may rely on REQ_FLUSH to ensure proper data
> synchronization, so the switch should do the right thing when required.
> The code is modeled on dm-stripe.
So you don't need to process REQ_FLUSH at all. REQ_FLUSH doesn't impose
any request synchronization, it only flushes the hardware cache - and
there is no cache. I would remove REQ_FLUSH support if it has no use.
dm-switch is used directly on host adapters (or maybe dm-mpath that is
connected on host adapters), there is no intermediate layer that would
need REQ_FLUSH.
Mikulas
> > Is there some write-back cache in the host adapter?
>
> No, there is not at this time.
>
> > Is there some write-back cache in the individual storage nodes? (i.e. does
> > the node signal end of I/O before it finishes writing data to the disk)
>
> No, there is not at this time. The iSCSI-backed SD devices we are
> ultimately using do not advertise any write-back cache ('sdparm
> --get=WCE' returns 0).
>
> > Another thing - please resend your code with "Signed-off-by". Read the
> > meaning of "Signed-off-by" in Documentation/SubmittingPatches, agree to
> > the terms and append "Signed-off-by: Jim Ramsay <jim_ramsay@xxxxxxxx>" to
> > the code. It is a legal requirement, so that you certify that the code is
> > under the open source license and that you have the right to distribute
> > the code.
>
> Will do shortly. Thanks!
>
> --
> Jim Ramsay
>
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