Re: [dm-devel] [PATCH] reworked dm-switch target

[Jim Ramsay <jim_ramsay dell com>]

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.

> 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 dell com>" 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