Re: [PATCH 5/5] dm: implement REQ_FLUSH/FUA support

[Kiyoshi Ueda <k-ueda@ct.jp.nec.com>]

Hi Tejun, Mike,

On 08/18/2010 01:51 AM +0900, Tejun Heo wrote:
> On 08/17/2010 04:07 PM, Mike Snitzer wrote:
>> NOTE: NEC has already given some preliminary feedback to hch in the
>> "[PATCH, RFC 2/2] dm: support REQ_FLUSH directly" thread:
>> https://www.redhat.com/archives/dm-devel/2010-August/msg00026.html
>> https://www.redhat.com/archives/dm-devel/2010-August/msg00033.html
> 
> Hmmm... I think both issues don't exist in this incarnation of
> conversion although I'm fairly sure there will be other issues.  :-)

The same issue is still there for request-based dm.  See below.


>>>     A related question: Is dm_wait_for_completion() used in
>>>     process_flush() safe against starvation under continuous influx of
>>>     other commands?
>> As for your specific dm_wait_for_completion() concern -- I'll defer to
>> Mikulas.  But I'll add: we haven't had any reported starvation issues
>> with DM's existing barrier support.  DM uses a mempool for its clones,
>> so it should naturally throttle (without starvation) when memory gets
>> low.
> 
> I see but single pending flush and steady write streams w/o saturating
> the mempool would be able to stall dm_wait_for_completeion(), no?  Eh
> well, it's a separate issue, I guess.

Your understanding is correct, dm_wait_for_completion() for flush
will stall in such cases for request-based dm.
That's why I mentioned below in
https://www.redhat.com/archives/dm-devel/2010-August/msg00026.html.

    In other words, current request-based device-mapper can't handle
    other requests while a flush request is in progress.

In flush request handling, request-based dm uses dm_wait_for_completion()
to wait for the completion of cloned flush requests, depending on
the fact that there should be only flush requests in flight owning
to the block layer sequencing.

It's not a separate issue and we need to resolve it at least.
I'm still considering how I can fix the request-based dm.

Thanks,
Kiyoshi Ueda