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 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
