[dm-devel] SCSI Hardware Handler and slow failover with large number of LUNS
Chandra Seetharaman
sekharan at us.ibm.com
Mon Apr 6 18:21:38 UTC 2009
Thanks for the response Mike.
On Mon, 2009-04-06 at 10:43 -0500, Mike Christie wrote:
> Chandra Seetharaman wrote:
> > Hello All,
> >
> > During testing with the latest SCSI DH Handler on a rdac storage, Babu
> > found that the failover time with 100+ luns takes about 15 minutes,
> > which is not good.
> >
> > We found that the problem is due to the fact that we serialize activate
> > in dm on the work queue.
> >
>
> I thought we talked about this during the review?
Yes, we did and the results were compared to the virgin code (w.r.t rdac
handler) and the results were good (also I used only 49 luns) :
http://marc.info/?l=dm-devel&m=120889858019762&w=2
>
> > We can solve the problem in rdac handler in 2 ways
> > 1. batch up the activates (mode_selects) and send few of them.
> > 2. Do mode selects in async mode.
>
> I think most of the ugliness in the original async mode was due to
> trying to use the REQ_BLOCK* path. With the scsi_dh_activate path, it
> should now be easier because in the send path we do not have to worry
> about queue locks being held and context.
>
little confused... we still are using REQ_TYPE_BLOCK_PC
> I think we could just use blk_execute_rq_nowait to send the IO. Then we
> would have a workqueue/thread per something (maybe per dh module I
> thought), that would be queued/notified when the IO completed. The
> thread could then process the IO and handle the next stage if needed.
>
> Why use the thread you might wonder? I think it fixes another issue with
> the original async mode, and makes it easier if the scsi_dh module has
can you elaborate the issue ?
> to send more IO. When using the thread it would not have to worry about
> the queue_lock being held in the IO completion path and does not have to
> worry about being run from more restrictive contexts.
You think queue_lock contention is an issue ?
I agree with the restrictive context issue though.
So, your suggestion is to move everything to async ?
>
>
> >
> > Just wondering if anybody had seen the same problem in other storages
> > (EMC, HP and Alua).
>
> They should all have the same problem.
>
>
> >
> > Please share your experiences, so we can come up with a solution that
> > works for all hardware handlers.
> >
> > regards,
> >
> > chandra
> >
> > --
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> > dm-devel at redhat.com
> > https://www.redhat.com/mailman/listinfo/dm-devel
>
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