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Re: [dm-devel] SCSI Hardware Handler and slow failover with large number of LUNS



On Mon, 2009-04-06 at 13:54 -0500, Mike Christie wrote:
> > 
> > 
> >>> 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
> > 
> 
> But we only have one level of requests. I am talking about when we tried 
> to send a request with REQ_BLOCK_LINUX_BLOCK to the module to tell it to 
> send another request/s with REQ_TYPE_BLOCK_PC. Now we just have the 
> callout and then like you said we can fire REQ_TYPE_BLOCK_PC reuqests 
> from there.
> 
> I think when I wrote easier above, I meant to write a cleaner 
> implementation.

Now I understand.

> 
> 
> 
> >> 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 ?
> 
> 
> I think people did not like the complexity of trying to send IO with 
> soft irq context with spin locks held, then also having the extra 
> REQ_BLOCK_LINUX_BLOCK layering.

clear now :)

> 
> 
> > 
> >> 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 ?
> > 
> 
> Do mean vs #1 or would you want to seperate and send some stuff async 
> and synchronously?

The other option I was thinking about was to utilize the capability of
the underlying device. For example, for lsi rdac, we can batch up mode
selects and send fewer commands down the wire, which would speed things
up.

I understand that not all devices would have a feature like rdac.
Nevertheless, it doesn't matter as this will be _inside_ the hardware
handler and they _don't_ have to be the same solution.

If async is the best option for all other handler, we can as well do
async for rdac also, which will keep the handlers looking the same :)

> 
> >>
> >>> 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 redhat com
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> > 
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