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Re: [dm-devel] [PATCH 4/7] scsi_dh: add EMC Clariion device handler

Chandra Seetharaman wrote:
On Thu, 2008-04-17 at 12:14 -0500, Mike Christie wrote:
Chandra Seetharaman wrote:
On Wed, 2008-04-16 at 11:29 -0500, Mike Christie wrote:
Chandra Seetharaman wrote:
+static int send_cmd(struct scsi_device *sdev, int cmd)
+	struct request *rq = get_req(sdev, cmd);
+	if (!rq)
+	return blk_execute_rq(sdev->request_queue, NULL, rq, 1);
My only concerns are:

1. EMC and HP need to send a command to every device to transition them. Because we do blk_execute_rq from the dm multipath workqueue we can now only failover/failback for a couple devices at a time.

I am not sure if this is a big deal, because this the error handler path so it is going to be slower than the normal path. But it seems like
Yes. But...

pg_init() due to failover/failback will be sent only when I/O is
sent/resent to a multipath device, isn't it ? and we don't expect I/Os
to be sent to all the devices at the same time (all the time), do we ?

I am not sure what you mean by all the time, because I am talking about

What I meant was that we do not expect I/Os to be sent to all the
devices at all the times (pg_init will be sent only when I/Os fails on a
path, right ?).

Sorry for not being clear.

No problem.

failover times above. And for failover I think I said yes in the previous mail. For EMC we are currently sending failover commands to all the devices at the same time, because EMC does not do the controller failover RDAC does.

RDAC doesn't do controller failover. It also does per lun failover.

Oh yeah, I forgot.

So, as you pointed, is it a big deal ? :)

In the previous mail I specifically said users might care, because they are picky about failover times, real 3m39.728s
user    0m4.135s
sys     0m14.536s

so the answer is to your question is what I said before, maybe :) I said I am not sure, because I do not have any numbers for the failover times.

Since RDAC also does the failover per device (as is the case with EMC),
I ran tests on about 49 luns. I ran disktest on all the disks at the


same time and disabled/enabled the port to the preferred path to
generate failover and failback.

Let me know what do you think.

Here are the results:
Tests run in an idle system. With 49 luns and the following script:
for i in `ls -1 /dev/mapper/mpath*`
	disktest $i -L 4000 -t 100 -P X &
	sleep 1

Simple Run:

with patchset:		2.6.25-mm1:
real    3m30.122s	real    3m29.746s
user    0m4.069s	user    0m4.099s
sys     0m14.876s	sys     0m14.535s

Is this just a boot up test or a test just running IO but no failback/failover?

Failover Run:

with patchset:		2.6.25-mm1:
real    5m18.875s	real    5m31.741s
user    0m4.069s	user    0m3.883s
sys     0m14.838s	sys     0m13.822s

Ehh, I have no idea if this is good or bad. Does it mean it is talking 13 more seconds to complete?

Have you seen the type of thread on dm-devel or the iscsi list where people are concerned with getting the time the failure is detected to the time IO is running on a new path down from something like 10 to 5 seconds. One time the iscsi driver did not implement time2wait correctly and by fixing it we shaved only 2 seconds off and users were very happy with the extra 2 seconds. We added the nop timer stuff so we could get faster failovers. We have the fast io fail tmo so we can speed up the process even more. Shaving off a second here or there is really nitpicky and if I were you I would give me the middle finger :) It just seems like people expect better performance from this type of error.

If my comment is too nitpicky then I am fine with ignoring this for now. We just have to fix the emc short/long tress pass code then. I added another EMC guy to the thread so he can ping the other EMC devs to get going (I had sent them questions on how to handle it and have not got a response).

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