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[dm-devel] Question about EMC Clariion trespass in multipath

I've been told that dm multipath does not support trespassing LUNs on
Clariion, but after looking through the hardware handler for EMC
Clariion devices, I believe it does.  However, I'm confused about how it
determines when to issue the trespass command.

Since we are using RHEL 5.3, I am looking at the version of the
hardware handler, dm-emc.c

Looking at emc_pg_init(), which, according to the documentation, is
called "when a Priority Group is selected for use, but before any I/O is
sent to it" (http://lwn.net/Articles/123134), it looks like the only way
out of this function is by either failing the path via
dm_pg_init_complete(), or issuing a switch-over command.  So either a
path is failed or a trespass occurs every time that a priority group is
selected?  What am I missing here?

Any guidance would be appreciated.


 192static void emc_pg_init(struct hw_handler *hwh, unsigned bypassed,
 193                        struct path *path)
 195        struct request *rq;
 196        struct request_queue *q = bdev_get_queue(path->dev->bdev);
 198        /*
 199         * We can either blindly init the pg (then look at the
 200         * or we can send some commands to get the state here (then
 201         * possibly send the fo cmnd), or we can also have the
 202         * initial state passed into us and then get an update here.
 203         */
 204        if (!q) {
 205                DMINFO("emc_pg_init: no queue");
 206                goto fail_path;
 207        }
 209        /* FIXME: The request should be pre-allocated. */
 210        rq = emc_trespass_get(hwh->context, path);
 211        if (!rq) {
 212                DMERR("emc_pg_init: no rq");
 213                goto fail_path;
 214        }
 216        DMINFO("emc_pg_init: sending switch-over command");
 217        elv_add_request(q, rq, ELEVATOR_INSERT_FRONT, 1);
 218        return;
 221        dm_pg_init_complete(path, MP_FAIL_PATH);

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