[dm-devel] ALUA - rescan device capacity on zero sized block devices

[Bart Van Assche]

On 04/13/15 17:32, Thomas Wouters wrote:
> We're performing some tests with open-iscsi and multipath on two 3par
> servers and their peer persistence feature.
> 3par is a commercial storage solution that uses ALUA to allow failover.
> We have two connections from each 3par server to a linux server.
>
> Every 3par server has two network controllers, so on our linux server we
> initiate 4 iscsi connections.
> Multipath detects that two of these connections are active paths (both
> to the same 3par device, that is active at that point) and two are ghost
> paths, to the passive 3par device.
>
> At this moment we have four block devices, the active paths show the
> actual device size and the standby paths show the devices as zero sized:
>
> # multipath -ll
> 360002ac000000000000000420001510c dm-3 3PARdata,VV
> size=100G features='1 queue_if_no_path' hwhandler='1 alua' wp=rw
> |-+- policy='round-robin 0' prio=130 status=active
> | |- 48:0:0:123 sdc 8:32 active ready running
> | `- 50:0:0:123 sdb 8:16 active ready running
> `-+- policy='round-robin 0' prio=1 status=enabled
>    |- 49:0:0:123 sdd 8:48 active ghost running
>    `- 51:0:0:123 sde 8:64 active ghost running
>
> # cat /sys/block/sdb/size
> 209715200
> # cat /sys/block/sdc/size
> 209715200
> # cat /sys/block/sdd/size
> 0
> # cat /sys/block/sde/size
> 0
>
> As soon as we perform a switchover on the 3par systems, multipath
> detects the priority changes and switches paths but the new active paths
> fail.
> We believe this is because 3par doesn't allow us to read the capacity of
> the disk on a standby path - and we have proof of this in the logs:
>
> Apr 13 15:05:12 deb-3par-test kernel: [   40.079736] sd 5:0:0:0: [sdc]
> READ CAPACITY failed
>
> Unfortunately, once we perform the switchover on 3par, the capacity of
> those old ghost paths, now active paths, is not re-read.  The multipath
> device is therefore reduced to a size of 0 and the filesystem becomes
> unavailable.
>
> If we only login on the two active paths without starting multipath,
> perform a switchover, then login on the two new active paths and start
> multipath, we have four block devices with a non-zero size and we can
> perform switchovers at will without any issues.
>
> We've found some older discussions describing these issues on the scsi
> target-devel and dm-devel mailinglists:
> - http://permalink.gmane.org/gmane.linux.scsi.target.devel/6531
> - https://www.redhat.com/archives/dm-devel/2014-July/msg00156.html
>
> As far as we can conclude after reading these messages, it is correct
> behavior for disallowing READ CAPACITY on ghost paths.  However, once
> the path becomes active, we do need a reread of the capacity in order
> for the path to be functional...
>
> We've created a workaround for our issue but we're not sure we're going
> in the right direction.
>
> diff --git a/multipathd/main.c b/multipathd/main.c
> index f876258..ff32681 100644
> --- a/multipathd/main.c
> +++ b/multipathd/main.c
> @@ -1235,6 +1235,11 @@ check_path (struct vectors * vecs, struct path * pp)
>
> pp->chkrstate = newstate;
> if (newstate != pp->state) {
> +
> + if (newstate == PATH_UP && pp->size != pp->mpp->size ) {
> + sysfs_attr_set_value(pp->udev, "device/rescan", "1\n",2);
> + }
> +
> int oldstate = pp->state;
> pp->state = newstate;

Hello Thomas,

The above patch will trigger a rescan after every failover and failback. 
I'm afraid that will slow down failover and failback, especially if the 
number of LUNs is large. I would appreciate it if the capacity would be 
reexamined only if it is not yet known.

Thanks,

Bart.