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.
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