John Hughes wrote:
> and no_path_retry 5). The mdadm raid10 built on top of the multipath
> devices hangs, even /proc/mdstat hangs.
>
> You're saying that without queue_if_no_path multipath basicly won't
> work
> - mdadm will see I/O errors on multipath devices if a path fails?
No. It will see IO errors immediately if _all_ paths fail. With
no_path_retry nn, the intended behaviour is to wait nn cycles to see if
the array (at elast one path) reappears, and to fail thhe IO after nn
cycles.
Waht you report here is exactly what I observed too (my distro was a
SLES10). Apparently, some versions of multipath-tools seem to be buggy
with respect to no_path_retry count and seem to react as if you had
used "no_path_retry queue". AFAIR some weeks ago Hannes Reinecke
stated here that this is indeed a bug in some SuSE versions of
multipath_tools.
I succeeded to set up mdadm mirrors (and also lvm mirrors on a SLES11
machine) on top of dm_multipath by explicitely using "no_path_retry
fail" (edit multipath.conf and restart multipathd afterwards). With
these settings, path failures are handled as usually, and I could
survive a (simulated) raid array failure (i.e., all paths failed).
"no_path_retry fail" may contradict commercial raid system
manufacturers' recommendations ... but it seems to work for me.
Another idea which you might take into account: I do not know the raid
array which you are using. My attempts were done with EMC Clariion
arrays. If I simulate an array failure by just removing the host's
access rights to a lun within the array, I get a different behaviour
depending on the lun address - on a Clariion, removing, say, scsi
address 2:0:0:0 and 3:0:0:0 is not exactly the same as removing 2:0:0:1
and 3:0:0:1. A clariion exposes some kind of dummy lun 0 ("LUNZ") to
all hosts which dont have access rights to any real lun visible at
address 0. The consequence ist that removing a real lun 0 will not
result in not having a lun 0 o the scsi level; instead, it results in a
not_ready lun 0 (i.e. 2:0:0:0 and 3:0:0:0 are still visible at the scsi
layer!). Therefore I recommend to simulate site failures with luns !=0
best regards
Diedrich
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Diedrich Ehlerding, Fujitsu Technology Solutions GmbH, R GE TIS N IC2
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