On Sun, 2011-02-27 at 14:21 +0000, Menny_Hamburger@xxxxxxxx wrote: > From: Menny Hamburger <Menny_Hamburger@xxxxxxxx> > > When sd scan fails in apprehending capacity, cache_type or write protect flag > property from the device, it automatically assigns a default value to the > failed property. When rescanning, in case of transport/host error, this default > value is invalid since the problem is with the connection to the device and not in > the device itself that may (in most cases) still be intact. Applying a default value > when failing may lead to problems when connection is re-established since the default > value persists unless an additional rescan is performed. That's correct. Zero means we know there's something there but we couldn't get the necessary information. A zero size device can't be read from or written to. > This problem was witnessed when running in a iSCSI environment under multipath > (with I/O on the active path). In this case we get a ping-ping effect where > multipathd switches between alternate paths forever (until rescan) because the > path checker states that the device is OK, and I/O fails immediately because of > the 0 capacity (assigned to the device when rescanning while the device was > disconnected from the storage). > > Reproduction over ISCSI environment: > 1) dd if=/dev/dm-0 of=/dev/zero bs=64 count=10000 > 2) ifdown ethN, ethM, ethK, ... (where ethX is an interface from which the > machine establishes connection to the storage array). > 3) iscsiadm -m session -R > 4) ifup ethN, ethM, ethK, ... This really doesn't look like a good idea. It's a layering violation in that the SCSI mid layer now has to try to determine if certain command failures are the result of host disruption. The idea of believing a prior value if a READ_CAPACITY fails also doesn't look to be such a good one. This could lead to volume corruption if the disruption is part of an array configuration. The correct fix looks to be to initiate a rescan when the host is active via hotplug, and just teach the path checker about zero size devices? James -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
