Dell PowerVault MD3660i Administrator's Guide - Page 126
Troubleshooting, I/O may hang when a Device Mapper device is deleted before the volume is unmounted.
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• I/O may hang when a Device Mapper device is deleted before the volume is unmounted. • If the scsi_dh_rdac module is not included in initrd, slower device discovery may be seen and the syslog may become populated with buffer I/O error messages. • I/O may hang if the host server or storage array is rebooted while I/O is active. All I/O to the storage array should be stopped before shutting down or rebooting the host server or storage array. • With an MD Series Dense storage array, after a failed path is restored, failback does not occur automatically because the driver cannot auto-detect devices without a forced rescan. Run the command rescan_dm_devs to force a rescan of the host server. This restores the failed paths enabling failback to occur. • Failback can be slow when the host system is experiencing heavy I/O. The problem is exacerbated if the host server is also experiencing very high processor utilization. • The Device Mapper Multipath service can be slow when the host system is experiencing heavy I/O. The problem is exacerbated if the host server is also experiencing very high processor utilization. • If the root disk is not blacklisted in the multipath.conf file, a multipathing node may be created for the root disk. The command multipath -ll lists vendor/product ID, which can help identify this issue. • If upgrading from a previous version of SLES, uninstall and then reinstall the latest scsi_dh_rdac module on the updated SLES installation. Then update the kernel and install the MD Storage Manager from the DVD. Troubleshooting Question How can I check if multipathd is running? Answer Run the following command. /etc/init.d/multipathd status Why does the multipath -ll command output not show any devices? First verify if the devices are discovered or not. The command #cat / proc/scsi/scsi displays all the devices that are already discovered. Then verify the multipath.conf to ensure that it is been updated with proper settings. After this, run multipath. Then run multipath - ll, the new devices must show up. Why is a newly-mapped LUN not assigned Run rescan_dm_devs in any directory. This should bring up the a multipathing device node? devices. I removed a LUN. But the multipathing mapping is still available. The multipathing device is still available after you remove the LUNs. Run multipath -f to remove the multipathing mapping. For example, if a device related with /dev/dm-1 is deleted, you must run multipath -f /dev/dm-1 to remove /dev/dm-1 from DM mapping table. If multipathing daemon is stopped/restarted, run multipath -F to flush out all stale mappings. Failback does not happen as expected with the array. Sometimes the low level driver cannot auto-detect devices coming back with the array. Run rescan_dm_devs to rescan host server SCSI bus and re-aggregate devices at multipathing layer. 126
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