Q: What is and what causes “device name shifting syndrome” and how can I avoid or fix it?
A: In FreeBSD ( concept ) everything is a file, for each device a file is created in the /dev directory when the device is attached / detected by the kernel.
For controllers, the kernel will reserve device(s) name(s) even if hardware (HDD) is not attached, for example ad0 and ad2 (master's on primary and secondary ATA channel) but not ad1 (slave on primary ATA channel).
For da X (SCSI Direct Access device driver) the kernel will create a new file ( assignment ) when the device is attached/detected. The problem here is that the kernel will assign “the next available /dev/name” . Windows behaves in similar fashion.
Example: attach 3 USB disk's, da0, da1 da2. Create a mount point for each disk. Now disconnect disks 2 (da1) and 3 (da2) and connect back disk 3 and you will see that the new device name is da1 (previously was da2).
A solution exists, it uses the label on device, see Labeling Disk Devices, and GPT disks also have labels which can be used.
Prior to Version 9, the XigmaNAS WebGUI did not make use of labels. Now every effort is made to label disks so we have the “Import . . .” and “Clear Config . . .” buttons on WebGUI Tab; Disks > Management > HDD Management which can help sort out the confusion, however nothing is perfect. These buttons are working well so far with the latest versions, but when upgrading from older versions there may be problems. This is because older versions made no attempt to use labels. You may also encounter problems because SoftRAID and ZFS disks use metadata stored on the disks which may ignore or override device names and because diffrent controllers, Motherboards and BIOS may report devices in different orders.
Now that servers have larger numbers of drives and mix ZFS, SoftRAID and standard disks,device name shifting syndrome can be quite a headache . Reduce the pain by using labels and keeping good notes on what disks are used where and for what. If you have an unexpected problem, your notes may be all that can save you from loss of data. Oh, that's assuming you have no backup. You do have a backup right?
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