Ubuntu – Hot-swap drive got new name. If I add it back to the `md` array and it gets renamed on reboot, will the array still work

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One of the HDDs in my server's RAID config failed, so I took it out of the array and had the data center hot-swap it. They've done that, but now the new drive is /dev/sdc rather than /dev/sda. I suspect that if I reboot the server, it will be /dev/sda again, so I'm hesitant to add it back to the array as /dev/sdc because I don't want to lay a trap for myself to fall into on the next reboot. I'd just as soon not reboot the server if I don't need to (if I do need to, well, too bad for me).

If I add it as /dev/sdc, will there be a problem on reboot? Or is there some way to change the device name from /dev/sdc to /dev/sda without rebooting?

This is on Ubuntu 10.04 LTS. It's an md array ("Linux Software RAID"), where currently one of the devices (there are a couple of them) looks like this ("degraded" because I've removed the old /dev/sda from it):

# mdadm --detail /dev/md0
/dev/md0:
        Version : 00.90.03
  Creation Time : Sun Oct 11 21:07:54 2009
     Raid Level : raid1
     Array Size : 97536 (95.27 MiB 99.88 MB)
  Used Dev Size : 97536 (95.27 MiB 99.88 MB)
   Raid Devices : 2
  Total Devices : 1
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Thu Jun 30 09:31:16 2011
          State : clean, degraded
 Active Devices : 1
Working Devices : 1
 Failed Devices : 0
  Spare Devices : 0

           UUID : 496be7a5:ab9177ed:7792c71e:7dc17aa4
         Events : 0.112

    Number   Major   Minor   RaidDevice State
       0       8       17        0      active sync   /dev/sdb1
       1       0        0        1      removed

Best Answer

It's fine to go ahead and add it as /dev/sdc. Reading through the kernel md documentation, if the name changes on reboot, it doesn't matter. (Good design, that.) Here's why:

Boot time autodetection of RAID arrays

When md is compiled into the kernel (not as module), partitions of type 0xfd are scanned and automatically assembled into RAID arrays. This autodetection may be suppressed with the kernel parameter "raid=noautodetect". As of kernel 2.6.9, only drives with a type 0 superblock can be autodetected and run at boot time.

The kernel parameter "raid=partitionable" (or "raid=part") means that all auto-detected arrays are assembled as partitionable.

Although I didn't have md compiled into the kernel, my setup does the same thing as the above because it's auto-loading mdadm and the mdadm.conf is set up to scan all partitions for a superblock just like the kernel would:

# by default, scan all partitions (/proc/partitions) for MD superblocks.
# alternatively, specify devices to scan, using wildcards if desired.
DEVICE partitions

So it's fine to rebuild the array with /dev/sdc; the name probably will change to /dev/sda on reboot, but that won't cause any trouble if md is set up as above.

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