How-to change the name of an MD device (mdadm)

Question

I am trying to run Hadoop Wordcount on Tachyon. I followed this link. But once I run wordcount Jar with below command

hadoop jar HadoopWordCount-0.0.1-SNAPSHOT-jar-with-dependencies.jar edu.WordCount -libjars tachyon-client-0.7.1-jar-with-dependencies.jar tachyon://tachyon_ip:19998/wordcountsample/word2.txtaa /OUT/wcTachyon

I am getting below error.

Error: java.lang.RuntimeException: java.lang.ClassNotFoundException: Class tachyon.hadoop.TFS not found
    at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:2112)
    at org.apache.hadoop.fs.FileSystem.getFileSystemClass(FileSystem.java:2578)
    at org.apache.hadoop.fs.FileSystem.createFileSystem(FileSystem.java:2591)
    at org.apache.hadoop.fs.FileSystem.access$200(FileSystem.java:91)
    at org.apache.hadoop.fs.FileSystem$Cache.getInternal(FileSystem.java:2630)
    at org.apache.hadoop.fs.FileSystem$Cache.get(FileSystem.java:2612)
    at org.apache.hadoop.fs.FileSystem.get(FileSystem.java:370)
    at org.apache.hadoop.fs.Path.getFileSystem(Path.java:296)

I am new to Tachyon.

Please advice.

Solution 1

Warning: Please make a backup before following any advise given here :-)

Option #1:

Edit /etc/mdadm.conf:

ARRAY /dev/md1 level=raid0 num-devices=2 UUID=d89d9d45:9a420812:c44c77eb:7ee19756 devices=/dev/sdb8,/dev/sda8

I just entered your own output and changed 127 to 1. Reboot and it should be md1 now.

Option #2:

If you don't have mdadm.conf, and want it to assemble to md1 automagically, you can do this:

mdadm has a concept of a "preferred minor". E.g. once a certain minor number is assigned to an array, and stored in the superblock, it will be used each time this array is auto-assembled. Verify this:

mdadm --detail /dev/md127

If it says "Preferred Minor : 127" this this is your problem. You need to update the preferred minor. How to do this:

You cannot just change the preferred minor. You need to boot into recovery or single user-mode if this is your root device, unmount the filesystem, after that:

mdadm --stop /dev/md127
mdadm --assemble --update=super-minor /dev/md1 /dev/sdb8 /dev/sda8

Then you'll have your md127 assembled as md1. And it should stay this way in the future too.

With 1.x arrays:

Newer arrays don't store a 'Preferred Minor' number and do not support "--update=super-minor". Instead they store a name which can be changed with:

mdadm --stop /dev/md127
mdadm --assemble --update=name --name=2 /dev/md1 /dev/sdb8 /dev/sda8

Or you can specify 'hostname:2' to keep everything consistent with other arrays (lookup the current name with --detail).

Solution 2

Just to add to haimg's excellent answer. It may happen that even with a mdadm.conf, the system (at least for Ubuntu) may still auto-assemble the arrays prior to reading mdadm.conf. So when it finds it it just ignores the arrays that have just been assembled anyway.

To account for that, run sudo update-initramfs -u to regenerate the proper initrd.

Relevant links: http://ubuntuforums.org/showthread.php?t=1764861%29%3a

Solution 3

None of the other answers worked for me but in Centos I used the following guide. The issue is that /etc/mdadm.conf is not really used at boot time and only gets updated when a new kernel is installed or initramfs is manually recreated. So essentially you just need:

dracut --force

source: http://realtechtalk.com/mdadm_change_wrong_device_name_md127_fix_and_solution_in_Linux-1783-articles

Solution 4

Version 0.9 superblocks don't have a name, they were introduced in version 1. /dev/md127 is not the name of the array, it's the name of the array's device file, which is a direct consequence of the raid block device's minor number. This minor number is chosen rather arbitrarily when the array is assembled.

The UUID provides a unique identifier for the array, if you need one. If you prefer a human-readable or structured name, put an LVM physical volume in the array.

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Comments

[XXL]

I am aware of the possibility of --homehost from the later version, that is why I have deliberately stated that this is a v0.90 metadata array. However, that is not my intention nor is it compatible. Looking through my posts I also somewhat can not notice where I mentioned that /dev/md127 was the name of the array. I think I have provided the exact details of what I want to do. So the question still remains, how do I change it and where is this "minor number" stored? I presume it is part of the superblock? Will it ruin the superblocks checksum, when hex-edited?

[Gilles 'SO- stop being evil']

@XXL That number is not stored anywhere, it is chosen rather arbitrarily when the array is assembled.

[XXL]

hmm.. I do not understand. Then why do I always get /dev/md127 when i mdadm -Esv? Shouldn't I get something else, that is random, other than a static name? That is if it is really "arbitrary"..

[XXL]

very good! just what i was looking for. so the trick was to also update the super-minor.. thank you!

[Vlad]

You're a life-saver! I used this to unbrick a WD NAS device which uses md.

[nh2]

This fixed the problem for me (after another reboot) on Ubuntu 16.04. In my case it was because I had recreated the array, and it had a new UUID. After updating it in mdadm.conf, rebooting alone was not sufficient; updating the initramfs did the trick.

[yahol]

+1 for the last part of the solution which was very helpful when moving raid array from one machine to another. 'hostname:2' and changing md127 to md1 was exactly what i needed to keep names consistent