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Old 06-29-2009, 03:36 PM   #1
deibertine
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md0 - what is it?


I have a set up a new CentOS 64 bit, with 4 250GB hard drives (No RAID configured).
Linux version 2.6.9-78.ELsmp (mockbuild@builder10.centos.org) (gcc version 3.4.6 20060404 (Red Hat 3.4.6-10)) #1 SMP Thu Jul 24 23:54:48 EDT 2008

However I noticed whenever I do an fdisk -l, I see this:

Disk /dev/sda: 250.0 GB, 250000000000 bytes
255 heads, 63 sectors/track, 30394 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Start End Blocks Id System
/dev/sda1 1 9561 76798701 da Non-FS data
/dev/sda2 9562 22947 107523045 83 Linux
/dev/sda3 * 22948 22960 104422+ 83 Linux
/dev/sda4 22961 30394 59713605 5 Extended
/dev/sda5 22961 29958 56211403+ fd Linux raid autodetect
/dev/sda6 29959 30394 3502138+ 82 Linux swap

Disk /dev/sdb: 250.0 GB, 250000000000 bytes
255 heads, 63 sectors/track, 30394 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Start End Blocks Id System
/dev/sdb1 * 1 9561 76798701 da Non-FS data
/dev/sdb2 9562 22947 107523045 83 Linux
/dev/sdb3 22948 29958 56315857+ fd Linux raid autodetect
/dev/sdb4 29959 30394 3502170 82 Linux swap

Disk /dev/sdc: 250.0 GB, 250000000000 bytes
255 heads, 63 sectors/track, 30394 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Start End Blocks Id System
/dev/sdc1 * 1 9561 76798701 da Non-FS data
/dev/sdc2 9562 22947 107523045 83 Linux
/dev/sdc3 22948 29958 56315857+ fd Linux raid autodetect
/dev/sdc4 29959 30394 3502170 82 Linux swap

Disk /dev/sdd: 250.0 GB, 250000000000 bytes
255 heads, 63 sectors/track, 30394 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Start End Blocks Id System
/dev/sdd1 * 1 9561 76798701 da Non-FS data
/dev/sdd2 9562 22947 107523045 83 Linux
/dev/sdd3 22948 29958 56315857+ fd Linux raid autodetect
/dev/sdd4 29959 30394 3502170 82 Linux swap

Disk /dev/md0: 172.6 GB, 172680806400 bytes
2 heads, 4 sectors/track, 42158400 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Disk /dev/md0 doesn't contain a valid partition table



Also in fstab md0 is mounted:
/dev/md0 / ext3 defaults 1 1

I am not too sure on what is /dev/md0 is for?
Can I delete/disable/unmount it?

Not sure if the system can boot without it.

Please advise.

Thanks!
DB
 
Old 06-29-2009, 03:52 PM   #2
Uncle_Theodore
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Well, apparently you DO have software RAID configured. And since your root partition is assigned to /dev/md0, you most certainly CAN'T remove it.
Look at the output of
mdadm --detail /dev/md0
cat /proc/mdstat

To see what's going on.
 
Old 06-29-2009, 04:30 PM   #3
NeddySeagoon
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deibertine,
Quote:
Also in fstab md0 is mounted:
/dev/md0 / ext3 defaults 1 1
shows that md0 is your root filesystem.
You can neither unmount it nor remove it.

Kernel raid devices are not normally partitioned so
Quote:
Disk /dev/md0 doesn't contain a valid partition table
is expected
 
Old 06-29-2009, 05:01 PM   #4
deibertine
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That's the biggest fear that I've suspected is I have in fact RAID configured.

What's weird is I specifically stated "No RAID" on the SaS Utility but then it still did RAID.

Is it possible that the OS image might have re-configured the setup for RAID?

I have Dell l precision t7400.

Anyway that I can get around this and disable RAID?

I just want 4 plain disk (JBOD) to setup.

Trying to avoid re-doing the OS install again.

Any advise is much appreciated.
 
Old 06-29-2009, 05:59 PM   #5
chrism01
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Location: Sydney
Distribution: Centos 6.5, Centos 5.10
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To start with use Uncle Theodore's cmds so we can see exactly what type of RAID is implemented and which disks are involved.
You can then disable the RAID with the right cmds, something like

mdadm --fail /dev/md0 /dev/sda1
mdadm --remove /dev/md0 /dev/sda1

then you'd have to change the type of your boot disk to Linux (83) instead of fd0, using fdisk.
You can have a look at the man page as well.
I'd definitely advise a backup first!
 
Old 06-30-2009, 11:00 AM   #6
deibertine
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Here are the results:

# mdadm --detail /dev/md0
/dev/md0:
Version : 00.90.01
Creation Time : Wed Jun 24 07:12:08 2009
Raid Level : raid5
Array Size : 168633600 (160.82 GiB 172.68 GB)
Device Size : 56211200 (53.61 GiB 57.56 GB)
Raid Devices : 4
Total Devices : 4
Preferred Minor : 0
Persistence : Superblock is persistent

Update Time : Tue Jun 30 08:52:20 2009
State : clean
Active Devices : 4
Working Devices : 4
Failed Devices : 0
Spare Devices : 0

Layout : left-symmetric
Chunk Size : 256K

UUID : a24af5e9:03b53220:23ded730:0c9a4a97
Events : 0.234992

Number Major Minor RaidDevice State
0 8 5 0 active sync /dev/sda5
1 8 19 1 active sync /dev/sdb3
2 8 35 2 active sync /dev/sdc3
3 8 51 3 active sync /dev/sdd3

# cat /proc/mdstat
Personalities : [raid5]
md0 : active raid5 sdd3[3] sdc3[2] sdb3[1] sda5[0]
168633600 blocks level 5, 256k chunk, algorithm 2 [4/4] [UUUU]

unused devices: <none>
 
Old 06-30-2009, 02:29 PM   #7
NeddySeagoon
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deibertine,

Its kernel software raid level 5. You cannot dismember that easily without a reinstall.
You could drop one device out of the raid set, reformat it and copy the install over but its unlikely to save much time.

You will also need to change some of the setup, like /etc/fstab and grub.conf so the system does not look for root on /dev/md0
 
Old 06-30-2009, 06:16 PM   #8
deibertine
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I found the problem, well it's not really a problem but it's an enhancement of our image.

It turned out that our image is set to operate in LVM RAID5 - wasnt aware of this as I did not create the image.

It's complicated but due to specifications within the supported product.... :-)

Thanks to all who posted.
 
  


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