Question about a mismatch between documentation and how mdadm currently works.

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Question about a mismatch between documentation and how mdadm currently works.

Kevin O'Gorman
I'm fooling around with RAID on Linux for the first time.  Lots to learn.  I have one issue that
keeps coming up with creating RAID arrays with mdadm.

The documentation I have all says to create arrays with something like
mdadm --create /dev/md0 (and some more stuff)
and what follows really does indicate they expect the array to appear as /dev/md0.  What
happens when I try it is that the array is created as /dev/md127.  If I create two arrays, the
next one is /dev/md126.

Likewise, if I assemble a RAID array, they go to the high numbers no matter what my command-line says.

What's up with this?  Is it historical?  Is something just ignoring that part of the command line?  Is that part even required?

++ kevindf


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Re: Question about a mismatch between documentation and how mdadm currently works.

Tom H-4
On Sat, Apr 1, 2017 at 9:50 PM, Kevin O'Gorman <[hidden email]> wrote:

>
> I'm fooling around with RAID on Linux for the first time. Lots to
> learn. I have one issue that keeps coming up with creating RAID arrays
> with mdadm.
>
> The documentation I have all says to create arrays with something like
> mdadm --create /dev/md0 (and some more stuff) and what follows really
> does indicate they expect the array to appear as /dev/md0. What
> happens when I try it is that the array is created as /dev/md127. If I
> create two arrays, the next one is /dev/md126.
>
> Likewise, if I assemble a RAID array, they go to the high numbers no
> matter what my command-line says.
>
> What's up with this? Is it historical? Is something just ignoring that
> part of the command line? Is that part even required?

# mdadm --create /dev/md0 -l 1 -n 2 /dev/sdb /dev/sdc
# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5]
[raid4] [raid10]
md0 : active raid1 sdc[1] sdb[0]
 1047552 blocks super 1.2 [2/2] [UU]
#

A random shot in the dark: is your hostname set properly?

Or, more usefully, does running "examine" output your actual hostname.
So for my array above, mdadm -E /dev/sd{b,c}

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Re: Question about a mismatch between documentation and how mdadm currently works.

Kevin O'Gorman
It seems to be something else: I was having so much going wrong that I was really confused and not connecting dots properly.

I/O errors were causing the arrays to be degraded or dropped.  When re-assembled using the
instructions I had, they were assigned the identities I mentioned.  Now that I seem to have the
I/O errors under control (it was a power distribution problem), things are matching the
documentation more closely.

Sorry for the alarm.  I seem to be in the steep part of at least one learning curve.

On Mon, Apr 3, 2017 at 12:51 PM, Tom H <[hidden email]> wrote:
On Sat, Apr 1, 2017 at 9:50 PM, Kevin O'Gorman <[hidden email]> wrote:
>
> I'm fooling around with RAID on Linux for the first time. Lots to
> learn. I have one issue that keeps coming up with creating RAID arrays
> with mdadm.
>
> The documentation I have all says to create arrays with something like
> mdadm --create /dev/md0 (and some more stuff) and what follows really
> does indicate they expect the array to appear as /dev/md0. What
> happens when I try it is that the array is created as /dev/md127. If I
> create two arrays, the next one is /dev/md126.
>
> Likewise, if I assemble a RAID array, they go to the high numbers no
> matter what my command-line says.
>
> What's up with this? Is it historical? Is something just ignoring that
> part of the command line? Is that part even required?

# mdadm --create /dev/md0 -l 1 -n 2 /dev/sdb /dev/sdc
# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5]
[raid4] [raid10]
md0 : active raid1 sdc[1] sdb[0]
 1047552 blocks super 1.2 [2/2] [UU]
#

A random shot in the dark: is your hostname set properly?

Or, more usefully, does running "examine" output your actual hostname.
So for my array above, mdadm -E /dev/sd{b,c}

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Kevin O'Gorman
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