Re: bug report: mdadm-devel-2 , superblock version 1

On Sunday July 24, pml@dtbb.net wrote:

Neil Brown wrote:

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On Sunday July 17, pml@dtbb.net wrote:

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# uname -a
Linux server 2.6.12.3 #3 SMP Sun Jul 17 14:38:12 CEST 2005 i686 GNU/Linux
# ./mdadm -V
mdadm - v2.0-devel-2 - DEVELOPMENT VERSION NOT FOR REGULAR USE - 7 July 2005

...

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root@server:~/dev/mdadm-2.0-devel-2# cat /proc/mdstat
Personalities : [raid5]
md1 : active raid5 sdc2[3] sdb2[1] sda2[0]
     128384 blocks level 5, 64k chunk, algorithm 2 [3/3] [UUU]

unused devices: <none>

** mdstat mostly okay, except sdc2 is listed as device3 instead of 

Hmmm, yes....  It is device number 3 in the array, but it is playing
role-2 in the raid5.  When using Version-1 superblocks, we don't moved
devices around, in the "list of all devices".  We just assign them
different roles. (device-N or 'spare').

So if I were to add (as an example) 7 spares to a 3 disk raid-5 array, 
and later removed them for use elsewhere, a raid using a v1.x superblock 
would keep a permanent listing of those drives even after being
removed? 

No.  A device retains it's slot number only while it is a member of
the array.  Once it is removed from the array it is forgotten.  If it
is re-added, it appears simply as a new drive and is allocated the
first free slot.

   Is there a possibility (for either asthetics, or just keeping things 
easier to read and possibly diagnose at a later date during manual 
recoveries) of adding a command line option to "re-order and remove" old 
devices that are marked as removed, that could only function if the 
array was clean, and non-degraded?  (this would be a manual feature we 
would run, especially if automatically doing this might actually confuse 
us during times of trouble-shooting?)

I'd rather change the output of mdadm to display the important
information (role in array) more prominently that the less important
info (position in array).

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   Number   Major   Minor   RaidDevice State
      0       8        2        0      active sync   /dev/.static/dev/sda2
      1       8       18        1      active sync   /dev/.static/dev/sdb2
      2       0        0        -      removed

      3       8       34        2      active sync   /dev/.static/dev/sdc2

** reports version 01.00.01 superblock, but reports as if there were 4 
devices used

Ok, this output definitely needs fixing.  But as you can see, there
are 3 devices playing roles (RaidDevice) 0, 1, and 2.  They reside in
slots 0, 1, and 3 of the array.

Depending on your answer to the first question up above, a new question 
based on your comment here comes to mind... if we assume, as you say 
above that it is normal for v1 superblocks to keep old removed drives 
listed, but down here you say the output needs fixing, which output is 
wrong in the example showing 0,1,2,3 devices, with device #2 removed, 
and device 3 acting as raiddevice 2 ?  If the v1 superblocks are 
designed to keep removed drives listed, then the above output makes 
sense.. now that you've pointed out the "feature".

It should look more like:

    Number   Major   Minor   RaidDevice State
       0       8        2        0      active sync   /dev/.static/dev/sda2
       1       8       18        1      active sync   /dev/.static/dev/sdb2
       3       8       34        2      active sync   /dev/.static/dev/sdc2

i.e. printing something that is 'removed' is pointless.  And the list
should be sorted by 'RaidDevice', not 'Number'.

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root@server:~/dev/mdadm-2.0-devel-2# ./mdadm -A /dev/md1
Segmentation fault

** try to assemble the array

This is not how you assemble an array.  You need to tell mdadm which
component devices to use, either on command line or in /etc/mdadm.conf
(and give --scan).

I failed to mention that I had an up to date mdadm.conf file, with the 
raid UUID in it, and (I will have to verify this) I believe the command 
as I typed it above, works with the 1.12 mdadm.  The mdadm.conf file has 
a DEVICE=/dev/hd[b-z] /dev/sd* line at the beginning of the config file, 
and then the standard options (but no devices= line).  Does -A still 
need *some* options even if the config file is up to date??  (as I said, 
I'll have to verify if 1.12 works with just the -A).

mdadm won't look at the config file unless you tell it too (with
--scan or --configfile).  At least that is what I intended.

Also, if -A requires some other options on the command line, should it 
not complain, instead of segfaulting? :D

Certainly!

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root@server:~/dev/mdadm-2.0-devel-2# ./mdadm -D /dev/md1
mdadm: md device /dev/md1 does not appear to be active.

** check if its active at all

root@server:~/dev/mdadm-2.0-devel-2# ./mdadm -A /dev/md1 /dev/sda2 
/dev/sdb2 /dev/sdc2
mdadm: device 1 in /dev/md1 has wrong state in superblock, but /dev/sdb2 
seems ok
mdadm: device 2 in /dev/md1 has wrong state in superblock, but /dev/sdc2 
seems ok
mdadm: /dev/md1 has been started with 3 drives.

** try restarting it with drive details, and it starts

Those message are a bother though.  I think I know roughly what is
going on.  I'll look into it shortly.

Is this possibly where the v1 superblocks are being mangled, and so it 
reverts back to the v0.90 superblocks that it finds on the disk?

I'm not sure until I look carefully through the code.

NeilBrown