Re: Failed JBOD RAID on old NAS, how to diagnose/resurrect?

[Chris Green <cl@isbd.net>]

On Tue, Mar 10, 2020 at 02:20:02PM -0700, Song Liu wrote:
> On Sun, Mar 8, 2020 at 7:15 AM Chris Green <cl@isbd.net> wrote:
> >
> > Well I've got it working again but I'm very confused as to *why* it
> > failed the way it did.
> >
> > A 'cat /proc/mdstat' produced:-
> >
> >     Personalities : [linear] [raid0] [raid1]
> >     md4 : active raid1 sda4[0]
> >           973522816 blocks [2/1] [U_]
> >
> >     md1 : active raid1 sdb2[0] sda2[1]
> >           256960 blocks [2/2] [UU]
> >
> >     md3 : active raid1 sdb3[0] sda3[1]
> >           987904 blocks [2/2] [UU]
> >
> >     md2 : active raid1 sdb4[0]
> >           973522816 blocks [2/1] [U_]
> >
> >     md0 : active raid1 sdb1[0] sda1[1]
> >           1959808 blocks [2/2] [UU]
> >
> > So md2 and md4 (the main parts of the two 1Tb disk drives) seemed to
> > be OK from the RAID point of view.  But I noticed that the block
> > device for /dev/md4 didn't exist:-
> >
> >     ~ # ls -l /dev/md*
> >     brw-r-----    1 root     root       9,   0 Sep 29  2011 /dev/md0
> >     brw-r-----    1 root     root       9,   1 Sep 29  2011 /dev/md1
> >     brw-r-----    1 root     root       9,  10 Sep 29  2011 /dev/md10
> >     brw-r-----    1 root     root       9,  11 Sep 29  2011 /dev/md11
> >     brw-r-----    1 root     root       9,  12 Sep 29  2011 /dev/md12
> >     brw-r-----    1 root     root       9,  13 Sep 29  2011 /dev/md13
> >     brw-r-----    1 root     root       9,  14 Sep 29  2011 /dev/md14
> >     brw-r-----    1 root     root       9,  15 Sep 29  2011 /dev/md15
> >     brw-r-----    1 root     root       9,  16 Sep 29  2011 /dev/md16
> >     brw-r-----    1 root     root       9,  17 Sep 29  2011 /dev/md17
> >     brw-r-----    1 root     root       9,  18 Sep 29  2011 /dev/md18
> >     brw-r-----    1 root     root       9,  19 Sep 29  2011 /dev/md19
> >     brw-r-----    1 root     root       9,   2 Sep 29  2011 /dev/md2
> >     brw-r-----    1 root     root       9,  20 Sep 29  2011 /dev/md20
> >     brw-r-----    1 root     root       9,  21 Sep 29  2011 /dev/md21
> >     brw-r-----    1 root     root       9,  22 Sep 29  2011 /dev/md22
> >     brw-r-----    1 root     root       9,  23 Sep 29  2011 /dev/md23
> >     brw-r-----    1 root     root       9,  24 Sep 29  2011 /dev/md24
> >     brw-r-----    1 root     root       9,  25 Sep 29  2011 /dev/md25
> >     brw-r-----    1 root     root       9,  26 Sep 29  2011 /dev/md26
> >     brw-r-----    1 root     root       9,  27 Sep 29  2011 /dev/md27
> >     brw-r-----    1 root     root       9,  28 Sep 29  2011 /dev/md28
> >     brw-r-----    1 root     root       9,  29 Sep 29  2011 /dev/md29
> >     brw-r-----    1 root     root       9,   3 Sep 29  2011 /dev/md3
> >     brw-r-----    1 root     root       9,   5 Sep 29  2011 /dev/md5
> >     brw-r-----    1 root     root       9,   6 Sep 29  2011 /dev/md6
> >     brw-r-----    1 root     root       9,   7 Sep 29  2011 /dev/md7
> >     brw-r-----    1 root     root       9,   8 Sep 29  2011 /dev/md8
> >     brw-r-----    1 root     root       9,   9 Sep 29  2011 /dev/md9
> >
> >
> > The fix was simply to use 'mknod' to create the missing /dev/md4, now
> > I can mount the drive and see the data.
> >
> > What I don't understand is where /dev/md4 went, how would it have got
> > deleted?  I have yet to reboot the system to see if /dev/md4
> > disappears again but if it does it's not a big problem to create it
> > again.
> >
> > Should the RAID block devices get created as part of the RAID start
> > up? Maybe there's something gone awry there.
> 
> Do you have proper /etc/md.conf?
> 
There is no /etc/md.conf or anything that I can see related to RAID
configuration anywhere in the system.

-- 
Chris Green