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I have a RAID5 on SYnology nas system that has crashed. In first the system prevent to one disk has been removed from the raid with the degraded status. After a shudown to unplug the disk and restart the nas, the raid status has changed to failed (CLean, degraded satus is still visible in raid configuration of mdadm) but in gui the size of raid array is 0byte :( and impossible to repair this!

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  • Can you post the contents of /proc/mdstat? – maxf Aug 24 '16 at 14:29
  • Hi, Yes I wil post the content of mdstat this evening because i have not access to the nas at this time. If I remember correctly the status of md5 indicate Clean,degraded with a missing disk (disk not appear anymore). on list sdd[5] – Greg186320 Aug 24 '16 at 15:03
  • Did you ask your Synology support? – Nils Aug 24 '16 at 15:04
  • Hi, Yes I wil post the content of mdstat this evening because i have not access to the nas at this time. If I remember correctly the status of md5 indicate Clean,degraded with a missing disk (disk not appear anymore). The partition list should be this one: sdc[5] sdd[5] sde[5] sdf[5] sdg[5] sdh[5] And the actual list is sdd[5] sde[5] sdf[5] sdg[5] sdh[5] Other thing is the [UUUUUU] indication I have found that indicated a normal Raid status but since the failure this status is [_UUUUU], I think it's indicate that the first disk in raid array has been removed. – Greg186320 Aug 24 '16 at 15:26
  • No, I have not asking to Synology support because it's a virtual machine. and the physical synology is an 2 bay only and have no issue. – Greg186320 Aug 24 '16 at 15:27
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Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] 
md2 : active raid1 sda3[0] sdb3[1] 
     21392384 blocks super 1.2 [2/2] [UU] 

md3 : active raid5 sdd5[1] sdh5[5] sdg5[4] sdf5[3] sde5[2]
     9743376960 blocks super 1.2 level 5, 64k chunk, algorithm 2 [6/5] [_UUUUU]

md1 : active raid1 sda2[0] sdb2[1] sdd2[7] sde2[6] sdf2[5] sdg2[4] sdh2[3]
     2097088 blocks [12/7] [UU_UUUUU____]

md0 : active raid1 sda1[0] sdb1[1] sdd1[7] sde1[6] sdf1[5] sdg1[4] sdh1[3]
     2490176 blocks [12/7] [UU_UUUUU____] unused devices: <none>

It's a good thing mdadm --create errored out on you: that command is more likely to destroy your array than save it. Since it failed early, it likely didn't do any damage.

/dev/md3 is only missing one device, so it should be possible to recover it. Running mdadm --run /dev/md3 followed by mdadm --readwrite /dev/md3 should make it usable; at this point, replace the failed disk and add the new one to the array with mdadm --manage /dev/md3 --add /dev/<insert new disk here>. The md subsystem will then start rebuilding the array.

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