RAID1 /dev/md4 lost one disk partion /dev/sdb4 after reboot, WHY??

[L]

On Sun, Mar 29, 2009 at 5:18 PM, Bruno Wolff III <bruno at wolff.to> wrote:
> On Sun, Mar 29, 2009 at 10:00:07 +1100,
>  L <yuanlux at gmail.com> wrote:
>> Hi
>>
>> I have RAID1 system,  the partition /dev/md4 is in raid1 with
>> /dev/sda4 /dev/sdb4. In order to check /dev/sdb4 I remove it from
>> /dev/md4
>
> There are ways of checking disks without pulling them out of service.
> smartctl can do surface scans while leaving the disk functional.
>
>> The system did sync md4, this lasted ~ 6 hours. however, after reboot,
>> /proc/mdstat shows /dev/md4 has one disk /dev/sda4 active.
>
> 6 hours is a long time for a resync. Was the system under heavy load?
> Are an IO errors showing up in your logs?
>
>> How to let system hold both partitions /dev/sda4 and /dev/sdb4 for
>> /dev/md4 after reboot?
>
> From what you described it looked like you were correctly readding the
> disk and waiting for the sync to complete before rebooting.
>


The  md re-sync on /dev/md4 (/dev/sda4 /dev/sdb4) was completed, after
reboot, the partition /dev/sdb4 on /dev/md4 was missed again, as shown
from mdstat

Personalities : [linear] [raid1]
md4 : active raid1 sda4[0]
     484327552 blocks [2/1] [U_]

md1 : active raid1 sdb1[1] sda1[0]
     2939776 blocks [2/2] [UU]

 md2 : active raid1 sdb2[1] sda2[0]
     104320 blocks [2/2] [UU]

 md3 : active raid1 sdb3[1] sda3[0]
     987904 blocks [2/2] [UU]

 unused devices: <none>

 any suggestion??

thanks

Y