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Old 11-06-2008, 05:08 AM   #1
Registered: Feb 2007
Posts: 34

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RAID1 how does it mirror

Ok i've setup RAID1 of my system, where "sda1" will mirror itself on "sdb1", i did that yesterday, and when i type cat /proc/mdstat, it was showing me something is sync etc... Now what i want to knw is, how does the sync work i.e. How does the system know when to mirror?? Is there command line for it?? To be more specific how do i get "sda1" to sync/mirror onto "sdb1" every 12 hours?? Can anyone guide me please.

In root mailbox i got this message, should i worry or is it normal??
This is an automatically generated mail message from mdadm
running on

A DegradedArray event had been detected on md device /dev/md1.

Faithfully yours, etc.

P.S. The /proc/mdstat file currently contains the following:

Personalities : [raid1]
md0 : active raid1 sdb1[1] sda1[0]
10241280 blocks [2/2] [UU]

md1 : active raid1 sda2[0]
478142528 blocks [2/1] [U_]

unused devices: <none>
Old 11-06-2008, 05:18 AM   #2
LQ Newbie
Registered: Nov 2008
Location: South-West UK
Distribution: Fedora, RHEL 4 and 5, Ubuntu 7 and AIX
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What is the system? What version of what OS?

If its basic software RAID 1 then it's all done behind the scenes (constant mirror of one disk to another - so if you lose one disk the other will still run and no data will be lost) and as far as you or me need to know, everytime a write is made to disk it is instantly mirrored to your second disk (well maybe not instantly but near enough)...

You can run the command mdadm --query --detail /dev/md0 for more info.

Thats all I can say as far as the information you've given and what that means to me!

As for the error - degraded means one of the disks is already missing from the array, are there any more errors related to this? This basically would mean your RAID wasnt working if this was the case.

Last edited by DT0X; 11-06-2008 at 05:24 AM.
Old 11-06-2008, 05:39 AM   #3
Registered: Feb 2007
Posts: 34

Original Poster
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I am not sure what you mean by "What is the System?" The OS is centos5.
But yesterday as soon as i finished the installation everything worked fine, it was sync nicely then i tested by unplug 1 of the HD and both of them was working. Today i've installed a few bits and transferred a few files. Tested again checked on the other HD to see if it was mirrored there but none of file and updates was present there.
[root@mau ~]# mdadm --query --detail /dev/md0
Version : 00.90.03
Creation Time : Wed Nov 5 19:12:13 2008
Raid Level : raid1
Array Size : 10241280 (9.77 GiB 10.49 GB)
Device Size : 10241280 (9.77 GiB 10.49 GB)
Raid Devices : 2
Total Devices : 1
Preferred Minor : 0
Persistence : Superblock is persistent

Update Time : Thu Nov 6 15:34:09 2008
State : clean, degraded
Active Devices : 1
Working Devices : 1
Failed Devices : 0
Spare Devices : 0

UUID : 0e31d364:7b6cf8e7:cebf8568:6ac74d63
Events : 0.32

Number Major Minor RaidDevice State
0 0 0 0 removed
1 8 17 1 active sync /dev/sdb1
It seems that my raid doesn't work anymore. How do i get it back?
Old 11-06-2008, 05:53 AM   #4
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I just used the word system because it was how you began your first post, sorry to confuse.

To add your disk back into the array see this document towards the bottom with raidhotadd etc...

Looks like someone had similar issues (albeit on Debian) here



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