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Old 12-14-2005, 04:19 AM   #1
Registered: Nov 2003
Location: Belgium
Distribution: Mandriva
Posts: 91

Rep: Reputation: 15
Another RAID question 5 vs 1


I am wondering about something. I am installing a Backup server. Well actually I am reinstalling it because I had a disk failure on my RAID 1 system. It were 2 disks of 250GB of which one died. Since I made also some lame install stupidities I had the idea of just reinstalling the whole system and taking out the good disk. I was able to mount that disk on an other machine with mount -t ext3 /mnt/sda1 /mnt/raid because it was a perfect mirror.

My real questions are the following. I have now 3 disks of 250 GB which allows me to use a raid 5 system. Question one is : would you consider this since I only have 3 disks and not more, or would you go back for raid 1.
Question 2 is the following. Assume one of my disks fails, then the raid will be out of order until a spare disk is installed and with raidhotadd resyncronised. But if I take out the two good disk, would I be able to build the third disk on an other system. Or if I have 3 good disks, would I be able to read the RAID on an other system. Because my idea is that if you add a raid system (with data) to another computer, it will be formatted and lost! But that would be actually very stupid so I think you should be able to read raiddisk on an other computer ... but how.

Thanks in advance

Old 12-14-2005, 08:54 PM   #2
Registered: Apr 2003
Location: Reading, Berkshire, IN-GER-LUND!!!
Distribution: Mandriva 2006
Posts: 228

Rep: Reputation: 30
First off, RAID1 requires an even number of disks. RAID5 will work with three, and personally, that's what I'd do. With three 250GB drives, you'll have a 500GB RAID5 array (the capacity of a RAID5 array is (N-1)*S where N is the number of drives and S is the capacity of each drive)

RAID5 can tolerate ONE disc faliure per array. The array will continue to work, but slowly as the data on the failed disk will need to be rebuilt "on the fly" from the parity data on the remaining disks.

If you're using standard Linux software RAID then yes, you can rebuild the failed disc on another system and move an array to another system provided that you copy the RAID configuration files to the new system. For recent distributions that's /etc/mdadm.conf. Also, make sure the individual devices in the array have the same /dev entries in the two systems. Else, you'll need to edit the configs to reflect any changes. It is strongly reccomended that you use the same version of the RAID software on both systems.

For example, a while ago I moved a software RAID0 array from one system to another after moving the /etc/mdadm.conf file. On the new system, the two SATA discs from the old system appeared as /dev/sdb and /dev/sdc, whereas in the old system they were /dev/sda and /dev/sdb. Because of this I edited the etc/mdadm.conf file to set the individual devices to /dev/sdb1 and /dev/sdc1. Then started the array as normal with mdadm --assemble /dev/md0. This procedure is the same for all RAID levels.

Hope this helps!


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