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Old 06-20-2011, 02:49 AM   #1
Shodan486
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Lightbulb RAID 5 questions

Finally resurrected my home server and got RAID 5 on it working, got few questions about the array itself, but first the config:

Mobo: Supermicro P3TDLR+
CPUs: 2x Pentium III Tualatin 1,4GHz
RAM: 2x1GB PC133 ECC / buffered

Storage (IDE) : 1x HP DVD-ROM (PATA)
1x IBM DeathStar 80GB (PATA)

(SATA): PCI-X FastTrak x4300 SATAII Hot Swappable
4x Seagate 1TB in RAID5 (SATAII)

And the last one :

More details here : http://www.supermicro.nl/products/ch...C733TQ-645.cfm

Setting up the system (with XP SP2 32bit) was easy, tweaks and stuff done, ALL OK. What bothers me is the array:

RAID 5 gave me a total of 3TB of space and 1TB for the parity - that was expected. Placed 128KB block mode, write-back cache processing, created one 2TB partition and one 1TB partition, since 32bit OS supports only 2TB partitions (but I think some othe OSes can go higher on 32bit somehow.), formatted and copied some .avi files on it from my IDE HDD to perform this test:

Wanted to see the RAID5 3+1 redundancy, so during the copy back of those avis to the IDE drive I unplugged one of those drives - ALL OK, copy back was still running, so I unplugged a second one, which displayed the read error - ALL OK. Plugged the drives back, cancelled the errors and tried copying once more - this time it was stragne, at least I understand it like this (and maybe wrongly): The array was back online, but when I wanted to copy again, I got only 3 drives working, but the copying still worked. When restarted, the BIOS of the controller prompted me for action, because one of the drive could not be found or failed - I thought that was the part of rebuilding my array with parity's assistance, but there was no such option that could allow me to do so - I only had the information table and the capability to delete (thus losing data according to the warning table) and create the logical devices. First I thought the drive really failed, but after deleting the LDEV and creating a new one, the same array, same attributes, everything seemed fine, the drive was back online and arranged into the array. What really bothers me that the .avi files were still there !!!

WHAT? I thought the data are lost when I try to delete and create a new LDEV, no matter if I change the attributes or not. Please clarify.

Also I'd like to ask if there is any logic in RAID failures, for instance I have my 0-3 drives, and let's say I wanna take out the 2nd and the 4th drive (1 & 3), which one of these should NOT be working, or ''ungroup'' itself? Because I have a feeling that when I tried particularly remove the drives no. 1 & 2 and in that order, the no. 1 failed, while the no. 2 worked with the rest of the drives - shouldn't that be vice versa? Or this happens only when I write to the array. Really I'm out of knowledge when grouping / ungrouping drives etc. Should I use some specific software for this type of procedure?
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Old 06-20-2011, 06:54 AM   #2
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Default Re: RAID 5 questions

If you drop two drives in a RAID 5, your data is hosed....
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Old 06-20-2011, 08:07 AM   #3
Shodan486
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Default Re: RAID 5 questions

Quote:
Originally Posted by dood View Post
If you drop two drives in a RAID 5, your data is hosed....
Ehm...What?
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Old 06-20-2011, 08:20 AM   #4
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Default Re: RAID 5 questions

RAID 5 (block-level striping with distributed parity) distributes parity along with the data and requires all drives but one to be present to operate; drive failure requires replacement, but the array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. The array will have data loss in the event of a second drive failure and is vulnerable until the data that was on the failed drive is rebuilt onto a replacement drive. A single drive failure in the set will result in reduced performance of the entire set until the failed drive has been replaced and rebuilt.


Meaning, if you lose more than one drive, you lose your data.
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Old 06-20-2011, 09:31 AM   #5
Per Hansson
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Default Re: RAID 5 questions

Yes, and the controller will mark a drive as failed.
So when you removed two drives you will obviously crash the array, since the maximum failed number of drives is only 1 drive.

Then when you put them back in the controller will still know that these drives failed before, so since they are "unreliable" he will not reuse them unless you tell it that the drive is ok and you want to use it.

Deleting the array and creating a new one with the same parameters does not delete the data on the drives, this can be a quick workaround for the above problem if the RAID controller is being troublesome about allowing a "failed" drive to be reused.

Of course it goes without saying that no initialization procedures or such may be done, then all data will be lost. (But on your 1TB drives a full initialization (writing zeroes to all sectors) takes about 4 hours....)
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Old 06-20-2011, 05:27 PM   #6
Uranium-235
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Default Re: RAID 5 questions

with raid 6 (dual redundant) you can do that, but you also loose another tb of data, and your sata card probably does not support it.

Personally, I like raid 10 if you have 4 drives, speed and redundancy
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Old 06-20-2011, 06:44 PM   #7
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Default Re: RAID 5 questions

Quote:
Originally Posted by Shodan486 View Post
WHAT? I thought the data are lost when I try to delete and create a new LDEV, no matter if I change the attributes or not. Please clarify.
The data will still be there until it is overwritten, usually a zero fill or an algorithm format *something like a DoD-S wipe* will do it.

Also as others have said, raid 5 is only 1 disk redundancy system and
raid is not and should never be used as your only backup storage system.

I have a six disk raid 5 file-server which also rsync it's data to three independent *2,1.5,1.5TB* disk. So if my array is dead, I still have the data to restore it and it goes the other way around. If I lose one of the independent disk I still have the raid to restore that disk.
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