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Old 04-01-2005, 08:58 AM   #1
boozer_2
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Raid 5 Operation Questions


I have a Raid 5 array up and running across 5 IDE 160GB hard drives. Since I am a newbie to Linux and Raid 5 I had some questions about how Raid 5 works.

I know one disk can fail, and the array will still be okay, but lets say one of my IDE controllers fail, and two drives drop from the array. Each of the hard drives are okay, but is the Raid array gone now and I've lost all data? Or will everything be okay once i replace the IDE and get the two drives back online?

What if an ide controller fails (or some other wierd issue) on power up and I lose 2 drives? Will the RAID 5 array fail to initialize during boot since 2 drives are missing, or will it initialize regardless, and then since 2 drives have failed, corrupt all my data.

If I replace a failed drive, and another drive fails while the array is reconstructing, will this cause a loss of data?

Thanks in advance for your help!
 
Old 04-01-2005, 09:27 AM   #2
Technoslave
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Since it's a PC and not big hardware, you're not cross connecting your IDE controllers for redundancy, you're doing it in order to speed up operations to disk a little, as well as to be able to have that many disks running at once.

So, yes, if one IDE controller does fail then the two disks that you have on there will cause the RAID to fail .. what happens from there I don't know, if you can manage to power off and replace the IDC controller, everything may be ok, it's hard to tell.

Yes, if a drive fails while rebuilding, then the entire array is hosed, welcome to the world of RAID 5.

The only good part is that it's much more rare for a controller to die than it is for an HD to go.
 
Old 04-01-2005, 09:29 AM   #3
ironwalker
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with 5 disks you can use 2 as hotspare.
 
Old 04-01-2005, 10:29 AM   #4
boozer_2
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Thanks for the replies

Anyone know what happens if an ide controller fails (or some other wierd issue) on power up and I lose 2 drives? Will the RAID 5 array fail to initialize during boot since 2 drives are missing, or will it initialize regardless, and then since 2 drives have failed, corrupt all my data. I would think that the software would be smart enough to see that 2 drives are missing and therefore not try to initiate the raid array... but I don't know. Anyone have an answer on this?

Thanks!
 
Old 04-01-2005, 02:58 PM   #5
Technoslave
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It shouldn't corrupt, but it's a computer, you never know.
 
Old 04-01-2005, 05:23 PM   #6
Jason1625
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With Raid 5, it needs one drive to start up. Are you booting off the array or are you booting off another with a RAID tower? in raid 5, if two drives fail, you lose everything. With 5 drives you might be better to do a 1+0 with a hot spare.

The way raid 5 works is like so

1 2 *
3 * 4
* 5 6

is the sort of format it goes on, with the * being the parity bit. if you use all five drives in the array it will be like this
1 2 3 4 *
5 6 7 * 8
9 10 * 11 12
13 * 15 16 17
* 18 19 20 21

You might want to look either 1+0 or into maybe raid 6+ as they are better tailored to more drives, Raid 5 works best under 3 drives.

Last edited by Jason1625; 04-01-2005 at 05:26 PM.
 
Old 04-01-2005, 08:39 PM   #7
Electro
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You need atleast 3 drives for RAID 5 to work. Yes, it will still work if two out of five drives fail. Three of the drives will have partial parity information to rebuild the two drives that have failed. Though four drives is the minimal you should use for RAID 5 if do not want to take a chance that it might fail.

Always and I do mean always schedule backups on a daily basis. 700 to 800 gigabytes is very hard to backup and it costs more money to keep it up.

Keep the drives cool so they do not fail sooner than you might think.
 
Old 04-03-2005, 12:18 AM   #8
boozer_2
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I did not realize I could lose 2 drives on a 5 drive array and still have a working array. Are you serious about that? Just everything I have read stated that if you lost 2 drive, you lost it all, regardless of the array size, unless you had hot spares.

As far as automatic drive initialization in Linux on boot, is there a definitive way linux will act if it is missing 2+ drives from the array? Pretend only 1 of four drives is powered, will it still try and initialize the array even though devices are missing? If so, sounds like it might be best to use a script to verify all devices are present before initializing the array. If you do something dumb, like forgetting power to a couple of the drives, this would prevent the array from initializing and destroying the raid array when there was no need for that to happen.

I am not booting off the array, i have a separate drive for the OS.
 
Old 04-04-2005, 03:43 AM   #9
Jason1625
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Well, I've tested this before, but in RAID 5 because of the way it works with parity with hotspares you -can- lose two, but not at the same time.

When one fails, it takes time for the other two to use the paritity information to write the data, or in the case of a hot spare, time for it to copy over using the parity drive. The way raid 6 works is that there are two parity peice of data per go, it's like this 1, 2, 3, *, *, etc

Really raid 5 is good, but only for the cost effective way to get a limited ammount of data redudancy for not having tons of harddrives. Your best bet is 1, worse being 0.

0+1 is always good for a hybrid array, it goes like this

1 2 1 2
3 4 3 4
5 6 5 6
7 8 7 8

I've used raid 5 array on a IBM X series, what happens is when one drive fails, it starts trying to get your attention. Thats only with three drives though, when I've used the raid tower, it just rebuilds the array tells you one has failed, but no serious warnings load up.

Last edited by Jason1625; 04-04-2005 at 03:45 AM.
 
Old 04-04-2005, 07:10 AM   #10
floppywhopper
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Check out this link

http://www.acnc.com/04_00.html

floppy

Last edited by floppywhopper; 04-04-2005 at 07:11 AM.
 
  


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