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Potential Failure with RAID 5 with 3 Disks

Posted on 2003-11-24
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I have a server that has 3 disks on a hardware RAID 5 configuration.  Since RAID 5 requires a 3 disk minimum, if 1 disk fails, does the RAID 5 "dissappear" and the server stops working, or does it keep running long enough for a replacement drive to be put in for rebuilding?  Is this allowed for hot swappable drives?

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Question by:Joe_27
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by:Luc Franken
Luc Franken earned 150 total points
ID: 9812185
Hi Joe_27,

It'll just keep on working, if the controller supports hotswap, you can even pull the failing harddrive out and insert a new one when the system is up and running (some controllers will even rebuild the raid automatically) You can always choose to use a hot-spare drive, so when one drive fails, the hot-spare will take over and you can replace the failing drive, the new drive will then become the hot-spare drive.

Greetings,

LucF
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by:Joe_27
ID: 9812340
So if 1 drive fails on a RAID 5-3 Disk System, the server can still function with the other 2?  Is all data retained when the 2 drives are running?
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by:chicagoan
chicagoan earned 100 total points
ID: 9812510
The array will reconstruct the data from parity information on the remaining good drives.
This will be a big hit on performance until you replace the failed drive and it is reconstructed.. You can add a hot spare toyour array which would rebuild automatically if one of the drives fail.
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by:Luc Franken
ID: 9812551
>So if 1 drive fails on a RAID 5-3 Disk System, the server can still function with the other 2?
Yes (p.s. I suggest you use RAID 5 as RAID 3 gives a lot of stress to the parity drive)

>Is all data retained when the 2 drives are running?
Yes

LucF
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by:Joe_27
ID: 9812817
Even with the 2 drives, it will remain in a RAID 5 mode?
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by:Luc Franken
ID: 9813078
sort of.... You've lost the fail safe function of RAID5, but the speed is still sustained and all data is still there. If one drive fails you should replace it as soon as possible, as when a second drive fails you've lost all data....

LucF
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by:chicagoan
ID: 9813348
It's known as 'degraded' mode and the data is there but I'm not sure I agree with the speed issue:

IEEE 14th International Parallel and Distributed Processing Symposium (IPDPS'00)
May 01 - 05, 2000
Cancun, Mexico
Nitin Muppalaneni, VERITAS Software Inida Pvt. Ltd.
K. Gopinath , Indian Institute of Science


Redundant Arrays of Inexpensive Disks (RAID) is a popular technique used to improve the reliability and performance of secondary storage. Of various levels of RAID discussed in \cite {bib:raid}, RAID1 and RAID5 have become more popular. Mirroring or RAID1 maintains multiple copies of the data, generally provides best performance and is easier to configure. Rotating parity scheme or RAID5 is the least expensive RAID scheme with good large update performance. It suffers from poor small update performance and performance drops sharply when a disk fails and the array enters degraded mode. Configuring RAID5 is more involved.This paper presents the design and implementation of a host-based driver for a multi-tier RAID storage system, currently with 2 tiers: a small RAID1 tier and a larger RAID5 tier. Based on access patterns, the driver automatically migrates frequently accessed data to RAID1 while demoting not so frequently accessed data to RAID5. The prototype provides reliable persistence semantics for data migration between the tiers using ordered updates. Mechanisms are separated from policies through an API so that any desired policy can be implemented in trusted user processes. Finally, we present comparison of the performance of our system with comparable systems using striping and RAID5.
 
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by:Luc Franken
ID: 9813426
>It suffers from poor small update performance and performance drops sharply when a disk fails and the array enters degraded mode.
I sort of agree with you, but.... .... most of the times users of the server won't even notice the difference, that's what I was pointing to with "but the speed is still sustained" bit of a misplaced thing if I look at it now :(
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by:chicagoan
ID: 9813557
as long as we notice it before another drive goes belly up!
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asgarali earned 150 total points
ID: 9830438
yes the raid 5 requires a minimum of 3 disks, if one dsk fails the server is still up and running without any problems and raid config is not disappeared  you could replace the defectiive hardrrive with a good one it will rebuild and the raiid set wiill be fine.  the only error it would show u iis the raid is degraded,  the hotswappable is allowed iif the server has all hotswappable drives in it.  you could also have one hotspare configured in the raid so that if any one fails the hotspare will  become a part of raid 5 and the one defect can be reverted to a hotspare when u have the replacement.  its not advisable to have the disks running for long enough coz sometimes  iif u have  one more hard disk fail then it would be a  big problem.


stack
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Expert Comment

by:asgarali
ID: 9830439
yes the raid 5 requires a minimum of 3 disks, if one dsk fails the server is still up and running without any problems and raid config is not disappeared  you could replace the defectiive hardrrive with a good one it will rebuild and the raiid set wiill be fine.  the only error it would show u iis the raid is degraded,  the hotswappable is allowed iif the server has all hotswappable drives in it.  you could also have one hotspare configured in the raid so that if any one fails the hotspare will  become a part of raid 5 and the one defect can be reverted to a hotspare when u have the replacement.  its not advisable to have the disks running for long enough coz sometimes  iif u have  one more hard disk fail then it would be a  big problem.


stack
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by:Gerald Connolly
Gerald Connolly earned 100 total points
ID: 9838615
Joe,

AS others have said RAID 5 requires a minimum of three disks. But only two of them hold your data, if one of these disks fails, the third (the so-called "Parity Disk" ) holds information that once combined with the remaining data disk allows for the data from the failed drive to be reconstructed, thereby preserving your data if one disk fails.

so your data looks like this:-

    Disk1          Disk2           Disk3
DataBlock1  DataBlock2  ParityBlockof1&2
DataBlock3  DataBlock4  ParityBlockof3&4
etc

In most RAID5 implementations the Parity block is cycled around the Disks to alleviate the overhead of using a dedicated Parity disk. If that were to happen and you lost a data disk, every disk access would be hit with reconstruction of the data from the failed disk, an overhead that your RAID controller could do with-out!

You can see that with a 3 disks you only get your useable capacity from 2 disks, so 3*18GB = 54GB raw capacity but you only get 36GB useable, a 33% loss, with 4 disks its 72GB raw and 54GB useable, a 25% loss and so on, but beware that although more spindles means the RAID5 overhead reduces, it also means proportionally more work if a disk fails and data needs to be reconstructed, as every remaining disk and the parity needs to be read to reconstruct the failed data, so in a 12 disk RAID5 set,it means 11 disk reads to get the data from a failed drive.

G
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