[CentOS] RAID 5 setup?

Les Mikesell lesmikesell at gmail.com
Thu Mar 25 21:56:34 UTC 2010

On 3/25/2010 4:43 PM, Boris Epstein wrote:
> On Thu, Mar 25, 2010 at 5:27 PM, Hakan Koseoglu<hakan at koseoglu.org>  wrote:
>> On Thu, Mar 25, 2010 at 9:07 PM, Boris Epstein<borepstein at gmail.com>  wrote:
>>>> Note: RAID5 is not really recomended for such large disks.  You run the
>>>> risk of a complete data loss if one disk fails and the another disk
>>>> fails during the rebuild.
>>> Why is the size a factor here? Why would this be OK with smaller
>>> disks? How would you partition this instead?
>> As the disks get bigger, rebuild time also increases and the
>> performance of the disks don't increase linearly with their storage.
>> This means that when you are rebuilding a disk, the chances of one of
>> your other disks failing becomes significantly large. Most suggest
>> RAID6 these days as a minimum, mirroring and striping appears to be
>> the most popular.
> You surely do have a point there. However, it is still not all that
> likely that a disk will fail during the rebuild time in question (what
> are we talking? some hours max?)

The common problem is that there are unused portions of the drives that 
go bad but are unnoticed for a long time.  Then one fails badly enough 
to get kicked out of the raid. Then when you rebuild, you have to 
reconstruct parity for even the unused parts of the drive and you hit 
previously unnoticed bad spots in the process.  I think the last Centos 
update added some sort of raid scan as a cron job that might detect bad 
spots earler, but I'm not sure what it actually does.

   Les Mikesell
    lesmikesell at gmail.com

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