[CentOS] How to test RAID-5

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Thu Mar 9 11:42:31 UTC 2006

Fajar Priyanto wrote:
> On Thursday 09 March 2006 05:07 pm, Simone wrote:
>> Hi, once you remove sda, you need to setup grub on sdb to get it
>> working. Just boot from the CentOS cd1 into linux rescue mode, and it
>> should autodetect your system and mount it under /mnt/sysimage.
>> Once you're in,
>> chroot /mnt/sysimage
>> /sbin/grub --> you get a prompt grub>
>> In my case I read in my grub.conf:
>> title CentOS (2.6.9-
>>         root (hd0,0)
>> so at the grub> prompt I would type
>> grub> root (hd0,0)
>> grub> setup (hd0)
>> grub> quit
>> You can then exit, reboot and your system should be up and running.
>> Hope this helps
> Thanks Simone,
> It works now. Anyway, can you give me an insight how RAID-5 works in 
> preventing the loss of data?
> I'm testing it by copying a large file (20MB) into /root, and when I unplugged 
> a drive, it seems that the data is still Ok. How does RAID-5 do this?
> Thank you,
It uses parity information that it stores across all three drives. In 
your setup 1/3 of each drive is used for the parity information. 
Basically any two drives then have enough parity information to recreate 
the date on the third drive. With raid 5 the usable storage is C * (N 
-1) where C is the capacity of the drives and N is the number of them. 
So if you have 3 * 250GB disks the usable storage is 500GB. If two disks 
die at the same time kiss all your data good bye (although this is a 
pretty rare scenario)

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