[CentOS] Re: Raid5 issues

[Toby Bluhm]

Ruslan Sivak wrote:
> Toby Bluhm wrote:
>> Ruslan Sivak wrote:
>>> Feizhou wrote:
>>>> Ruslan Sivak wrote:
>>>>> Feizhou wrote:
>>>>>>
>>>>>>> I do have a SIL3114 chipset, and I think it's supposed to be 
>>>>>>> supported by device mapper.  When I go to rescue mode, I see it 
>>>>>>> loading the driver for SIL3112, but nothing appears under 
>>>>>>> /dev/mapper except control.  Are there instructions somewhere on 
>>>>>>> getting it to use my controller's raid?
>>>>>>
>>>>>> Your controller only has a bios chip. It has no raid processing 
>>>>>> capability at all.
>>>>>>
>>>>>> You need to use mdadm. anaconda should be able to let you create 
>>>>>> to mirrors and then create a third array that stripes those md 
>>>>>> devices,
>>>>>> _______________________________________________
>>>>> Anaconda doesn't let me create a stripe raid set on top of a 
>>>>> mirror set.  And it doesn't detect it when I do it manually.
>>>>> Also the bios chip presents additional issues.  I believe when I 
>>>>> don't have a raid array set up, it won't boot at all.  When I have 
>>>>> it on raid10, I had trouble booting, and when I have it on 
>>>>> concatenation, everything works fine, until a drive is replaced.  
>>>>> At that point, i have to recreate the array, as concatenation is 
>>>>> not a fault tolerant set, and at this point I seem to lose all my 
>>>>> data.
>>>>
>>>> It won't boot at all without a raid array setup? That sounds really 
>>>> funny.
>>>>
>>> Actually I'm not 100% sure on this, but I think this is the case.  I 
>>> believe the first time I set it up as a raid10, assuming that linux 
>>> will just ignore it.  I installed centos by putting boot on a raid1, 
>>> and root on LVM over 2 raid1 sets.  I had trouble getting it to boot.
>>>>> Is there a way to get it to use the raid that's part of the bios 
>>>>> chip?  
>>>>
>>>> Repeat after me. There is no raid that is part of the bios chip. It 
>>>> is just a simple table.
>>> Yes, I know this is fakeraid, aka softraid, but I was hoping that 
>>> using the drivers would make it easier to support raid 10 then with 
>>> mdadm, which seems to be impossible to get to work with the 
>>> installer.  I'm not even sure why the raid10 personality is not 
>>> loaded, as it seems to have been part of the mdadm since version 1.7.
>>>>> Something about device mapper?
>>>>
>>>>
>>>> You need the fake raid driver dmraid if you are going to set up 
>>>> stuff in the bios. What version of centos are you trying to 
>>>> install? libata in Centos 5 should support this without having to 
>>>> resort to the ide drivers.
>>>> _________________________________
>>> I'm trying to install centos 5 - the latest.  How would I go about 
>>> using dmraid and/or libata?  The installer picks up the drives as 
>>> individual drives.  There is a drive on the silicon image website, 
>>> but it's for RHEL4, and I couldn't get it to work.  I'm open to 
>>> using md for raid, or even LVM, if it supports it.  I just want to 
>>> be able to use raid10, as I can't trust raid5 anymore.
>>>
>>
>> IIRC you had two out of four new disks die? So maybe it would be more 
>> accurate to say it's your hardware you don't trust. Raid5 is used 
>> without problems by ( I assume ) many, many people, myself included. 
>> You could have a raid10 and still lose the whole array if two disks 
>> that in the same mirror die at once. I guess no software in the world 
>> can really overcome bad hardware. That's why we do backups :)
>>
>> Anyway, perhaps excersizing /stressing the disks for a few days 
>> without error would make you feel more confident about the HDs.
>>
>
> Actually, 2 disks did not die.  Due to the fact that it was a new raid 
> 5 array (or for whatever reason), it was rebuilding the array.  One of 
> the drives had a media error, and this caused the whole array to be lost.
> This is exactly what this article warns about:
>
> http://www.miracleas.com/BAARF/RAID5_versus_RAID10.txt


The article doesn't seem to mention the fact that if a disk in a mirror 
set dies and the remaining disk within the set starts to have data 
corruption problems, the mirror will be rebuilt from corrupted data.

I don't know what you can do at this point, though. Perhaps make 2 
separate mirrors and rsync them?  You could keep copies of  changes that 
way.