[CentOS] Re: Raid5 issues

[Ruslan Sivak]

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

Russ