Re: Re: Raid5 issues

[Ruslan Sivak <rsivak@xxxxxxxxxxxxx>]

Toby Bluhm wrote:
> 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.
While this is true, it's far more likely that there will be a media 
error (i.e. bad sector), and that the system will notice it.  With raid 
5, it will just kick out the drive, and you can say bye bye to your 
data.  With raid 10, if it happens on one of the disks in the other set, 
you don't have a problem, and if it happens to the disk in the same set 
(not very likely), I'm not sure what the outcome will be, but hopefully 
it can recover?  I have just had a windows drive have a whole bunch of 
bad sectors and I was still able to boot to windows, and copy most of 
the data off.  I can't imagine Linux being any worse. 
> 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.
I know there is a raid10 personality for md.  I saw it in the source 
code.  I see people's boot logs all over the web that say this:

md: linear personality registered as nr 1
md: raid0 personality registered as nr 2
md: raid1 personality registered as nr 3
md: raid10 personality registered as nr 9
md: raid5 personality registered as nr 4

Why does CentOS5 not support the raid10 personality?  Do i need to custom compile md?  Do I need to custom compile the kernel?   

Russ





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