Re: Upgrading a software RAID

[Bill Davidsen <davidsen@tmr.com>]

Maxime Boissonneault wrote:
>
> Robin Hill a écrit :
>> On Mon May 25, 2009 at 01:05:15PM -0400, Maxime Boissonneault wrote:
>>
>>  
>>> Hello,
>>> I am using Ubuntu Hardy with 3x500GB drives and the following RAID
>>> configuration :
>>>
>>> /boot is on a 100MB RAID1
>>> / is on a 30GB RAID0
>>> /home is on a 906GB RAID5
>>>
>>>     
>> Ouch - why RAID0 for /?  If you lose a single drive then all the
>> configuration, etc. is down the drain.  I'd suggest rethinking this
>> while you're going through the rebuild process anyway.
>>
>>   
> That is why I do backups of / on /home regularily. I first did a RAID0 
> for performance. I did some testing on performance of RAID0,1,5 and 
> RAID1 was pretty lousy (I put the results of my tests here : 
> http://cqed.physique.usherbrooke.ca/~mboisson/htpc.php?sec=raid_test )
> I guess I should have used a RAID5, but RAID1 seems like a terrible 
> idea considering the really bad performances.
>
>>> I want to replace the 3 drives by 3 1TB drives.
>>>
>>> Here is how I planned to do it :
>>> 0- Backup my /home on some external disk.
>>> 1- backup / with something like :
>>> sudo tar cvpzf /backup.tgz --exclude=/media --exclude=/proc
>>> --exclude=/lost+found --exclude=/backup.tgz --exclude=/mnt 
>>> --exclude=/sys
>>> --exclude=/home /
>>> mv /backup.tgz $1
>>>     
>> You'll need to do the tar in single user mode (init 1) to ensure there's
>> no open files when you're backing up (or use a bootable CD).
>>
>>   
> I used this backup before to restore the system and it seemed to work 
> pretty well.
>>> 2- Replace 1 disk
>>> 3- Boot and let the RAID1 and RAID5 reconstruct
>>>     
>> You'll have to boot to a CD then - your root FS is trashed at this
>> point.
>>
>>  
>>> 4- Replace 1 other disk
>>> 5- Boot and let the RAID1 and RAID5 reconstruct again
>>> 6- Replace the last disk
>>> 7- Boot and let the RAID1 and RAID5 reconstruct one last time
>>> 8- Boot and restore the backup on the RAID0 / partition.
>>> 9- Resize the /home partition to 1 TB.
>>>
>>> I suspect there will be a problem replacing the primary disk, but I 
>>> guessed
>>> that I could solve this simply by changing which is the primary disk 
>>> in the
>>> BIOS.
>>>
>>> Is there any other problem that will or could happen ?
>>>
>>>     
>> See above - your current plan is pretty much doomed I'm afraid.
>>
>>  
>>> For example, I am not sure if the raid manager is on the /boot 
>>> partition or
>>> on the /. I guess if it is on /, it won't work at all since the raid 
>>> manager
>>> itself won't be able to run ?
>>> Also, is it possible to boot and access a command line to restore 
>>> the backup
>>> with a failed / partition ?
>>>
>>> I am also unsure about how I should proceed to resize the /home 
>>> partition.
>>> Is this done through mdadm ?
>>>
>>>     
>> No, you'll have to delete & recreate the partition using fdisk.
>>
>>  
>>> Please enlight me on any problems that I will have.
>>>
>>>     
>>
>> An easier option (assuming you want to keep the arrays as currently
>> setup) would be to boot from a CD and do a full copy of each of the
>> 500GB disks to the 1TB disks.  You'll then just need to resize the
>> last partition (assuming /home is set up on the last partition on the
>> disks anyway) and grow the array and filesystem.
>>
>> A _better_ option (if possible) would be to install all the drives in
>> the system concurrently, then you can boot from CD and create arrays on
>> the new drives and copy the data across.  You'll also need to update
>> mdadm.conf (and the initrd if you're using one) to indicate the new
>> array IDs.
>>
>> HTH,
>>     Robin
>>   
>
> I can not install more drives in the computer. It is a home theater 
> computer in a small case. I was expecting to be able to let the raid 
> manage the copies itself.
>
> If the / was on a RAID5, would it be able to boot with 2 disks ?
> If so, is it possible to convert my RAID0 to a RAID5 ?
> For example, I could boot on a CD, backup / onto /home, delete the 
> RAID0 array and recreate it as RAID5, then restore the backup. Would 
> this work ?

Based on my testing (somewhat old now) and regular use, I would say 
raid10 is probably your best bet. It's fast and secure, and with the -f2 
option for "far" copies it's able to give high transfer rates.

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc

"You are disgraced professional losers. And by the way, give us our money back."
    - Representative Earl Pomeroy,  Democrat of North Dakota
on the A.I.G. executives who were paid bonuses  after a federal bailout.



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