Topic: Install in degraded Raid5?

[NickCritten]

Hi all,

This may be a stupid question, but is it possible at install time to get SME to install onto 3 HDD's in degraded RAID5?


The reason behind this is that I currently have 2x400Gig SATA drives in RAID1, and I'd like to expand this to a 4x400Gig RAID5 Array, and have bought 2 more 400Gig drives towards this end.

I was thinking that I could maybe take one of the mirrors out, as my backup, then use three drives to create a degraded Raid 5, then transfer my data from the backup, then wipe and add it to the array.

Is this feasible in any way?


If not, can anyone suggest a workaround, as I don't have sufficient space elsewhere for my data, and backing up 380Gig to DVD's will be a headache.

Cheers,

[Gaston94]

Hi,
you can perform a "sme nospare" installation ==> with 3 disk you'll have a Raid5 config
But I am not sure joinning the fourth disk  will be possible, in the way your are looking at, because of some restriction in the version of the tools included in SME
- and you have to be aware a raid5 on 4 disks without spare one, is not the best choice (as a data security point of view)

As far as I look at this point, any try to join a new disk to an existing RAID 5 put this one as a spare disk. My notes say that the feature you are looking were requiring at least a 2.6.17 kernel and a 2.3.1 mdadm versions == BUT I did NOT check these versions)

My choose would have be to install SME system on two "small" disks (RAID1) an then add a raid5 array which would have been mounted under /home/e-smith/files

Another thing which can be done, might be to expand your existing RAID 1 array on your new disks. This as been described some days ago in the forum

Anything you would try regarding the disk settings. If you have such tricky operations to perform, a good idea is to check the scenario at the best cost and risk, for instance :
 - register at vmware and install a free vmware server on your desktop
 - create a virtual server , pre-create five disks (fully allocated - require some more time, but usefull later))
 - try different installation 3,4 5 disks
(you need to have at least 2GB disks size for a 7.x installation)

That way you will see exactly what is happening in each scenario
That way you will be able to test the data recovery operation (copying data from you "backup" disk)
you'll see in real world what can be the required action to access data from a SME raid+lvm system from another raid+lvm system (this can be done, but this is not direct from the box)

That way you'll be able to browse safely and not stressed the wiki, the forums for any errors, issues, you were not expecting

All of this with zero risks for your existing datas

G.

[NickCritten]

Hi Gaston,

Thanks for the reply.

I've since found several places where it mentions adding a fourth drive to a 3 drive RAID 5... I hadn't previously thought this possible so I'll be giving this ago.

As you suggested I was planning to dry-run the procedure on my VMWare box - I'll just have to clear off some space first.

- and you have to be aware a raid5 on 4 disks without spare one, is not the best choice (as a data security point of view)

Hmmm. True, however in my experience running raid5 on three disks, with or without a hot spare is a bad idea as the disks get thrashed and throughput is no good. In a four drive setup, if data security were my highest concern I'd be going for RAID 0+1, this gives much higher resilience and throughput that 3xR5 + Spare.
As it happens I'm more interested in capacity than resilience - This is my home media server.  Loss of the data in the event of a simultaneous failure of two drives would be a drag, but not suicide inducing.

Another thing which can be done, might be to expand your existing RAID 1 array on your new disks. This as been described some days ago in the forum

I may very well give that a go - again I hadn't thought that possible.

If I had sufficient dosh I'd get 8 Disks and run RAID6 with a hot spare, but I'm not that rich!   

Many Thanks again.

[SoftDux]

Hi

I'm trying to accomplish the same as note above. I currently have 3x 250GB HDD's which I want to setup as RAID 5 (with sme nospare) option, then later when I have more cash purchase 3 / 4 more drives (so I can have 1 spare), and 4U case (using a 2U case now), so ideally I'd like to increase the space available to it's max (and have a hot spare).

How well does this operation work?

[Gaston94]

Hi SoftDux,
I don't think you can manage your installation, from the normal SME 7 process.

The reason is that you have to indicate at the raid device creation, how many physical devices will participate (the extension of a soft raid device cannot be achieved with the current kernel and mdadm version).

I don't think the goal deserve the work, a backup restore operation once you have your new hardware, should be choiced instead.

G.

[SoftDux]

Ok, so are you saying that even if I used any other Linux, and I had 3HDD setup as RAID5, I won't be able to add more drives to the RAID set & extend the space?

[Gaston94]

Hi,
I am not exactly saying you cannot. I am 95% sure you cannot within the current revision of package included  in SME7.
The resize option for mdadm raid managed is working for kernel above 2.6.17 (not sure of the revision, but this should be correct) and mdadm 2.3.x).
You'll find some success story reports with some Google search

G.