CentOS - Sep 2009 - Software Raids Questions (I have only ever used hardware?)

Hey List;

I have no experience with software RAIDs; at work we only use hardware
RAIDs and I'm looking to implement, probably a RAID 5 set up at home
for a media server however I have a few questions;

I have three 1TB drives in various places; one is inside a USB caddy,
one is inside my PC and in is inside my existing media centre.

Is it possible to add these three drives to another one to give me
4TBs of space in a RAID setup without having to wipe the drives as in
my experience which is only with hardware RAIDs, I have normally
formatted all the disks before creating the RAID? The thing is, if
that is the case I will need to transfer 3TB of stuff somewhere (I
have an idea where, if this were the case), make the RAID then
transfer it all back but I really don't want to do that as I'm sure
you can imagine.

Also, if the above where possible; in the future could I then keep
adding more drives and expanding the RAID?

Note: Obviously I know for this to be a RAID 5 I would need extra
drives but the RAID level is undecided, but provisionally I think it
will be RAID 5.

On a side note, I cobbled together my new media centre running Ubuntu
but I might move it back to CentOS, it was originally CentOS and that
is my favoured distro, but I would rather not now it is running Ubuntu
happily I'm just wondering, is this all achievable in Ubuntu? Granted
people on the CentOS mailing list might not know that, but if anyone
knows that it is all achievable in CentOS then I would move back?

Thanks for reading.

Regards,
James ;)

-- 

Ted Turner  - "Sports is like a war without the killing." -
http://www.brainyquote.com/quotes/authors/t/ted_turner.html

few things;

i'm not hip to the latest barney phife shareware utils that may allow  
some kind of dynamic raiding bs, but generally, no, you must format  
those drive to the desired fs after raiding them so back up all data u  
want housed (or hozed) on that new raid.

basic math is that for a raid 5, subtract 1 drives worth of space and  
thats your usable space.

so if u have 4x1tb drives, raw space is 3tb in a raid 5.

md (software raid mechanism or watever u wanna call it) allows u to  
grow so if your raid is formatted with an fs that allows u to grow,  
then u r good to go (lvm will allow u to grow).


On Sep 27, 2009, at 1:03 PM, James Bensley wrote:
> Hey List;
>
> I have no experience with software RAIDs; at work we only use hardware
> RAIDs and I'm looking to implement, probably a RAID 5 set up at home
> for a media server however I have a few questions;
>
> I have three 1TB drives in various places; one is inside a USB caddy,
> one is inside my PC and in is inside my existing media centre.
>
> Is it possible to add these three drives to another one to give me
> 4TBs of space in a RAID setup without having to wipe the drives as in
> my experience which is only with hardware RAIDs, I have normally
> formatted all the disks before creating the RAID? The thing is, if
> that is the case I will need to transfer 3TB of stuff somewhere (I
> have an idea where, if this were the case), make the RAID then
> transfer it all back but I really don't want to do that as I'm sure
> you can imagine.
>
> Also, if the above where possible; in the future could I then keep
> adding more drives and expanding the RAID?
>
> Note: Obviously I know for this to be a RAID 5 I would need extra
> drives but the RAID level is undecided, but provisionally I think it
> will be RAID 5.
>
> On a side note, I cobbled together my new media centre running Ubuntu
> but I might move it back to CentOS, it was originally CentOS and that
> is my favoured distro, but I would rather not now it is running Ubuntu
> happily I'm just wondering, is this all achievable in Ubuntu? Granted
> people on the CentOS mailing list might not know that, but if anyone
> knows that it is all achievable in CentOS then I would move back?
>
> Thanks for reading.
>
> Regards,
> James ;)
>
> -- 
>
> Ted Turner  - "Sports is like a war without the killing." -
> http://www.brainyquote.com/quotes/authors/t/ted_turner.html
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> http://lists.centos.org/mailman/listinfo/centos

On 28/09/2009, at 5:33 AM, James Bensley wrote:
> Hey List;
>
> I have no experience with software RAIDs; at work we only use hardware
> RAIDs and I'm looking to implement, probably a RAID 5 set up at home
> for a media server however I have a few questions;
>
> I have three 1TB drives in various places; one is inside a USB caddy,
> one is inside my PC and in is inside my existing media centre.
>
> Is it possible to add these three drives to another one to give me
> 4TBs of space in a RAID setup without having to wipe the drives as in
> my experience which is only with hardware RAIDs, I have normally
> formatted all the disks before creating the RAID? The thing is, if
> that is the case I will need to transfer 3TB of stuff somewhere (I
> have an idea where, if this were the case), make the RAID then
> transfer it all back but I really don't want to do that as I'm sure
> you can imagine.
>
> Also, if the above where possible; in the future could I then keep
> adding more drives and expanding the RAID?
>
> Note: Obviously I know for this to be a RAID 5 I would need extra
> drives but the RAID level is undecided, but provisionally I think it
> will be RAID 5.
>
> On a side note, I cobbled together my new media centre running Ubuntu
> but I might move it back to CentOS, it was originally CentOS and that
> is my favoured distro, but I would rather not now it is running Ubuntu
> happily I'm just wondering, is this all achievable in Ubuntu? Granted
> people on the CentOS mailing list might not know that, but if anyone
> knows that it is all achievable in CentOS then I would move back?
>
> Thanks for reading.
>
> Regards,
> James ;)
>
Hi James,

I'm going to try and answer all your questions concisely because the  
main thread seems to have turned into an debate more than anything else.

Someone has already stated that the total capacity of a RAID5 array is  
that of all disks minus 1. So with your four 1TB drives you can have  
3TB of space.

Moving all the data elsewhere is probably the easiest option to take  
and also the fastest. You could in theory create a degraded RAID5  
array with two drives then migrate data across while growing the array  
at the same time then growing the array each time all data from a  
drive has been moved. This would take very long, require a lot of  
work, and I would therefore recommend against it.

Down the track you can add more drives to a RAID5 array and "grow" the
array. This process also takes a long time. You do not need to be  
using LVM on top of mdadm to complete this. Once an array has finished  
growing you can use resize2fs to increase the size of an ext3 file  
system. It can even be done with the filesystem mounted, though  
personally I think it's best to do a forced fsck.ext3 on the file  
system then run the resize2fs on it while it is offline.

The process to grow a RAID5 array is to add a new drive to the array  
(which adds it as a hot spare) then run a --grow including the --raid- 
devices=5 argument (which is what you'd use if you were to expand your  
4 drive RAID5 system to a 5 drive array down the track).

If you were using LVM on top you'd need to use pvresize and then  
lvresize to increase the size of your logical volumes, and then resize  
the file system.

One thing I would strong suggest is that you get a UPS for your  
server. In my experience RAID5 is much more prone to problems if you  
experience power loss, compared to RAID1 or even a single disk system.

I hope that helps.

Regards,
Oliver


> -- 
>
> Ted Turner  - "Sports is like a war without the killing." -
> http://www.brainyquote.com/quotes/authors/t/ted_turner.html
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> http://lists.centos.org/mailman/listinfo/centos