[CentOS] RAIDs and JBOD?

Les Mikesell lesmikesell at gmail.com
Tue Nov 10 22:14:18 UTC 2009

James Bensley wrote:
> I have some questions regarding a new home server I am going to build
> in the hopefully very near future (ASAP, I just need to finish
> planning everything and this is the penultimate hurdle), I will be
> creating a software RAID...
> Lets say I have three drives "knocking" around which are all 1TB SATA
> II drives but each made by a different manufacturer. I am going to
> guess that these couldn't be used in a RAID 5? Or could they?
> However could a similar result of 2TBs of data with redundancy be
> achieved with JBOD?

If you use software raid to combine the JBOD's, yes.

> Also regarding RAID 5, three drives of data to one for parity is the
> max ratio I believe? I.e. to expand this by adding another data drive,
> the original parity drive would no longer cover this and another would
> be required, is this correct?

Yes, but if I were doing it I'd either run 2 drives in RAID1 or get 
another drive and either have 2 RAID1 mount points or a RAID 0+1.  The 
advantage of RAID1 is that you can recover the data from any single disk 
and it still runs full speed even with a missing disk.  RAID5 works but 
there is a performance hit and a big one when a disk is bad.

> One more question about hot swappable drives, I understand that you
> can create RAID arrays with and without hot swappable drives but I am
> confused by this concept. I'm my experience with RAIDs I have only
> every delt with a RAID 1 that has degraded. I simply set the drive as
> offline, replaced it, set it to online and the RAID rebuilt itself all
> without restart the server and operation was never interrupted. So we
> can presume the server had hot swappable drives enabled yes? (It was a
> hardware RAID). With a software RAID is this still achievable?

Sata drives all but a few controllers are designed to be hot swappable 
but you need a special drive bay that permits swapping.  It probably 
doesn't matter for a home server where you can shut it down for repair 
anyway.   With software raid, after the drive is recognized (either 
hotswap or reboot) you need to fdisk a matching partition and then use 
an 'mdadm --add ...' command to sync a new drive into the raid.

   Les Mikesell
    lesmikesell at gmail.com

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