[CentOS] RAIDs and JBOD?

Les Mikesell lesmikesell at gmail.com
Wed Nov 11 03:57:36 UTC 2009

James Bensley wrote:
> Thanks all for the promptness of your responses and the details you
> have provided it is greatly appreciated.
> Since this is a home media server performance isn't imperative and
> mirroring and RAID 10/0+1 are too expensive so I am going to use my
> three existing drives of different manufactures as they are all
> reasonably new (each was purchased at different times this year) and
> throw in two more giving four drives for data and one for parity. That
> will suffice in terms of storage size (4TBs) and a ratio of four data
> drives to one parity is as far as I feel comfortable in terms of
> hardware redundancy.
> I have read a few articles about mdadm and I have devised the
> following strategy in my head and am looking for some confirmation of
> its theoretical success:
> Two of my existing three drives are full of data. I will purchase two
> more drives to go with my existing blank drive and set them up as a
> RAID 5 > copy my existing data on to the new file system one drive at
> a time and after each drive has been copied I will add said drive and
> use mdadm --grow to then incorporate that drive into the RAID before
> adding the next drive. Can anyone point out a flaw in this plan or
> more preferred method for doing this, or have I, dare I say it, got it
> right?

The flaw is that you don't have a backup.  And if you grow the space on a raid 
you also have to resize the filesystem to use it.

> Also I was initially going to get a PCI-E SATA card to connect up all
> these drives and use mdadm to make a software RAID, for this
> particular setup is that ill advised or do people think this will
> suffice (simple because my budget is low and hardware RAID controller
> cards are more expansive, in my experience but if you know of a good
> bargain I'm all ears!). Just quickly, this brings me back to the issue
> of a hot swappable drive. Up time isn't critical as its a home server
> so I don't believe a hotspare is needed, in the event of a drive
> failure I can shutdown the server and fire up single user mode and
> have the RAID file system dismount and then replace the failed drive
> and rebuild the array, is this correct?

Yes, but again, backups are a good thing...  If you hit a bad sector on the 
other drives during the rebuild, you lose - and this is moderately likely since 
the rebuild has to read the whole disk, part of which probably hasn't been 
accessed in a long time.

   Les Mikesell
    lesmikesell at gmail.com

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