[CentOS] RAIDs and JBOD?

Drew drew.kay at gmail.com
Tue Nov 10 21:50:32 UTC 2009

> 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?

They can in fact. There might be minor differences of a few sectors
between the drives but md RAID will account for those by using the
'smallest' (lowest sector count) drive as the base.

> However could a similar result of 2TBs of data with redundancy be
> achieved with JBOD?

JBOD (Just a Bunch Of Disks) has no redundancy. The redundancy enters
when you assign those disks to RAID sets.

> 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?

No. The parity will be the same size regardless of how many drives are used.

> 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?

All hot swappable drives allow you to do is replace them without
having to completely shutdown the machine. In hardware raid this is
often built in such that you can replace a drive without telling RAID
ahead of time and it will compensate. IBM xSeries servers are a good
example. Software RAID can also do this but you have to tell the RAID
system that you plan to remove the drive and then tell it when you add
a new drive back. Hot swapping disks is also dependent upon the drive
controller supporting hotswap.


"Nothing in life is to be feared. It is only to be understood."
--Marie Curie

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