My cutomer wants to set up a OS/apps server with two 72GB drives
configured with RAID 1+0. The way I understand RAID 1+0 is
that if there was a drive failure the whole OS/apps is lost because you
actually need at least 4 drives to set up a RAID 1+0 by splitting
them up into two arrays. Am I right or am I wrong?
Thanks.
I think that you are right. Raid 1+0 will be a stripe of mirrors. So you carry out mirroring for the two disks and you need atleast two more disks to carry out the striping. Here's a link that discusses Raid 1+0 versus Raid 0+1. Also check this page. Says very clearly that Raid 1+0 requires a min of four disks to implement.
Yes you need 4 disks at a minimum, but no, the loss of a single disk should not take the shebang down. Not even for raid 0+1. And certainly not for raid 1+0.
With only 2 drives, you really can't do raid 0+1 or raid 1+0, unless you take the view that each drive counts as a striped array with a 72 GB wide stripe. So then you simply mirror the drives and then the loss of one drive is still not a system killer.
What Perderabo said, although I disagree with calling a sigle disk a stripe.
In a two disk 1+0 each disk would be a single disk concatenation, rather than a single disk stripe, it is a little pedantic to make the destinction with only two disks, but the difference only becomes apparrent when adding extra disk to the setup.
Thanks guys. This reassures my logic and hopefully the
customer will go with 4 drives instead of two.
But as stated they probably could go with just 2 drives but lose
the true effect of the stripping and increased redundancy.