[GRLUG] RAID levels

[Mike Williams]

Last night at The Warehouse we had a fairly involved discussion about 
the various RAID levels. I realized, while churning it over in my head 
again a little while ago, that one of my assumptions was not quite 
correct. I thought that in RAID 6, since you have an extra drive, that 
the system doesn't need to do parity reconstruction like RAID 5 does 
once you lose the first drive. Since even in RAID 6 there is actual data 
on that drive, it still has to do the reconstruction so you will still 
lose performance once the first drive failed. That being the case, would 
Casey's music box be better off in RAID 5 with a hot spare rather than 
RAID 6 without? RAID 6 has the advantage that the array wouldn't go down 
until the 3rd drive failure,while a RAID 5 array would go down on the 
2nd. However, under RAID 5 with a hot spare the rebuild of the array 
could start immediately, vastly reducing the time during which the array 
is running in degraded mode. Thoughts?

For those who weren't part of the discussion, we have an array of 5 
disks to RAID. RAID Level 0 is striping and isn't actually redundant: 
very fast, 5 disks worth of storage, but a failure of any disk takes 
down the array (Bad Idea). RAID 1 is mirroring and uses half the 
available storage space. In the case of 5 disks, we would get 2 disks 
worth of storage plus have a hot spare. It would take either 2 or 3 
drive failures before the rebuild finishes to take down that array. RAID 
5 is like RAID 0 but with one drive's worth of redundancy. Any 2 drive 
failures will take down the array, it slows down significantly when the 
first one fails, and you get 4 drives' worth of storage. RAID 6 is 
similar, but can weather 2 failing drives but you only get 3 drives of 
storage (the same as RAID 5 with a hot spare). A "hot spare" is an extra 
drive that is installed and powered on but not used until a live drive 
fails, at which point it gets a copy of that drive's data and takes its 
place.

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