[GRLUG] RAID levels

[scott.tanner at comcast.net]

I'd say RAID 6 is the better option compared to RAID5 with hot spare. While the spare will give you a jump start on the recovery process, you're still at risk of loosing everything if another drive dies before the rebuild is complete - which can be a couple of days with multi-terabyte arrays. RAID 6 will keep you protected during a single disk failure while rebuilding. Since you're willing to sacrifice a disk as a hot-spare, you might as well get some use out of it. 

Also agree about the performance in degraded performance - negligible for most people. 


Regards, 
Scott 

----- Original Message -----
From: "Richard Nienhuis" <richardnienhuis at gmail.com> 
To: "Mailing List for LUG in greater Grand Rapids, MI area." <grlug at grlug.org> 
Sent: Friday, September 30, 2011 6:55:53 PM 
Subject: Re: [GRLUG] RAID levels 

Performance issues are going to be inconsequential for a machine playing music. In degraded mode there is probably still enough performance. Also hot spare isn't going to spare you from the time to rebuild the array. 


On Fri, Sep 30, 2011 at 4:59 PM, Mike Williams < knightperson at zuzax.com > wrote: 


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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