Re: [Bacula-users] Bacula and 16 bay JBOD

Not really, RAID6+0 only requires 8 drives minimum you can create two RAID6's of 4 drives each and stripe them together. This has a benefit as multi-layer based parity raids increases random write iops performance. But the main issue is array integrity, mainly with large capacity drives as they have UBR rates in 10^14 or 10^15 range but when you consider their capacity this dominates availability calculations for an array well over any 'hard' failure calculations. Generally with 1TB+ drives with 10^14 UBR rates I would be hard pressed to use more than 6 drives in a raid group (4D+2P), with 10^15 you may get by with 8D+2P but you have to choose your own risk level.

Personally, I don't like building arrays that have a probability of not reading a sector in a single sub array greater than 5% (ideally it should be less than 1% but you're not going to get that unless you're talking 10^16 and small drives (<500GB).

This still doesn't address the silent errors that happen which is the main thrust behind file systems like ZFS and/or T10 DIF (fat sectors) which do checking to make sure what sector your requesting is the sector you're getting.

On 2011-03-18 08:22, Alan Brown wrote:

Phil Stracchino wrote:

With RAID6, you can survive any one or two disk failures, in degraded
mode.  You'll have a larger working set than RAID10, but performance
will be slower because of the overhead of parity calculations.  A third
failure will bring the array down and you will lose the data.

There's always RAID60, but that requires a lot of drives.




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