On Mar 26, 2013, at 11:44 PM, Bill Arlofski <waa-bacula AT revpol DOT com> 
wrote:

> On 03/26/13 22:33, Wood Peter wrote:
>> Hi,
>> 
>> I'm planning to use Bacula to do file level backup of about 15 Linux
>> systems. Total backup size is about 2TB.
>> 
>> For Bacula server I'm thinking to buy Dell PE R520 with 24TB internal
>> storage (8x 3TB disks) and use virtual tapes.
> 
> Hi Peter.   First, you're going to want some RAID level on that server and
> RAID0 is not (IMHO) RAID at all.  :)
> 
> At a minimum you're going to want to set up (for example) RAID5 in the server
> which will give you a maximum of 7 x 3 = 21 TB since the equivalent of 1 full
> drive is used for parity, spread across all of the drives in a RAID5 array.
> 
> Having said that, RAID5 does not have the best write speeds, but other RAID
> levels that will give more redundancy and better write speeds will use
> significantly more drives and give you less total storage. You may need to
> spend some time to consider your redundancy and read/write throughput
> requirements.
> 
> Also, you will generally want to configure at least one drive as a hot spare
> so the RAID controller can automatically and immediately fail over to it in
> the case of a drive failure in the array.
> 
> So that takes away at least 3 more TB, so now your down to 18TB total storage
> with a minimally configured RAID5 array with 1 hot spare.
> 
> Just some things to consider. :)


Another thing to consider is that with large capacity drives (3 TB) combined 
into large RAID-5 arrays there is an increased likelihood of a catastrophic 
array failure during an array rebuild due to an initial drive failure.  For 
this reason, RAID-6 would be preferred for such large arrays.

Reliability of large RAID arrays is one of the motivations behind raidz3 
(triple-parity redundancy) in ZFS.  See, e.g., 
http://queue.acm.org/detail.cfm?id=1670144 for details.

Cheers,

Paul.


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