[OSX-Users] Re: More backup questions, was: Re: backing up while on road away from time capsule

[Jules Field]

On 17/09/2011 18:31, Hugh Glaser wrote:
> Sorry, I should have made it clearer - I am talking about the standard OSX software RAID and extendable disk facilities using Disk Utility.
> As I said, there seems to be remarkably little about it, especially recently.
As I said, I have only ever needed to do a simple 2 disk mirror (RAID 1) 
with that, so don't know what else it can do. It should be documented in 
the OS X Server documentation on www.apple.com, though.
>
> And of course whenever anyone asks, there are loads of the usual "helpful" replies saying "Don't do that, it's not a backup; use TM; buy a hardware RAID; etc." and almost zilch of people who have actually used it.
Agreed, they all account for different failure modes. Most people tend 
to believe that the failure modes they have thought of are the only ones 
that are likely to happen. In truth there are a wide variety of failure 
modes; "the disk going pop" is but one of very many. And you may well 
get many failures at the same time.

Hence my love of RAID 6 with on-site (but geographically well separated) 
server+filestore duplication in addition to off-site tape backup and 
archival with backup logs (detailing all the commands needed to 
reconstruct the data) going to a separate off-site mailbox run by a 
separate company.

And no, that's not pie-in-the-sky unnecessary duplication: it's 
precisely what we do with the ECS undergrad filestore. And it's also 
very cheap to implement, so we can do it in ECS.

Jules.


> Thanks for not doing that, Jules; as always just addressing the question is really helpful.
:-)
>
> On 17 Sep 2011, at 18:16, Jules Field wrote:
>
>>
>> On 17/09/2011 15:11, Hugh Glaser wrote:
>>> Thanks.
>>> I was wondering if I gave it 3 disks to start with it would offer me the RAID 5 option - one can hope :-)
>> Yes, you can do RAID 5 on 3 disks, no problem. You'll get 2 disks of usable space. But they will have to be wiped to start with when the RAID volume is built.
>>> But I think I did see somewhere where they say the only options are the ones given (0&   1).
>> Ah, if it can only do striping (0) and mirroring (1), then you're obviously out of luck. Not much of a RAID system if it can't even do RAID 5. RAID 6 is the best, but not many small systems support it, and it's only really necessary when you've got multi-Tbyte RAID volumes.
>>> And presumably if I gave it 3 and said Stripe, it would Stripe across all three, not change to RAID 5.
>> Yes, should do a 3 disk stripe (0).
>>> Can you hazard a guess on Q1, based on your vast experience?
>>> Or don't you guess? :-)
>> It depends entirely on the system. It is perfectly possible, yes. But whether your particular RAID controller supports it is another matter. A lot of software RAID systems do.
>>> What really pisses me off is forums that don't date the messages - I keep finding a post on OSX RAID, only t6o realise it is talking abut panther, or earlier.
>> :-( Often Google will tell you the date it read the page, which will give an indication of the most recent date it has changed.
>>
>> Jules.
>>
>> P.S. On Mac OS X Server, I have only ever needed a simple 2-disk mirror, so I've never investigated its abilities beyond that.
>>
>>
>>> On 17 Sep 2011, at 14:23, Jules Field wrote:
>>>
>>>> On 16/09/2011 19:14, Hugh Glaser wrote:
>>>>> On 15 Sep 2011, at 11:45, Jules wrote:
>>>>>
>>>>>> Lion will continue to do Time Machine backups to spare space on your laptop's own hard disk while it's away from its Time Capsule. Obviously this is no protection against disk failure, but it does provide protection against accidental file deletion or corruption.
>>>>> Wow. I didn't see that. That is really cool - just what I want it to do.
>>>>>
>>>>> New questions (there seems to be precious little about this on the net):
>>>>> I think I'm going to Mirror RAID my TV programmes, etc. disk, as protection against device failure.
>>>>> 1) Can I create a Mirror RAID Set using Concatenated Disk Sets as the components?
>>>>> 2) If I add more than two Disks (or Concatenated Disk Sets) to the RAID Set, will it do the clever stuff to give me RAID 5, or whatever? I am guessing not - it will just do multiple mirrors?
>>>> It won't do (2), you would only ever end up with more faces to the mirror. Converting a RAID 1 mirror into a RAID 5 requires a total reorganisation of the data, they are *very* different beasts. AFAIK nothing will automatically convert a RAID1 to RAID5 just because you add more disks, I've certainly never seen anyone's product attempt that trick. If it were to do it, you would lose all redundancy (and therefore safety) while it attempted to do it, and it will take a very long time to achieve.
>>>>
>>>>
>>>> Jules
>>>>
>>>> -- 
>>>> sysjkf at ecs.soton.ac.uk
>>>>
>> Jules
>>
>> -- 
>> sysjkf at ecs.soton.ac.uk
>>

Jules

-- 
sysjkf at ecs.soton.ac.uk