Il 18/03/2011 00:57, Phil Stracchino ha scritto:
> On 03/17/11 18:46, Marcello Romani wrote:
>> Il 16/03/2011 18:38, Phil Stracchino ha scritto:
>>> On 03/16/11 13:08, Mike Hobbs wrote:
>>>> Hello, I'm currently testing bacula v5.0.3 and so far so good. One
>>>> of my issues though, I have a 16 bay Promise Technologies VessJBOD. How
>>>> do I get bacula to use all the disks for writing volumes to?
>>>>
>>>> I guess the way I envision it working would be, 50gb volumes would be
>>>> used and when disk1 fills up, bacula switches over to disk2 and starts
>>>> writing out volumes until that disk is filled, then on to disk3, etc..
>>>> eventually coming back around and recycling the volumes on disk 1.
>>>>
>>>> I'm not sure the above scenario is the best way to go about this, I've
>>>> read that some people create a "pool" for each drive. What is the most
>>>> common practice when setting up a JBOD unit with bacula? Any
>>>> suggestions or advice would be appropriated.
>>>
>>> That scheme sounds like a bad and overly complex idea, honestly.
>>> Depending on your data load, I'd use software RAID to make them into a
>>> single RAID5 or RAID10 volume. RAID10 would be faster and, if set up
>>> correctly[1], more redundant; RAID5 is more space-efficient, but slower.
>>>
>>>
>>> [1] There's a right and a wrong way to set up RAID10. The wrong way is
>>> to set up two five-disk stripes, then mirror them; lose one disk from
>>> each stripe, and you're dead in the water. The right way is to set up
>>> five mirrored pairs, then stripe the pairs; this will survive multiple
>>> disk failures as long as you don't lose both disks of any single pair.
>>>
>>>
>>
>> Hi Phil,
>> that last sentence sounds a little scary to me: "this will survive
>> multiple disk failures *as long as you don't lose both disks of any
>> single pair*".
>> Isn't RAID6 a safer bet ?
>
> That depends.
>
> 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.
>
> With RAID10 with sixteen drives, you can survive any one drive failure
> with minimal performance degradation. There is a 1 in 15 chance that a
> second failure will be the other drive of that pair, and bring the array
> down. If not, then there is a 1 in 7 chance that a third drive failure
> will be on the same pair as one of the two drives already failed. If
> not, the array will still continue to operate, with some read
> performance degradation, and there is now a just less than 1 in 4 chance
> (3/13) that if a fourth drive fails, it will be on the same pair as one
> of the three already failed. ... And so on. There is a cumulative 39%
> chance that four random failures will fail the entire array, which rises
> to 59% with five failures, and 78% with six. (91% at seven, 98% at
> eight, and no matter how many leprechauns live in your back yard, at
> nine failures you're screwed of course. It's like the joke about the
> two men in the airliner.)
>
> But if the array was RAID6, it already went down for the count when the
> third drive failed.
>
>
>
> Now, granted, multiple failures like that are rare. But ... I had a
> cascade failure of three drives out of a twelve-drive RAIDZ2 array
> between 4am and 8am one morning. Each drive that failed pushed the load
> on the remaining drives higher, and after a couple of hours of that, the
> next weakest drive failed, which pushed the load still higher. And when
> the third drive failed, the entire array went down. It can happen.
>
> But ... I'm running RAIDZ3 right now, and as soon as I can replace the
> rest of the drives with new drives, I'll be going back to RAIDZ2.
> Because RAIDZ3 is a bit too much of a performance hit on my server, and
> - with drives that aren't dying of old age - RAIDZ2 is redundant
> *enough* for me. There is no data on the array that is crucial *AND*
> irreplaceable *AND* not also stored somewhere else.
>
> What it comes down to is, you have to decide for yourself what your
> priorities are - redundancy, performance, space efficiency - and how
> much of each you're willing to give up to get as much as you want of the
> others.
>
>
Phil,
that was an interesting read. Thanks for your detailed response.
(Your last paragraph is of course the definitive word on the subject.)
Now that I think about it, I realize I didn't fully take into account
the high number of drives we're talking about. Probably if using RAID6 a
spare drive is to be considered. Or, better yet, a mirror machine...
But then we're back to "it depends", I guess :-)
Oh, and BTW, maybe it's time for me to move past these old limited raid
levels and investigate ZFS and those intriguing RAIDZx arrays...
Marcello
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