Il 18/03/2011 19:01, Mehma Sarja ha scritto:
> On 3/17/11 4:57 PM, Phil Stracchino wrote:
>> 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.
>>
>>
> There is one more thing to think about and that is cumulative aging.
> Starting with all new disks is a false sense of security because as they
> age, and if they are in any sort of RAID/performance configuration, they
> will age and wear evenly. Which means they will all start to fail
> together. It is OK to design a system and assume one or two simultaneous
> drive failure - when the drives are relatively young. After 3 years of
> sustained use, like email storage, you are at higher risk no matter
> which RAID scheme you have used.
>
> Mehma
This is an interesting point. But what parameter should one take into
account to decide when it's time to replace an aged (but still good)
disk with a fresh one ?
Marcello
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