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 ------------------------------------------------------------------------------ Colocation vs. Managed Hosting A question and answer guide to determining the best fit for your organization - today and in the future. http://p.sf.net/sfu/internap-sfd2d _______________________________________________ Bacula-users mailing list Bacula-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bacula-users
