Thanks for your considered response Leslie On Thu, Feb 3, 2011 at 12:25 AM, Leslie Rhorer <lrhorer@xxxxxxxxxxx> wrote: >> So keeping the drive size fixed at 2TB for the sake of argument, do >> people agree with the following as a conservative rule of thumb? >> Obviously adjustable depending on financial resources available and >> the importance of keeping the data online, given the fact that >> restoring this much data from backups would take a loooong time. This >> example is for a money-poor environment that could live with a day or >> two of downtime if necessary. >> >> less than 6 drives => RAID5 >> 6-8 drives ==> RAID6 >> 9-12 drives ==> RAID6+spare >> over 12 drives, start spanning multiple arrays (I use LVM in any case) > > That's pretty conservative, yes, for middle of the road > availability. For a system whose necessary availability is not too high, it > is considerable overkill. For a system whose availability is critical, it's > not conservative enough. So maybe for my "money-poor environment that could live with a day or two of downtime" I'll add a drive or two as my own personal rule of thumb. Thanks for the feedback. > That assumes the RAID1 array elements only have 2 members. With 3 > members, the reliability goes way up. Of course, so does the cost. Prohibitively so for my situation, at least for the large storage volumes. My OS boot partitions are replicated on every drive, so some of them have 20+ members, but at 10GB per 2TB, not expensive 8-) >> depending on luck, whereas RAID6 would allow **any** two (and >> RAID6+spare any *three*) drives to fail without my losing data. So I > > That's specious. RAID6 + spare only allows two overlapping failures. Well yes, but my environment doesn't have pager notification to a "hot-spare sysadmin" standing by ready to jump in. In fact the replacement hardware would need to be requested/purchase-ordered etc, so in that case the spare does make a difference to resilience doesn't it. If I had the replacement drive handy I'd just make it a hot spare rather than keeping it on the shelf anyway. >> On my lower-end systems, a RAID6 over 2TB drives takes about 10-11 >> hours per failed disk to rebuild, and that's using embedded bitmaps >> and with nothing else going on. > > I've never had one rebuild from a bare drive that fast. This wasn't a bare drive, but a re-add after I'd been doing some grub2 and maintenance work on another array from SystemRescueCD, not sure why the member failed. It's not a particularly fast platform, consumer SATA2 Hitachi drives attached to mobo Intel controller, ICH7 I believe. cat /proc/mdstat was showing around 60k, while the RAID1s rebuild at around double that. Would the fact that it was at less than 30% capacity make a difference? -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
