Miles Nordin wrote: >>>>>> "djm" == Darren J Moffat <[EMAIL PROTECTED]> writes: >>>>>> "bf" == Bob Friesenhahn <[EMAIL PROTECTED]> writes: >>>>>> > > djm> Why are you planning on using RAIDZ-2 rather than mirroring ? > > isn't MTDL sometimes shorter for mirroring than raidz2? I think that > is the biggest point of raidz2, is it not? >
Yes. For some MTTDL models, a 3-way mirror is roughly equivalent to a 3-disk raidz2 set, with the mirror being slightly better because you do not require both of the other two disks to be functional during reconstruction. As the number of disks in the set increases, the MTTDL goes down, so a 4-disk raidz2 will have lower MTTDL than a 3-disk mirror. Somewhere I have graphs which show this... http://blogs.sun.com/relling/entry/zfs_raid_recommendations_space_performance > bf> The probability of three disks independently dying during the > bf> resilver > > The thing I never liked about MTDL models is their assuming disk > failures are independent events. It seems likely to get a bad batch > of disks if you buy a single model from a single manufacturer, and buy > all the disks at the same time. They may have consecutive serial > numbers, ship in the same box, u.s.w. > You are correct in that the models assume independent failures. Common failures for independent devices (eg vintages) can be modeled using an adjusted MTBF. For example, we sometimes see a vintage where the MTBF is statistically significantly different than other vintages. These can be difficult to predict and any such predictions may not help you make decisions. Somewhere I talk about that... http://blogs.sun.com/relling/entry/using_mtbf_and_time_dependent > You can design around marginal power supplies that feed a bank of > disks with excessive ripple voltage, cause them all to write > marginally readable data, and later make you think the disks all went > bad at once. or use long fibre cables to put chassis in different > rooms with separate aircon. or tell yourself other strange disaster > stories and design around them. But fixing the lack of diversity in > manufacturing and shipping seems hard. > My favorite is the guy who zip-ties the fiber in a tight wad at the back of the rack. Fiber (and copper) cables have a minimum bend radius specification. In fiber cables, small cracks can occur which, over time, become larger and cause attenuation. If you are really interested in diversity, you need to copy the data someplace far, far away, as many of the Katrina survivors learned. But even that might not be enough diversity... http://blogs.sun.com/relling/entry/diversity_revisited http://blogs.sun.com/relling/entry/diversity_in_your_connections > For my low-end stuff, I have been buying the two sides of mirrors from > two companies, but I don't know how workable that is for people trying > to look ``professional''. It's also hard to do with raidz since there > are so few hard drive brands left. > I agree, and do the same. > Retailers ought to charge an extra markup for ``aging'' the drives > for you like cheese, and maintian several color-coded warehouses in > which to do the aging: ``sell me 10 drives that were aged for six > months in the Green warehouse.'' > I just looked at our field data for disks through last month and would say that aging won't buy you any assurance. We are seeing excellent and improving reliability. Mind you, we are selling enterprise-class disks from the top bins :-) Meanwhile, thanks Miles for being a setup guy :-) -- richard _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
