Hi Phil. On Thu, 2013-07-04 at 17:43 -0400, Phil Turmel wrote: > > The focus is absolutely on data security/resilience,... and not at all > > on performance. > This particular statement trumps all other considerations. Sarcasm? (*Sheldon Cooper hat on*) > Triple-copy raid10 > across four drives can match that resiliency, with dramatically better > performance, but with a substantial cost in capacity. hmm I've briefly thought about that as well (just forgot to mention it)... for some reason (probably a non-reason) I've always had a bad feeling with respect to that uneven mixing (i.e. three copies on four disks), AFAIU that would look like (each same number being the same chunck: +---------+ +---------+ +---------+ +---------+ | sda | | sdb | | sdc | | sdd | +---------+ +---------+ +---------+ +---------+ 0 1 2 0 1 3 0 2 3 1 2 3 4 5 6 4 5 7 4 6 7 5 6 7 8 9 10 8 9 11 8 10 11 9 10 11 And that gives me again, any 2 disks... but so much better performance? With 4x 4TiB disks,.. RAID6 would give me 16/2 TiB... and the above would give me 16/3 TiB?! Quite a loss... And AFAIU it doesn't give me any better resilience than RAID6 (by tricks like probabilities or so)? Can it be grown? Like when I want to use the 5th bay? What would it be then, still any 2 out of 5? > Two-failure resilience is vital to completing recovery after replacing a > failed drive, particularly when the read error rates of consumer-grade > drives are involved. Well,... I have enterprise disks, and I have backups on different media,... but nevertheless,... I wouldn't "risk" RAID5 for my precious data > In your specific case, raid6 has one additional advantage: making future > expansion to the fifth bay a reliable, simple, no downtime event. Ah... so I couldn't online/offline grow a RAID10 with n/f/o=3 ? > In your situation, I would use raid6. To mitigate the performance hit > on occasional random-access work, I would use a small chunk size (I use > 16k). That will somewhat hurt peak linear performance, but even > bluray-equivalent media streams only amount to 5 MB/s or so. That would > be 80 IOPS per device in such a four-drive raid6. I think RAID6 will be what I go for, at least unless the RAID10 with three blocks gives me any resilience bonus, which I can't see right now. Any ideas about the layout? i.e. left-symmetric-6, right-symmetric-6, left-asymmetric-6, right-asymmetric-6, and parity-first-6 ? I'd have even one more question here: Has anyone experience with my idea of intentionally running devices of different vendors (Seagate, WD, HGST)... for resilience reasons?... Does it work out as I plan, or are there any hidden caveats I can't see which make the resilience (not the performance) worse? Thanks :), Chris.
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