On Tue, Nov 10, 2009 at 4:06 PM, tsuraan <[email protected]> wrote: >> 3/ The md-raid6 recovery code assumes that there is always at least >> two good blocks to perform recovery. That makes the current minimum >> number of raid6 members 4, not 3. (small nit the btrfs code calls >> members 'stripes', in md a stripe of data is a collection of blocks >> from all members). > > Why would you use RAID6 on three drives instead of mirroring across > all of them? I agree it's an artificial limitation, but would anybody > use a RAID6 with fewer than 4 drives?
Here is some text I wrote on a local linux-users-group list a few months ago, on a thread talking about cost/reliability trade-off on small arrays. (it doesn't seem to be in a public archive) Lets also consider another configuration: Raid 0: 4 * 1TB WD RE3s = $640; 4TB; $0.160/GB WD1002FBYS (1TB WD RE3) has a spec MTBF of 1.2 million hours. Lets assume a mean time to replace for each drive of 72 hours, I think thats a reasonably prompt response for a disk at home. Raid 0 1.2million hours/4 = 34.22313483 yrs MTBF $4.675/TB/MTBF_YEAR Raid 5 1.2million_hrs * (1.2million_hrs/(4*3*72)) = 190,128 yrs MTBF $0.00112/TB/MTBF_YEAR Raid 0+1 (1.2million_hrs * 1.2million_hrs / (2 * 72))/2 = 570,386 yrs MTBF $.00056102/TB/MTBF_YEAR Raid 6 1.2million_hrs*1.2million_hrs*1.2million_hrs/(4*3*2*72*72) = 1,584,404,390 yrs MTBF $0.00000020/TB/MTBF_YEAR -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
