<i>&gt;You&#x27;re getting caught up on redundancy for a given total number of disks, and in the process missing the point entirely.</i><p>That is the only point. Let me state it like this: you have N disks of which you want to get M capacity out of (M &lt; N), what&#x27;s the RAID configuration that maximizes redundancy. My assertion is that parity raid is strictly better than RAID10 at doing that, you haven&#x27;t really refuted it.<p><i>&gt;From there, my point should be more clear. Just like how you can constantly add more and more parity blocks (and disks to handle them), I can constantly widen the mirror. </i><p>Of course you can, and for any given number of disks you&#x27;ll always be behind on redundancy. You can of course just throw more hardware at the problem and if your choices are between RAID6 (2-parity) and RAID10 then RAID10 eventually wins because you need more parity disks than RAID6 provides to take advantage of the extra hardware. Since most block RAID implementations only do 2 or at most 3-parity, RAID10 ends up being the only practical solution for lots of disks. But for a 4 or 5 disk setup (most NAS applications for example) RAID6 or ZFS 3-parity support is a better choice if the performance tradeoff is workable (as it is in most NAS).<p><i>&gt;because with a n-disk array, a RAID1 will always be at least as redundant as the equivalent parity-based RAID </i><p>This is a red herring, yes there is the lower bound of no implementation with N disks can survive the loss of all N disks, and if you want the space of only a single disk RAID1 of N disks is the obvious choice.<p><i>(really, it&#x27;ll always be able to survive one more disk failure than an n-1 parity-block array; you can see this rather easily when comparing RAID5 with a three-way RAID1).</i><p>This is again wrong. A 3 disk RAID1 has the size of 1 disk and a 3 disk RAID5 has the size of 2 disks so they&#x27;re not the same. To make it comparable you need to make the array RAID6 at which point you have the same space and redundancy. You&#x27;d still do a RAID1 though as for the 1 disk size case there is no added benefit of parity.<p><i>&gt;RAID10&#x27;s advantages become more clear, since each mirror group added to the striped whole instantly bumps up capacity and the best-case without compromising the worst-case. Meanwhile, you do have the advantage of having more flexibility in the quantity of disks added, but doing so doesn&#x27;t help your best-case unless you feel like resuming the whole &quot;let&#x27;s make every disk a parity or mirror disk&quot; arms race.</i><p>This is missing the point entirely. The parity normal case is the same as the RAID10 best case and better than the worst case. There&#x27;s no real way around that.<p><i>&gt;Which is exactly why you&#x27;re incorrect. Since any disk failure is like any other, every single disk has to be unique in order for a parity-based array to benefit from any diversity</i><p>This is just not true. Having diversity means that failures become uncorrelated. Parity RAID explores that even better than RAID10. The property you keep bringing up is basically saying &quot;since RAID10 has worse redundancy than n-parity it benefits more from uncorrelated disk failures&quot; which is true but only by partially mitigating a problem that parity RAID doesn&#x27;t have.<p><i>&gt;This has been a fun discussion, and there is certainly lots of room for debate on the merits of mirroring v. parity, but I&#x27;m starting to think that we should just agree to disagree on this one.</i><p>I think that by now I&#x27;ve explained my point clearly enough and even repeated myself a bit so yeah, I&#x27;ll drop it here.