[aniou]

I'm sorry to say that but this article is not entirely true - an illustration "how does traditional raid 4/5/6 do it?" shows ONLY RAID 4. There is a big difference between RAID 4 and RAID 5/6 and former was abandoned a years (decades?) ago in favor of RAID 5 and - later - 6.

Of course, it gives "better publicity" for RAID-Z, but it is rather an marketing trick not engineering.

See https://en.wikipedia.org/wiki/Standard_RAID_levels

[mrighele]

Note that the article talks about the way the array is expanded, not how the specific level works.

In other words, what they are saying is that the traditional way to expand an array is essentially to rewrite the whole array from scratch, so if the old array has three stripes, with blocks [1,2,3,p1] [4,5,6,p2] and [7,8,9,p3] (with p1 and p2 being the parity blocks), the new array will have stripes [1,2,3,4,p1'], [5,6,7,8,p2'] and [9,x,x,x,p3'], i.e. not only has to move the blocks around, but also recompute essentially all the parity blocks.

IF I understand the ZFS approach correctly, the existing blocks are not restructured but only reshuffled, so the new layout will be logically still [1,2,3,p1] [4,5,6,p2] and [7,8,9,p3] but distributed on five disks so [1,2,3,p1,4] [5,6,p2,7,8], [9,p3,x,x,x]

It seems that this means less work while expanding, but some space lost unless one manually copies old data in a new place.

IF I got it right, I am not sure who is the intended audience for this feature: enterprise users will probably not use it, and power users would probably benefit from getting all the space they could get from the extra disk

[Elhana]

Power users would like to get all the space, but when the choice is either you buy just one hdd and get some space or 4+ hdds to replace old array with a completely new one and then left over with unused old one - most would pick first option.