On 17/10/17 20:04, Phil Turmel wrote:

No, it is*wrong*.  Writes in multiples of 4k and entirely within a
chunk are passes as-is to the devices.  For mirrors, all affected
devices get a copy of the request.  For parity raid, the 4k stripes
corresponding to those 4k blocks will be pulled into the stripe cache
for recalculation.  Not whole chunk-size stripes.  The stripe cache is
multiples of 4k, not multiples of the chunk size!

Writes smaller than 4k, or not aligned to 4k, will generate a
read-modify-write cycle of the 4k block involved.  Not the whole chunk.

It is more accurate to say that a chunk may be the*largest*  a request
can be before it is split between devices.

Okay, I think I need to update my understanding on this ... :-)

Let's say a chunk is 12K. That's three 4K blocks to drive 1, followed by 
three to drive 2 etc. Does that mean that each chunk is split across 
three stripes, or is the stripe all the 12K chunks one per drive?

In other words, does a stripe consist of one block per drive, or one 
chunk per drive?

(I'll put a "sic" on that page then, just to point out it's a 
misunderstanding by the original author. As I said, I'd rather not mess 
around with the page now.)

Cheers,
Wol