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
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