On Tue, Oct 17 2017, Anthony Youngman wrote: > 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? RAID5 would not allow a 12K chunk size (must be power of 2) but RAID0 would. Not sure about RAID10. I interpret "stripe" to mean "a set of chunks, one from each device". So if you had a RAID10 with a 12K chunk size and 3 devices, then a stripe would be 36K of space, 12K per device. This is primarily an address-space mapping. Think of it as a function from "array-address" to "device-index, device-address". 0 -> 0,0 512 -> 0,512 1024 -> 0,1024 .... 3072 -> 1,0 3584 -> 1,512 .... No imagine that the application always sends 512 I/O requests. Each I/O request is mapped through the above function and sent to the appropriate device with the new address. In practice, larger requests are allowed and the a split into sub-requests if the function isn't contiguous for the whole range of a particular request. > > In other words, does a stripe consist of one block per drive, or one > chunk per drive? One chunk per drive. Note that inside the md/raid5 code the word "stripe" usually means one PAGE per drive. This is an unfortunately historical accident. I sometimes use the word "strip" (no 'e') to mean one page (or one block) per device. A strip is not contiguous in the array address space. A stripe is. Thanks, NeilBrown > > (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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