> If you're going to cherry pick a portion of a commit header please > reference the commit id and use quotes or indentation to make it clear > what is being referenced, etc. Apologies. > Quite the tangent just to setup an a toy example of say: thinp with 256K > blocksize/chunk_sectors ontop of a RAID6 with a chunk_sectors of 128K > and stripesize of 1280K. I screwed up my math ... many apologies :/ Consider a thinp of chunk_sectors 512K atop a RAID6 with chunk_sectors 1280K. (Previously, this RAID6 would be disallowed because chunk_sectors could only be a power of 2, but 07d098e6bba removed this constraint.) -With lcm_not_zero(), a full-device IO would be split into 2560K IOs, which obviously spans both 512K and 1280K chunk boundaries. -With min_not_zero(), a full-device IO would be split into 512K IOs, some of which would span 1280k chunk boundaries. For instance, one IO would span from offset 1024K to 1536K. -With the hypothetical gcd_not_zero(), a full-device IO would be split into 256K IOs, which span neither 512K nor 1280K chunk boundaries. > > To be clear, you are _not_ saying using lcm_not_zero() is correct. > You're saying that simply reverting block core back to using > min_not_zero() may not be as good as using gcd(). Assuming my understanding of chunk_sectors is correct -- which as per blk-settings.c seems to be "a driver will not receive a bio that spans a chunk_sector boundary, except in single-page cases" -- I believe using lcm_not_zero() and min_not_zero() can both violate this requirement. The current lcm_not_zero() is not correct, but also reverting block core back to using min_not_zero() leaves edge cases as above. I believe gcd provides the requirement, but min_not_zero() + disallowing non-power-of-2 chunk_sectors also provides the requirement. > > But it's possible I'm misunderstanding the purpose of chunk_sectors, > > or there should be a check that the one of the two devices' chunk > > sizes divides the other. > > Seriously not amused by your response, I now have to do damage control > because you have a concern that you really weren't able to communicate > very effectively. Apologies.
