On Mon, Nov 30 2020 at 7:21pm -0500, John Dorminy <jdorminy@xxxxxxxxxx> wrote: > > 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.) Think you have your example messed up still. RAID 10+2 with 128K chunk_sectors, 1280K full stripe (io_opt). Then thinp stacked ontop of it with chunk_sectors of 1280K was usecase that wasn't supported before. So stacked chunk_sectors = min_not_zero(128K, 1280K) = 128K > -With lcm_not_zero(), a full-device IO would be split into 2560K IOs, > which obviously spans both 512K and 1280K chunk boundaries. Sure, think we both agree lcm_not_zero() shouldn't be used. > -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. RAID6 with chunk_sectors of 1280K is pretty insane... And yet you're saying full device IO is 1280K... So something still isn't adding up. Anyway, if we run with your example of chunk_sectors (512K, 1280K), yes there is serious potential for IO to span the RAID6 layer's chunk_sector boundary. > -With the hypothetical gcd_not_zero(), a full-device IO would be split > into 256K IOs, which span neither 512K nor 1280K chunk boundaries. Yeap, I see. > > 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. But your chunk_sectors (512K, 1280K) example is a misconfigured IO stack. Really not sure it worth being concerned about it. > I believe gcd provides the requirement, but min_not_zero() + > disallowing non-power-of-2 chunk_sectors also provides the > requirement. Kind of on the fence on this... think I'd like to get Martin's take. Using gcd() instead of min_not_zero() to stack chunk_sectors isn't a big deal; given the nature of chunk_sectors coupled with it being able to be a non-power-of-2 _does_ add a new wrinkle. So you had a valid point all along, just that you made me work pretty hard to understand you. > > > 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. Eh, I need to build my pain threshold back up.. been away from it all for more than a week.. ;) Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
